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The runoff change in a large river is significant for hydrological event monitoring, such as floods and droughts. In this paper, we attempt to invert the river flow change based on the surface displacements observed by the continuous global navigation satellite system (GNSS) stations in the Wuhan section of the Yangtze River (WYR). We establish a two‐dimensional model accounting for deformation from a line load in an elastic half‐space to estimate the annual water load change from the GNSS data. The results show that the inverted line load from the annual maximum displacement change observed by the GNSS is Ne=2.30×108N·m−1 ${N}_{e}=2.30\times {10}^{8}\mathrm{N}\cdot {\mathrm{m}}^{-1}$, which is in good agreement with the river load from the hydrological station. This paper demonstrates a low‐cost and practical method to river flow research in local areas, which is also a creative application for the GNSS. Plain Language Summary: The runoff change in large rivers is an important element for flood and drought monitoring. Hydrological stations are used to monitor the water load change. Because of the high cost of construction and maintenance, there are few hydrological stations in most areas, and the unpredictable noise of hydrological stations influences the stability and precision of the results. We use the vertical displacements observed by the global navigation satellite system (GNSS) to estimate the river load and find that the results agree with the calculated water load change from hydrological stations. Thus, the GNSS can be an alternative for runoff change research, with low cost and high efficiency. In this study, we find a new application for the GNSS to water runoff change in local areas. Key Points: We propose a new method for estimate the river load change using the continuous global navigation satellite system (GNSS) dataA simple two‐dimensional line load on Elastic half‐space can invert the river load with considerable accuracy in local areasThe forward modeling prove that the modeled displacement agrees with the observed displacement from the GNSS [ABSTRACT FROM AUTHOR]

Prompted by field observation of an aufeis‐induced channel avulsion along the Hula Hula River in June 2021, we use measurements of channel migration zone width along 15 rivers flowing north across the Arctic coastal plain in Alaska, USA. We differentiated sites with aufeis that covered ≥1 km2 in early summer during the period 2017–2021 from sites without such aufeis formation. All but four of the 28 sites with aufeis have widths greater than the 95% confidence interval and 20 sites fall outside of the 95% prediction interval for channel width based on drainage area. Pairwise comparison indicates that the population of aufeis sites have significantly wider channel migration zones (p < 0.0001) than non‐aufeis sites after accounting for drainage area. Seasonal aufeis facilitates lateral channel migration and associated heterogeneity. Loss of aufeis under warming climate may reduce habitat diversity in these river corridors. Plain Language Summary: Aufeis forms in Arctic river corridors when groundwater springs continue to provide water to the surface after the river is covered by ice. The resulting shelves of ice can be up to 3 m thick and extend across the channel and floodplain. As this ice slowly melts in early summer, it can divert river flow from the active channel onto the floodplain, creating new channels. This paper examines the resulting width of the floodplain across which active channel migration occurs. Comparing sites with and without aufeis, we find that sites with aufeis are significantly wider. Loss of aufeis with warming climate will reduce the habitat diversity of Arctic rivers. Key Points: Aufeis that persists into summer on rivers of the Alaskan Arctic coastal plain can force channel avulsionSites with repeated aufeis in successive years have wider channel migration zonesLoss of aufeis as climate warms may result in narrower and less diverse river corridors [ABSTRACT FROM AUTHOR]

Ganymede is the only Solar System moon known to generate a permanent magnetic field. Jovian plasma motions around Ganymede create an upstream magnetopause, where energy flows are thought to be driven by magnetic reconnection. Simulations indicate Ganymedean reconnection events may be transient, but the nature of magnetopause reconnection at Ganymede remains poorly understood, requiring an assessment of reconnection onset theory. We present an analytical model of steady‐state conditions at Ganymede's magnetopause, from which the first Ganymedean reconnection onset assessment is conducted. We find that reconnection may occur wherever Ganymede's closed magnetic field encounters Jupiter's ambient magnetic field, regardless of variations in magnetopause conditions. Unrestricted reconnection onset highlights possibilities for multiple X lines or widespread transient reconnection at Ganymede. The reconnection rate is controlled by the ambient Jovian field orientation and hence driven by Jupiter's rotation. Future progress on this topic is highly relevant for the JUpiter ICy moon Explorer mission. Plain Language Summary: Ganymede is the largest moon of Jupiter and the only Solar System moon that produces its own magnetic field. Ganymede's magnetic field is surrounded by Jupiter's much larger magnetic field, which flows around the moon like a river flowing around a rock. The boundary where Jupiter's magnetic field first encounters Ganymede's is called the magnetopause. At this boundary, energy and mass can move between the two magnetic fields through a process called magnetic reconnection. Our paper introduces a simple model of Ganymede's magnetopause and uses this model to show where reconnection can occur on the boundary. We find that reconnection can occur anywhere on the magnetopause for any plausible environmental conditions around Ganymede, so the locations where these energy‐releasing events occur may be particularly unpredictable. The rate of energy released by reconnection meanwhile depends on near‐Ganymede conditions, which change significantly as Jupiter rotates. These results will help inform the planning of the JUpiter ICy moon Explorer mission to Ganymede. Key Points: We create the first analytical model of conditions at Ganymede‐Jupiter magnetopause and assess magnetic reconnection onset theoryReconnection may occur anywhere on the magnetopause where Ganymede's closed magnetic field meets the ambient field of JupiterThe average reconnection rate at Ganymede exhibits a Jovian‐diurnal variation and hence is driven by Jupiter's rotation [ABSTRACT FROM AUTHOR]

Antecedent hydrological conditions are recorded through the evolution of dissolved lithium isotope signatures (δ7 ${\delta }^{7}$Li) by juxtaposing two storm events in an upland watershed subject to a Mediterranean climate. Discharge and δ7 ${\delta }^{7}$Li are negatively correlated in both events, but mean δ7 ${\delta }^{7}$Li ratios and associated ranges of variation are distinct between them. We apply a previously developed reactive transport model (RTM) for the site to these event‐scale flow perturbations, but observed shifts in stream δ7 ${\delta }^{7}$Li are not reproduced. To reconcile the stability of the subsurface solute weathering profile with our observations of dynamic stream δ7 ${\delta }^{7}$Li signatures, we couple the RTM to a distribution of fluid transit times that evolve based on storm hydrographs. The approach guides appropriate flux‐weighting of fluid from the RTM over a range of flow path lengths, or equivalently fluid residence times. This flux‐weighted RTM approach accurately reproduces dynamic storm δ7 ${\delta }^{7}$Li‐discharge patterns distinguished by the antecedent conditions of the watershed. Plain Language Summary: Storm events often cause characteristic shifts in stream solute chemistry. Interpreting these signals offers insight into the water‐rock interactions occurring within watersheds. Here, we use lithium stable isotopes and reactive transport modeling to relate how long water spends in a catchment, or how deep water infiltrates through a catchment, to the extent of chemical weathering. We show that the first significant storm after a dry season exports more chemically evolved water, while a wet season storm releases less evolved, shallower, and younger water. Our results indicate that stream flow δ7 ${\delta }^{7}$Li in small watersheds offers a sensitive record of hydrological conditions prior to the storm, reflecting subtle shifts in the efficiency of the Critical Zone to generate, transport, and ultimately export solutes. Key Points: Stream lithium stable isotope ratios (δ7Li) recorded at high frequency over storm events are sensitive to antecedent conditionsA reactive transport model cannot produce observed shifts in stream chemistry through variations in flow rate aloneFlux‐weighting of model fluid outputs based on time‐varying fluid transit time distributions describes stream δ7Li over storm hydrographs [ABSTRACT FROM AUTHOR]

We investigate whether geomorphic signatures of permafrost are embedded in planforms of river meanders, and we inquire as to how physical factors unique to permafrost environments are able to affect their dynamics. By exploiting satellite imagery, a data set of 19 freely‐meandering Arctic rivers is compared against an independent data set of 23 freely‐meandering streams flowing through temperate and tropical regions. Suitable dimensionless metrics are defined to characterize morphometric properties of meanders in terms of the spatio‐temporal distribution of curvature and channel width. Results show the absence of marked contrasts in the amplitude of bend‐curvature between the two data set. Differently, we find a permafrost signature in the channel width response, which manifests itself through larger values of the average bend‐width and by peaks of width fluctuations. Field data suggest that permafrost meanders tend to widen for increasing bend sinuosity, likely promoting a shift of their morphodynamic regime as final cutoff is approached. Plain Language Summary: One of the most striking impacts of climate warming in the Arctic region is permafrost thaw. Arctic rivers typically flow through perennially‐frozen floodplains, thus they are particularly susceptible to ground thawing. In order to understand the response of Arctic rivers to climate variability, basic knowledge about key differences with respect to non‐permafrost streams is needed. Despite recent studies which have emphasized the slower yearly movement rates distinguishing Arctic streams, we still do not understand whether permafrost‐affected rivers show distinctive features in their morphology due to specific physical mechanisms. By exploiting satellite imagery, we show that permafrost leaves a signature in the shape of meandering Arctic rivers. Specifically, their average bend‐width increases as sinuosity develops, while the amplitude of width oscillations is larger than that displayed by their non‐permafrost kin. Key Points: Permafrost is found to leave a morphological signature in the spatio‐temporal signal of channel width at the bend scalePermafrost meander bends show larger amplitude of width oscillations and widen as sinuosity increasesBend curvature as a standalone indicator does not provide evidence of any significant permafrost‐fingerprint [ABSTRACT FROM AUTHOR]

Sedimentary landforms on Earth and other planetary bodies are built through scour, transport, and deposition of sediment. Sediment connectivity refers to the hypothesis that pathways of sediment transport do not occur in isolation, but rather are mechanistically linked. In dryland river systems, one such example of sediment connectivity is the transport of fluvially deposited sediment by wind. However, predictive tools that can forecast fluvial‐aeolian sediment connectivity at meaningful scales are rare. Here we develop a suite of models for quantifying the availability of river‐sourced sediment for aeolian transport as a function of river flow, wind regime, and land cover across 168 km of the Colorado River in Grand Canyon, USA. We compare and validate these models using topographic changes observed over 10 years in a coupled river sandbar‐aeolian dunefield setting. The models provide a path forward for directly linking fluvial hydrology with the management and understanding of aeolian landscapes. Plain Language Summary: Landscapes on Earth are built from sediment, which is deposited by a variety of processes, especially water and wind. In rivers located in dry regions, sediment is deposited by flowing water during river floods. Once this sediment dries, it is often then moved by wind across the landscape. Although we frequently observe this sediment connectivity between water and wind, we are not currently able to predict when and where windblown transport of river‐deposited sediment may occur. We used maps of sand in Grand Canyon along 168 km of the Colorado River, along with records of river flow, wind speed, and vegetation cover, to develop four models that predict how much sediment is available for wind to move. We compared the results of these models to actual windblown sediment transport at a study site in Grand Canyon, finding that when models predicted more sand was available at the site, we also observed more landscape changes there which were due to wind. Our research makes it possible to understand how changes in river flows affect the amount of sediment moved by wind. Key Points: We developed four methods for modeling the extent of exposed, bare sand available for windblown transport in river valleysOur models employ data on river flow, land cover, wind velocity, and vegetation to predict exposed, bare sand extent at corridor scalesWhere models predict increased areal extent of bare sand, we also observe a greater volume of topographic changes driven by wind [ABSTRACT FROM AUTHOR]

Debris flows and floods generated by rainfall runoff occur in rocky mountainous landscapes and burned steeplands. Flow type is commonly identified post‐event through interpretation of depositional structures, but these may be poorly preserved or misinterpreted. Prior research indicates that discharge magnitude is commonly amplified in debris flows relative to floods due to volumetric bulking and increased frictional resistance. Here, we use this flow amplification to develop a metric (Q*) to separate debris flows from floods based on the ratio of observed peak discharge to the theoretical maximum water discharge from rainfall runoff. We compile 642 observations of floods and debris flows and demonstrate that Q* distinguishes flow type to ∼92% accuracy. Q* allows for accurate identification of debris flows through simple channel cross‐section surveys rather than through qualitative interpretation of deposits, and therefore should increase the performance of models and engineered structures that require accurate flow‐type observations. Plain Language Summary: Steep channels in rocky landscapes and recently burned areas carry a wide range of flow types, ranging from water floods to highly destructive debris flows composed of rock, sediment, other incidental debris, and water. Typically, the flow type that recently occurred in a channel is identified by interpreting the structure of deposits left behind by the flow. However, these deposits can be challenging to interpret, provided they are preserved at all. Prior research has shown that the shift in flow physics occurring in debris flows has the effect of amplifying the magnitude of the flow beyond that which is possible in floods. In this study, we take advantage of this flow amplification to develop a quantitative metric that allows for flow type to be identified without interpreting depositional structures. We demonstrate the metric to be highly accurate. This methodology should increase the accuracy of predictive models trained on such observations, which can help to better understand where these hazards exist and protect communities and infrastructure at risk. Key Points: To distinguish debris flows from floods, a metric Q* equal to the ratio of peak discharge to rainfall‐runoff discharge is developedQ* separates floods (Q* ≤ 1) from debris flows (Q* > 1) to ∼92% accuracy and is robust to uncertainties in the key parametersThe Q* metric should reduce misidentification of flow type and increase the performance of empirical‐hazard models and hazard mitigation [ABSTRACT FROM AUTHOR]

River flow models that consider satellite observations of the water surface elevation (WSE) require meaningful segmentation into reaches. Segmentation methods involve a trade‐off between the spatial sampling and error characteristics of satellite observations, both of which impact the flow model realism/accuracy. This paper investigates the spatial properties of WSE profiles (slope and concavity) and their link with hydraulic controls (HCs) and leverages this knowledge to advance river segmentation. Fine‐scale synthetic cases were analyzed to characterize HC effects on the remotely observable WSE properties. Following this, a wavelet‐based segmentation method was developed and subsequently tested on real rivers, including test cases with measurements characteristic of the Surface Water and Ocean Topography satellite mission. The results show that the local extrema of water surface (WS) concavity are good candidates for defining reach bounds while consistently preserving the HC signatures and local flow nonuniformities (deviations from equilibrium) from fine to large spatial scales. Key Points: The hydraulic visibility of river surface profile variabilities from space is investigatedHydraulic control signatures are depicted in local extrema of water surface curvatureA wavelet‐based river segmentation preserving hydraulic controls is proposed [ABSTRACT FROM AUTHOR]

Abstract: Seasonal streamflow forecasts facilitate water allocation, reservoir operation, flood risk management, and crop forecasting. They are generally computed by forcing hydrological models with outputs from general circulation models (GCMs) or using large‐scale climate indices as predictors in statistical models. In contrast, hybrid statistical‐dynamical forecasts (combining statistical methods with dynamical climate predictions) are still uncommon, and their skill is largely unknown. Here we conduct systematic forecasting of seasonal streamflow using eight GCMs from the North American Multi‐Model Ensemble, 0.5–9.5 months ahead, at 290 stream gauges in the U.S. Midwest. Probabilistic forecasts are developed for low to high streamflow using predictors that reflect climatic and anthropogenic influences. Results indicate that GCM forecasts of climate and antecedent climatic conditions enhance seasonal streamflow predictability; while land cover and population density predictors decrease biases or enhance skill in certain catchments. This paper paves the way for novel forecasting approaches using dynamical GCM predictions within statistical frameworks. [ABSTRACT FROM AUTHOR]

Glaciers and ice streams flowing over sediment beds commonly have a layer of ice‐rich debris adhered to their base, known as a "frozen fringe," but its impact on basal friction is unknown. We simulated basal slip over granular beds with a cryogenic ring shear device while ice infiltrated the bed to grow a fringe, and measured the frictional response under different effective stresses and slip speeds. Frictional resistance increased with increasing slip speed until it plateaued at the frictional strength of the till, closely resembling the regularized Coulomb slip law associated with clean ice over deformable beds. We hypothesize that this arises from deformation in a previously unidentified zone of weakly frozen sediments at the fringe's base, which is highly sensitive to temperature and stress gradients. We show how a rheologic model for ice‐rich debris coupled with the thermomechanics of fringe growth can account for the regularized Coulomb behavior. Plain Language Summary: Many glaciers move by sliding over sediment beds. As the glacier flows downslope, ice can infiltrate the underlying sediments, forming a layer of ice‐rich debris attached at the glacier's base. We investigated how this frozen fringe impacts glacier motion by simulating glacier slip in a cold‐room facility with a specialized ring shear device. We recreated glacier conditions, sliding ice over granular beds to form the fringe, and then assessed how frictional resistance at the slip interface varied under different stresses and ice speeds. We found that frozen fringe influences the relationship between ice speed and frictional resistance, known as the "slip law." As ice speed increased, basal friction increased to a threshold that matches the strength of the ice‐free sediment bed—mirroring the "regularized Coulomb slip law" inferred for clean ice over soft beds. We attribute this behavior to deformation in a weakly frozen zone at the base of the frozen fringe and show how this behavior can be incorporated into existing parameterizations of glacier slip. Key Points: Ring shear experiments show frozen fringe alters basal slip dynamics for soft‐bedded glaciersDeformation in a zone of weakly frozen sediments within the fringe leads to a regularized Coulomb slip response [ABSTRACT FROM AUTHOR]

Measurement of planktonic chlorophyll‐a—a proxy for algal biomass—in rivers may represent local production or algae transported from upstream, confounding understanding of algal bloom development in flowing waters. We modeled 3 years of chlorophyll‐a transport through a 394‐km portion of the Illinois River and found that although algal biomass is longitudinally widespread, most net production occurs at river control points in the upper reaches (up to 3.7 Mg chlorophyll‐a y−1 km−1). Up to 69% of the algal biomass in the upper river was a result of within‐reach production, with the remainder recruited from headwaters and tributaries. High chlorophyll‐a measured farther downstream was largely because of transport from source‐area control points, with substantial net losses of algal biomass occurring in the lower river. Modeling the often‐overlooked river transport component is necessary to characterize where, when, and why planktonic algae grow and predict how far and fast they move downstream. Plain Language Summary: Planktonic algae in rivers may accumulate during periods of high productivity stimulated by favorable light, temperature, nutrient, and flow conditions, which can disrupt ecological processes and affect human uses including recreation and drinking water supply. Planktonic algae observed in rivers may occur because of local growth or transport from upstream source areas. Therefore, considering both local and upstream conditions may improve early warnings of potentially harmful blooms. Along a 394‐km stretch of the Illinois River, we found that most of the algae grew in the upper reaches and was then transported to downstream reaches, contributing to potential downstream harms such as excessive turbidity, organic carbon, biological oxygen demand, and algal toxins. We demonstrate how the often‐overlooked river transport component can be quantified to better identify where, when, and why algae grow in river networks. Key Points: Planktonic algal biomass is pervasive in the Illinois River, yet production is favored at certain locations and timesMost planktonic algal biomass was produced in upper‐reach control points that supplied downstream areasTransport analysis using local and upstream data improves understanding of river algal blooms [ABSTRACT FROM AUTHOR]

Drought has affected much of the western United States since about the year 2000, including the Upper Colorado River Basin (UCRB). Using a time series of UCRB streamflow derived from a tree‐ring based reconstruction of UCRB streamflow for the years 1 CE through 1905 CE, together with naturalized UCRB streamflow values for 1906 CE through 2021 CE, we identify 51 drought events, including the 2000–2021 drought. Although the recent 2000–2021 drought has been relatively severe, it is not the most severe of the UCRB drought events we identified. Results also indicate that natural variability combined with projected climate warming can result in UCRB drought events that are more severe than any drought since 1 CE. Plain Language Summary: A long and severe drought has affected much of the western United States since about the year 2000 CE, including the Upper Colorado River Basin (UCRB). Comparing this drought to past UCRB droughts (during 1 CE through 2021 CE), we find that the 2000–2021 drought is not the most severe UCRB drought. The results also suggest that natural variability combined with projected climate warming could result in UCRB drought events that are more severe than any drought since 1 CE. Key Points: We identify 51 drought events for the years 1 CE through 2021 CE for the Upper Colorado River Basin (UCRB)Twelve of the 51 UCRB droughts we identify were more severe than the 2000–2021 droughtNatural variability and/or a 1°C temperature increase can trigger UCRB droughts that are more severe than the 2000–2021 drought [ABSTRACT FROM AUTHOR]

The riverine transport and deposition of mud is the primary agent of landscape construction and evolution in many fluvial and coastal environments. Previous efforts exploring this process have raised uncertainty regarding the effects of hydrodynamic and chemical controls on the transport and deposition of mud, and thus the constructions of muddy coastal and upstream environments. As such, direct measurements are necessary to constrain the deposition of mud by river systems. Here, we combine laboratory evidence and a field investigation in the Mississippi River delta to explore the controls on the riverine transport and deposition of mud. We show that the flocculation of mud, with floc diameters greater than 10 μm, in freshwater is a ubiquitous phenomenon, causing the sedimentation of mud to be driven by changes in local hydrodynamics, and thus providing an explanation for how river systems construct landscapes through the deposition of mud in both coastal and upstream environments. Plain Language Summary: Muddy landscapes are some of the most common and vital environments on Earth's surface, such as floodplains, deltas, and estuaries. Due to their small size, and thus easily suspendable nature, mud particles are thought to be extremely difficult to deposit under typical flow conditions in rivers. As such, the means by which rivers deposit mud has been the subject of much study and debate. Canonically, the deposition of cohesive sediment occurs as rivers approach the ocean, where increases in salinity induce the aggregation of clay and silt particles to form heavier floccules, or "flocs." Recent studies have also explored the influence of organic material on mud aggregation. However, the means by which rivers are able to construct muddy landscapes upstream, in the absence of saline or organic‐rich water, remains uncertain. To address this discrepancy, we utilize a combination of laboratory evidence and a field investigation on the Mississippi River delta to show that all mud is transported as flocs, not as individual grains, and as such, the deposition of mud by river systems is controlled by the ability of the river flow to suspend existing mud flocs, and not by abrupt changes in water chemistry inducing flocculation in coastal regions. Key Points: Flocculation of mud in water is a ubiquitous phenomenonSalinity may have little to no effect on mud floc size as rivers enter marine environmentsDeposition of flocculated mud in river systems is driven by changes in hydrodynamic stress [ABSTRACT FROM AUTHOR]

High‐resolution regional climate model (RCM) simulations of global warming consistently predict larger percentage increases in precipitation in the lee of midlatitude mountain ranges than on their windward slopes, indicating a weakening of the orographic rain shadow. This redistribution of precipitation could have profound consequences for water resources and ecosystems, but its underlying mechanisms are unknown. Here we show that rain‐shadow weakening is just one manifestation of a more general decrease in the influence of orography on precipitation under global warming. We introduce a simple model of precipitation change based on this principle, and find that it agrees well with an ensemble of high‐resolution simulations performed over the western United States. We argue that diminished orographic influence can be explained by the unique vertical structure of orographically forced ascent, which tends to maximize in the lower atmosphere where condensation is thermodynamically less sensitive to warming. Plain Language Summary: Mountains supply about half of the fresh water used by humans globally and often regulate the seasonal cycle of stream flows by accumulating snowpack during the wet season and releasing it over the dry season. Regional simulations tend to predict large future changes in the spatial pattern of precipitation in mountainous regions, but the reasons for these changes are not well understood. Here we show that the predicted patterns of precipitation change in the western US can be explained by a simple mechanism that causes the influence of mountains to weaken with increasing temperatures. We introduce a simple model of precipitation change based on this effect, and show that its predictions closely match those of far more sophisticated regional simulations of future warming over the western US. Key Points: Orographic influence on precipitation is reduced in simulations of global warmingFractional precipitation change scaled by warming is reproducible with a simple model [ABSTRACT FROM AUTHOR]

We demonstrate that river discharge can be estimated by deriving water surface velocity estimates from satellite‐derived video imagery when combined with high‐resolution topography of channel geometry. Large Scale Particle Image Velocimetry (LSPIV) was used to map surface velocity from 28 s of 5 Hz satellite video acquired at a 1.2 m nominal ground spacing over the Darling River, Tilpa, Australia, during a 1‐in‐5‐year flood. We stabilized and assessed the uncertainty of the residual motion induced by the satellite platform, enhancing our sub‐pixel motion analysis, and quantified the sensitivity of image extraction rates on computed velocities. In the absence of in situ observations, LSPIV velocity estimates were validated against predictions from a calibrated 2D hydrodynamic model. Despite the confounding influence of selecting a surface velocity depth‐averaging coefficient, inference of discharge was within 0.3%–15% compared with gauging station measurements. These results provide a valuable foundation for refining satellite video LSPIV techniques. Plain Language Summary: Estimates of river flow are needed to manage water resources and flood risk. However, many of the world's rivers are not gauged, limiting hydrological understanding of river response to changing environmental conditions and storm events. We demonstrate that satellite video can be used to map velocity by tracking surface water features from one video frame to the next, and scaled to compute discharge where river geometry is known. Using a video of a flood on the River Tilpa, Australia, our results agree with ground‐based measurements to within 0.3%–15%. The ability to deploy satellites to acquire video anywhere globally could contribute to measuring discharge on ungauged rivers. Key Points: Satellite video acquired along 12.6 km of the River Darling, Australia, at 5 Hz for 28 s during a 1‐in‐5‐year storm eventSatellite video‐based velocities coupled with high resolution topography estimate riverine discharges to within 15% of in situ gauge dataParametrization of non‐contact velocimetry and choice of a depth‐averaging coefficient (α) influence the accuracy of discharge estimates [ABSTRACT FROM AUTHOR]

Ongoing runoff declines in the Colorado River Basin have been shown to be predominately driven by decreasing albedo from warming‐driven snow‐cover loss, especially in late‐spring (hereafter snowmelt‐radiation feedback). Here, we explore the feedback's impact on annual runoff sensitivity to warming across the western U.S. (WUS) using hydrologic model simulations. For 1°C uniform warming, we show that runoff is most sensitive to warming in modestly snow‐covered, interior mountain headwaters, especially the Rocky Mountains. Runoff sensitivities are most associated with the snowmelt‐radiation feedback in basins with runoff coefficients between 0.2 and 0.6, where runoff sensitivity increases with more snow and lower winter temperature. In aggregate, ∼48% of WUS runoff sensitivity is attributable to the snowmelt‐radiation feedback and is especially pronounced in the warming‐sensitive river basins (annual runoff decreases >5%/°C). We also show that the feedback's impact decreases with increasing temperature, which has unresolved implications for streamflow declines in a less‐snow future. Plain Language Summary: Regional climate warming is driving strong runoff changes in the western U.S. (WUS), especially the Upper Colorado River Basin (UCRB). Previous work showed that warming‐related snow cover reductions lead to more solar radiation absorption and evapotranspiration, which largely explain ongoing runoff declines in UCRB. Here, we assess the impact of this snowmelt‐radiation feedback on warming‐induced runoff changes across WUS. In a warmer world, we find that the largest annual runoff sensitivities are in the interior mountainous WUS with modest snow cover. The snowmelt‐radiation feedback explains over half of the warming‐induced runoff changes in warming‐sensitive WUS basins and about half of WUS' overall runoff sensitivity. In areas influenced by the snowmelt‐radiation feedback, both runoff sensitivity and the feedback's contribution become smaller with higher temperatures, suggesting a potentially slower rate of streamflow decline as temperatures rise in a warmer future. Key Points: Snowmelt‐radiation feedback accounts for ∼1/2 of warming‐driven runoff decline across the Western U.S. (WUS)Runoff sensitivities are most linked to snowmelt‐radiation feedback in river basins with runoff coefficients in the range 0.2–0.6Runoff sensitivities to warming are largest in modestly snow‐covered, interior mountainous parts of WUS, especially the Rocky Mountains [ABSTRACT FROM AUTHOR]

Although most deltas are expected to lose land due to climate change and urbanization, tide‐dominated deltas have been suggested to gain land. The processes of land change in such deltas are, however, not well understood, and tide‐dominated deltas with all their known morphological attributes have not been simulated before. Our Delft3D simulations successfully reproduce tide‐dominated delta morphology, and show that tide‐dominated deltas with seaward widening and stable channels, elongate tidal bars and a funnel shape form when bidirectional tidal flow occurs along the whole delta. In contrast, tide‐influenced but river‐dominated deltas protrude from shoreline, and are efficient at gaining land by mouth bar deposition and bifurcations. These differences are a function of tidal‐fluvial interactions expressed by month‐averaged seaward and lateral velocities. Identification of delta types is thus critical for predicting future land change. We propose morphological metrics to differentiate river‐ and tide‐dominated deltas and test metrics on 40 modern deltas. Plain Language Summary: River deltas build extensive coastal lands that are one of the most economically and ecologically valuable environments on Earth. While many deltas lose land globally due to climate change and urbanization, deltas significantly influenced by tides are suggested to gain land. Here we utilize numerical modeling to explore the relative formative roles of river and tidal flows, and quantify how the land loss and gain processes differ in river‐ and tide‐dominated deltas. We show that deltas where tidal currents dominate throughout the whole delta build land with a low efficiency, because they form narrow elongate bars and seaward widening distributary channels. In contrast, deltas where tidal currents only dominate in seaward reaches are more efficient at gaining land as they build lobate deltas through mouth bar formation. We propose morphological criteria to distinguish river‐ and tide‐dominated deltas, so more accurate predictions of future land loss and gain can be made. We successfully test these criteria on 40 modern deltas, and show that identification of delta types and understanding the relative roles of river and tidal processes are critical for accurately predicting future change in deltas. Key Points: Delft3D modeling quantifies the formative roles of river discharge and tidal amplitude in formation of river‐ versus tide‐dominated deltasWe simulate for the first time how river‐tide interactions lead to channel widening and tidal bar formation in tide‐dominated deltasUnderstanding distinct land loss, gain, and formative processes in river‐ versus tide‐dominated deltas are critical for predicting future change [ABSTRACT FROM AUTHOR]

Accurate observation of hillslope groundwater storage and instantaneous recharge remains difficult due to limited monitoring and the complexity of mountainous landscapes. We introduce a novel storage‐discharge method to estimate hillslope recharge and the recharge ratio—the fraction of precipitation that recharges groundwater. The method, which relies on streamflow data, is corroborated by independent measurements of water storage dynamics inside the Rivendell experimental hillslope at the Eel River Critical Zone Observatory, California, USA. We find that along‐hillslope patterns in bedrock weathering and plant‐driven storage dynamics govern the seasonal evolution of recharge ratios. Thinner weathering profiles and smaller root‐zone storage deficits near‐channel are replenished before larger ridge‐top deficits. Consequently, precipitation progressively activates groundwater from channel to divide, with an attendant increase in recharge ratios throughout the wet season. Our novel approach and process observations offer valuable insights into controls on groundwater recharge, enhancing our understanding of a critical flux in the hydrologic cycle. Plain Language Summary: Groundwater in hilly areas is an important source of water. The amount of rainfall that replenishes groundwater storage is known as groundwater recharge. Because groundwater recharge is challenging to measure directly, we applied a technique that makes it possible to use a more readily observable variable—streamflow, or the water flow in rivers and streams—to calculate how much water is stored in the hillslope as groundwater. This made it possible to use streamflow to estimate how much rainfall becomes groundwater recharge. By understanding the structure of the ground and how moisture is distributed, we were able to determine how the amount of recharge changes over the wet season. Our work improves understanding of how rainfall and plant water use affect groundwater recharge, which is important for managing water resources in mountain landscapes. Key Points: Increases in hillslope groundwater storage can be quantified from storage‐discharge relationsField measurements of groundwater and vadose zone (VZ) storage corroborate seasonality in recharge ratios (recharge per precipitation input)Recharge ratio increases with decreasing plant‐driven VZ (soil and rock) storage deficits, reflecting spatial variations in storage [ABSTRACT FROM AUTHOR]

Mixing models are used throughout earth and environmental science to quantify the relative contributions of sources to mixtures, based on chemical or isotopic tracers. Often, however, some end‐members are missing or their tracer distributions overlap, precluding the use of conventional mixing models. Here I show how these constraints can be overcome by exploiting the information contained in tracer time‐series fluctuations. This approach, ensemble end‐member mixing analysis (EEMMA), can potentially quantify many sources using a single tracer, even if their mean concentrations are indistinguishable. EEMMA can also quantify source contributions when some sources are unknown, and even infer the tracer time series of a missing source. Benchmark tests with synthetic data verify the reliability of this approach, thus expanding the range of mixing models that can be quantified using tracer time series. An R script is provided for the necessary calculations, including error propagation. Plain Language Summary: Chemical and isotopic tracers are widely used to partition mixtures (such as air masses, stream flows, xylem water, or biogeochemical fluxes) among potential contributing sources. These "mixing models" are widely used to improve process understanding throughout Earth and environmental science. However, because conventional mixing models are based on comparing the mean tracer concentrations in the sources and mixture, they typically require that all sources have been identified and sampled, their average tracer concentrations are distinctly different, and the number of sources does not exceed the number of independent tracers, plus one. Temporal fluctuations in tracer concentrations can provide additional information that loosens these constraints, thus expanding the range of feasible mixing models. Here I outline this approach and test its reliability using synthetic benchmark data. With this approach, in contrast to conventional mixing models, a single tracer can be used to quantify contributions from many different sources, even if their means overlap. Even when some sources are unmeasured or even unknown, this approach can estimate the remaining sources' mixing fractions, and even, given sufficiently accurate data, can estimate the mixing fraction and tracer signature of an unknown source. Thus, this approach expands the range of applications for mixing models. Key Points: Tracer fluctuations contain information that can help constrain mixing models, including those with overlapping end‐member signaturesThis approach can also constrain mixing models with many sources but only one tracer, or those in which some sources may be missingThe concepts and math behind this approach are outlined, and validated using benchmark tests; a script for the method is provided [ABSTRACT FROM AUTHOR]

Summer streamflow variations strongly affect water supply reliability and ecological functioning of western U.S. (WUS) streams. Traditional snow‐based forecasts of summer streamflow are becoming less accurate with warming‐induced reductions in winter snow accumulation. This reflects a rising importance of competing runoff‐generating processes in controlling summer streamflow variations, primarily an increasing role of rainfall in contrast to snowmelt. Here, based on a snowmelt‐rainfall tracking algorithm applied to two hydrological models, we show that cool‐season rainfall provides an important volumetric contribution to summer streamflow for many WUS streams in the current climate, and this contribution will increase under climate warming, especially in years with warm snow droughts and abnormally dry summers. We also show that seasonal rainfall (warm‐/cool‐seasons) dominates the variability of summer streamflow across ∼70% area of WUS. We show that an increasing warm‐season rainfall contribution to summer streamflow (largely replacing snowmelt) results in reduced summer streamflow predictability. Plain Language Summary: Summer streamflow is a critical water resource in the generally dry summers of the western U.S. (WUS), and is routinely forecasted using spring snowpack and/or winter total precipitation as primary predictors. However, climate warming leads to reduced snowpacks, exacerbates summer low flows, and reduces the accuracy of snow‐based summer streamflow forecasts. On the other hand, the role of winter rainfall as a control on summer streamflow increases in a warmer climate. Here, we explicitly quantify the contributions from cool‐season rainfall, warm‐season rainfall, and snowmelt to summer streamflow across the WUS, and how they change under a uniformly 1°C warmer climate. We show that the cool‐season rainfall contribution to summer streamflow increases under warming across WUS, especially in streams that currently have low‐to‐moderate snow contributions to runoff, and in years with anomalously warm winters and/or dry summers. We also show that the warm‐season rainfall contribution to summer streamflow increases widely, especially in the southern interior of WUS in a warmer climate, and that increasing warm‐season rainfall contribution to summer streamflow (largely replacing snowmelt) results in reduced summer streamflow predictability. Key Points: The cool‐season rainfall contribution to summer streamflow is greatest in low‐elevation coastal streams with dry summersClimate warming leads to an increased contribution of seasonal rainfall to summer streamflow as spring snowmelt contributions declineSummer streamflow predictability declines with reduced snowmelt and increased warm‐season rainfall contribution in a warmer climate [ABSTRACT FROM AUTHOR]

Climate change is expected to alter the distribution of flow discharge in rivers worldwide. We study the impact of climate‐driven flow changes on the shape of riverbed, and specifically on alternate bars, large deposits of gravel/sand that often form in rivers. We consider the illustrative example of the Alpine Rhine River, showing two nearby reaches with similar hydro‐morphological characteristics, but different channel width. Hydrological projections are obtained from literature, while the evolution of alternate bars is predicted through a novel, semi‐analytical model. Results show a remarkably different behavior of the two reaches: the upstream one, being wide enough for a full development of alternate bars, is resistant to flow alterations; the downstream reach, whose width is close to threshold conditions, is highly susceptible to future changes, showing a strong tendency to increase bar prominence. These findings reflect a general tendency of near‐threshold geomorphic systems to be vulnerable to anthropic stressors. Plain Language Summary: The worldwide alteration of the river flow induced by climate change is likely to significantly impact the bed morphology of embanked rivers, which is often characterized by the presence of alternate bars, namely repetitive sequences of large sediment deposits and scour zones. Bar formation is both a major issue for river management (due to local erosion at instream structures and increase of flood risk), and an important resource for sustaining biodiversity, because bar morphology templates rich habitats for river fauna and vegetation. We analyze the effect of climate change on river bars by considering existing state‐of‐the‐art projections of future flow discharge, and by implementing a mathematical model suitable to perform long‐term simulations, while keeping the essential ingredients to reproduce bar dynamics. Model results reveal a very different adaptation of the riverbed to climate change: relatively wide reaches are expected to maintain the current alternate bar characteristics, while reaches whose width is close to a critical threshold value are likely to experience a remarkable alteration in the next decades, which provides a noteworthy example of how near‐threshold geomorphic systems may be highly sensitive to climate change. Key Points: Climate‐driven changes of hydrological regime can have a significant impact on bar morphologyThe formation of alternate bars in channelized rivers is promoted by a decrease of frequency of high flowsSensitivity of bar morphology to climatic stressors depends on how far the river width is from the key morphodynamic threshold value [ABSTRACT FROM AUTHOR]

Deltas are fragile ecosystems threatened by sea‐level rise, sediment starvation, and subsidence. Erosional/depositional processes in these systems mainly depend on the sediment supply and the spatial divergence in bed shear stress induced by hydrodynamic forces. Thus, quantifying the spatiotemporal variability of the flow velocity field is essential for forecasting their fate. To calibrate/validate models, field measurements alone are not sufficient because such data only characterize the hydrodynamic conditions in localized areas. Remote sensing has potential to fill this data gap. We developed a methodology to estimate flow velocities from a map of suspended sediment concentration (SSC) measured by the NASA airborne spectrometer AVIRIS‐NG within the Wax Lake Delta, Louisiana. We extracted streaklines from remotely sensed SSC estimates, and quantified water fluxes and velocities based on the distance between them. Our study demonstrates that the velocity field in deltas can be estimated by leveraging the synoptic information offered by remote sensing. Plain Language Summary: River deltas are coastal environments particularly susceptible to sea‐level rise. Sediment deposition can counteract land loss from sea‐level rise, but we have a poor understanding of where and when land is built through sediment accretion. Improving the accuracy of hydrodynamic models is pivotal to predicting the fate of sediment that rebuilds sinking land, but we lack methods to test these models against observations over large areas. Here, we introduce a new method to measure water discharge, flow direction and speed from remotely sensed data. The method takes advantage of water flow patterns revealed in data collected from a NASA instrument flown on an airplane. Our study shows the utility of remotely sensed data to improve the performance and reliability of models that are needed to predict the fate of river deltas. Key Points: We propose a novel methodology to compute water fluxes and flow velocities in deltas from maps of suspended sediment concentrationOur method quantifies flow velocities based on the distance between streaklines, under the assumption that streaklines act as streamlinesFlow velocities obtained via our methodology match well with those derived from a numerical model in a sub‐region of the Wax Lake Delta [ABSTRACT FROM AUTHOR]

Groundwater is an invaluable global resource, but its long‐term viability as a resource for consumption, agriculture, and ecosystems depends on precipitation recharging aquifers. How much precipitation recharges groundwaters varies enormously across Earth's surface, yet recharge rates often remain uncertain. Here we use a global synthesis of field‐estimated recharge across six continents to show that globally recharge first‐order follows a simple function of climatic aridity. We use this relationship to estimate long‐term recharge in energy‐limited systems outside of permafrost regions. Our aridity‐based recharge estimates are consistent with the global field data but, on average, double previous estimates of global models. Our higher recharge estimates are likely caused by preferential groundwater recharge and discharge occurring at grid scales finer than global models. The higher recharge estimates suggest that more groundwater contributes to evapotranspiration and streamflow than previously represented by global hydrological models and global water cycle diagrams. Plain Language Summary: Groundwater is an essential resource for societies and ecosystems. The rate at which rainfall and snow replenish groundwater storage is important as it dictates the upper limit of sustainable groundwater use. Here we use measurements of groundwater recharge to show how climate determines groundwater recharge rates. Measured recharge rates, on average, strongly exceed those of models. This suggests there is more recharge globally than currently acknowledged. Consequently, also more groundwater recharge must get back to Earth's surface via river flow or water use of vegetation. Key Points: A global recharge data set indicates that climate strongly shapes the fraction of precipitation that will recharge groundwatersThis recharge data set indicates more recharge globally than existing global hydrological models suggestThus, more groundwater must contribute to evaporation and streamflow than represented by current global models and water cycle diagrams [ABSTRACT FROM AUTHOR]

Since 2013, extreme floods within the Santee River basin (North/South Carolina, USA) caused $1.5B in damage. The instrumental period, however, is too short to determine if recent extreme events are anomalous within a long‐term context. Here, we present reconstructions of storm‐, base‐, and total streamflow for the Santee River using a multi‐species tree‐ring network calibrated to flow data during the period 1923–2018. Tree‐ring data explained higher variance (r = 0.59; p < 0.01; 900–2018) of instrumental baseflow than total streamflow (r = 0.41; p < 0.01; 1500–2018) or stormflow (r = 0.26; p < 0.05; 1690–2018). Our reconstruction reveals a long‐term increase in baseflow over the past millennium. The North Atlantic subtropical high regulates baseflow in the Santee River (r = 0.45; p < 0.01). Recent high levels of baseflow may be connected to the position of the subtropical high, increasing the likelihood of flooding. Plain Language Summary: Tree rings are often used to estimate past levels of streamflow to better place modern extremes into a historical context. Streamflow has two components, baseflow (flow from groundwater) and stormflow (flow contributed from storms). Here, we use tree rings to estimate past streamflow of the Santee River, South Carolina, USA. We find that tree rings are better at representing variations in baseflow rather than stormflow. This connection to baseflow is likely because tree growth is related to long‐term changes in water supply throughout the growing season, and changes in baseflow are more likely to result in changes in annual growth. Additionally, we find that certain species are more sensitive to baseflow than others, with Taxodium distichum (bald cypress) being the most sensitive. We were able to reconstruct baseflow back 1,100 years and found that baseflows increased over this time. Higher levels of baseflow will result in larger flooding events, like the 2013 and 2019 events. The location of the western edge of a large atmospheric high pressure centered off the Atlantic Coast, the North Atlantic subtropical high, has a strong influence on baseflow levels and could explain the increasing trend in reconstructed baseflows. Key Points: Tree rings better represent baseflow compared to stormflow and total streamflowRapid 20th–21st century increases in baseflow escalate flooding susceptibility and are exceptional over the last 1,118 yearsRecent increases in baseflow are related to the position of the western flank of the North Atlantic subtropical high [ABSTRACT FROM AUTHOR]

Determining the flow regime of non‐perennial rivers is critical in hydrology. In this study, we developed a new approach using CubeSat imagery to detect streamflow presence using differences in surface reflectance for areas within and outside of a river reach. We calibrated the approach with streamflow records in the Hassayampa River of Arizona over 3 years (2019–2021), finding good agreement in the annual fractions of flowing days at stream gages (R2 = 0.82, p < 0.0001). Subsequently, annual fractions of flowing days were derived at 90 m intervals along the Hassayampa River, finding that 12% of reaches were classified as intermittent, with the remaining as ephemeral. Using a Hovmöller diagram, streamflow presence was visualized in unprecedented spatiotemporal detail, allowing estimates of daily fraction of flowing channel and annual fractions of flowing days. This new tool opens avenues for detecting streamflow and studying hydrological and biogeochemical processes dependent on water presence in drylands. Plain Language Summary: When and where streamflow occurs is critical for understanding Earth processes, as well as for policy and regulatory purposes. It is well‐known that most dryland rivers are non‐perennial and have high streamflow variability in space and time. However, there is a lack of observations to capture the dynamics of these river systems. Here, we present a new approach to determine the presence of streamflow using commercial small satellites which provide almost daily imagery at 3‐m cell sizes. Using this method, we compute streamflow presence over the Hassayampa River of Arizona. Our results highlight that the flowing fraction of the river varies from season to season and that the fraction of flowing days in the river varies greatly along its total length. This work suggests that the imaging capacity of small satellites can improve the detection of streamflow in dryland rivers as compared to current ground methods. Key Points: CubeSat satellite imagery can detect streamflow presence in dryland rivers at high spatiotemporal resolutionRobust differences in near infrared band signals between a river reach and the surrounding areas allow detection of flowing waterA Hovmöller diagram of streamflow presence provides a rapid visualization of spatiotemporal variations along the river length [ABSTRACT FROM AUTHOR]

A warmer climate impacts streamflows and these changes need to be quantified to assess future risk, vulnerability, and to implement efficient adaptation measures. The climate simulations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5), which have been the basis of most such assessments over the past decade, are being gradually superseded by the more recent Coupled Model Intercomparison Project Phase 6 (CMIP6). Our study portrays the added value of the CMIP6 ensemble over CMIP5 in a first North America wide comparison using 3,107 catchments. Results show a reduced spread of the CMIP6 ensemble compared to the CMIP5 ensemble for temperature and precipitation projections. In terms of flow indicators, the CMIP6 driven hydrological projections result in a smaller spread of future mean and high flow values, except for mountainous areas. Overall, we assess that the CMIP6 ensemble provides a narrower band of uncertainty of future climate projections, bringing more confidence for hydrological impact studies. Plain Language Summary: Greenhouse gas emissions are causing the climate to warm significantly, which in turn impacts flows in rivers worldwide. To adapt to these changes, it is essential to quantify them and assess future risk and vulnerability. Climate models are the primary tools used to achieve this. The main data set that provides scientists with state‐of‐the‐art climate model simulations is known as the Coupled Model Intercomparison Project (CMIP). The fifth phase of that project (CMIP5) has been used over the past decade in multiple hydrological studies to assess the impacts of climate change on streamflow. The more recent sixth phase (CMIP6) has started to generate projections, which brings the following question: is it necessary to update the hydrological impact studies performed using CMIP5 with the new CMIP6 models? To answer this question, a comparison between CMIP5 and CMIP6 using 3,107 catchments over North America was conducted. Results show that there is less spread in temperature and precipitation projections for CMIP6. This translates into a smaller spread of future mean, high and low flow values, except for mountainous areas. Overall, we assess that using the CMIP6 data set would provide a more concerted range of future climate projections, leading to more confident hydrological impact studies. Key Points: A comparison of hydrological impacts using Coupled Model Intercomparison Project version 5 (CMIP5) and Coupled Model Intercomparison Project Phase 6 (CMIP6) ensembles is performed over 3,107 catchments in North AmericaThe CMIP6 ensembles provide a narrower band of uncertainty for hydrological indicators in the futureIt is recommended to update hydrological impact studies performed using CMIP5 with the CMIP6 ensemble [ABSTRACT FROM AUTHOR]

The ever‐changing hydroclimatic conditions of the landscape induce ceaseless variations in the wet channel length (L) and the streamflow (Q) of a catchment. Here we use a perceptual model to analyze the links among (and the drivers of) four descriptors commonly used to characterize discharge and active length dynamics in streams, namely the L(Q) relationship and the cumulative distributions of local persistency, flowrate and active length. The model demonstrates that the shape of the L(Q) law is defined by the cumulative distribution of the specific subsurface discharge capacity along the network, a finding which provides a clue for the parametrization of L(Q) relations in dynamic streams. Furthermore, we show that L(Q) laws can be constructed combining the streamflow distribution with disjoint active length data. Our framework links previously unconnected formulations for characterizing stream network dynamics, and offers a novel perspective to describe the scaling between wet length and discharge in rivers. Plain Language Summary: Stream networks react to changing climatic conditions (wetting or drying) in the surrounding landscape. Consequently, the length of flowing channels and the corresponding streamflow vary through time owing to precipitation events and seasonal climatic patterns. Here we present a conceptual model that analyzes the controlling factors of four standard descriptors of streamflow and active length dynamics in streams, and identifies their mutual connections. The model provides insight on the physical processes that determine the joint changes of active length and discharge, investigating the impact of climate and landscape morphology on these standard descriptors. The approach formally describes how the relation between flowrate and active length originates from the spatial aggregation of local properties of the stream network (water supply and transport capacity). A set of mathematical relations is also derived, which can be used in practical applications to interpret and predict the relationship between wet length and catchment discharge ‐ even in cases in which synchronous measurements of these two variables are not available. Key Points: The shape of the active length (L) versus discharge (Q) relation mirrors the distribution of specific subsurface capacity along the networkThe L(Q) law can be reconstructed combining observations of active length and discharge collected during different time periodsWe derive exact analytical expressions that connect the L(Q) law to the distributions of local persistency, flowrate and active length [ABSTRACT FROM AUTHOR]

Climate change threatens biodiversity through global alteration of habitats, but efficient conservation responses are often hindered by imprecise downscaling of impacts. Besides thermal effects, warming also drives important ancillary environmental changes, such as when river hydrology evolves in response to climate forcing. Earlier snowmelt runoff and summer flow declines are broadly manifested in snow‐dependent regions and relevant to socioeconomically important cold‐water fishes. Here, we mechanistically quantify how climate‐induced summer flow declines during historical and future periods cause complex local changes in Chinook salmon (Oncorhynchus tshawytscha) habitats for juveniles and spawning adults. Changes consisted of large reductions in useable habitat area and connectivity between the main channel and adjacent off‐channel habitats. These reductions decrease the capacity of freshwater habitats to support historical salmon abundances and could pose risks to population persistence in some areas. Plain Language Summary: The large majority of climate change research for aquatic organisms has focused on stream water temperature and discounted the importance of flow changes associated with evolving precipitation regimes. Using a high‐resolution stream topographic map that supported a series of linked flow and fish habitat quality models, our research describes how observed historical flow reductions and projected future reductions affect local habitat quality, distributions, and connectivity for a salmon population. We demonstrated that as climate change continues to change flow regimes in snow‐dependent regions, it will cause fragmentation and reduce the overall volume of available habitats in headwater streams where salmon and other cold‐water fish species spawn and grow during early life stages. Key Points: Warming from climate change accelerates runoff and decreases streamflow during low flow periods in many snow dependent regionsExtended summer low flows reduce and fragment the capacity of freshwater habitats to support historical salmon abundancesClimate adaptation actions for salmon should consider hydrologic changes and floodplain connectivity besides increasing water temperatures [ABSTRACT FROM AUTHOR]

The ongoing 22‐year drought in the Upper Colorado River Basin (UCRB) has been extremely severe, even in the context of the longest available tree‐ring reconstruction of annual flow at Lees Ferry, Arizona, dating back to 762 CE. While many southwestern drought assessments have been limited to the past 1,200 years, longer paleorecords of moisture variability do exist for the UCRB. Here, gridded drought‐atlas data in the UCRB domain along with naturalized streamflow data from the instrumental period (1906–2021) are used in a K‐nearest neighbor nonparametric algorithm to develop a streamflow reconstruction for the Lees Ferry gage starting in 1 CE. The reconstruction reveals a second‐century drought unmatched in severity by the current drought or by well‐documented medieval period droughts in the UCRB. Although data are sparse, analysis of individual long tree‐ring records and other paleoclimatic data also support the occurrence of an exceptional second‐century drought. Plain Language Summary: The Colorado River drought we currently are experiencing is severe in the context of the 116‐year gage record (1906–2021), but how severe is it in a long‐term context? Existing tree‐ring based reconstructions of Colorado River streamflow have suggested that the 22‐year period 2000–2021 could be the worst drought in the southwestern United States in 1,200 years. The purpose of this study is to extend the Colorado River reconstruction back 2,000 years and to evaluate the current drought in a long‐term context. We find that an even more extreme drought occurred and persisted over much of the second century. Data are sparse this far back in time, but evidence from both tree‐ring data and paleoclimate data from lakes, bogs, and caves supports the existence and severity of this drought in the context of the last two millennia. Additional work is needed to learn more about this drought and its causes, but we now know that drought more persistent than even the well‐documented medieval period droughts occurred in the past, expanding our understanding of the range of natural climate variability. Key Points: A new tree‐ring based ensemble streamflow reconstruction spanning the last two millennia was developed for the Colorado RiverThe new reconstruction reveals a second‐century drought unmatched in severity by any past droughts in the Upper Colorado River BasinThe ongoing 22‐year period of low Colorado River streamflow is a rare event, but is not the most severe drought in the past 2,000 years [ABSTRACT FROM AUTHOR]

River dunes are wavelike features in sandy riverbeds that move slowly in a downstream direction. These bedforms are shaped by the interaction between flowing water and sand grains at the riverbed. Understanding river dune dynamics is essential to determine sediment transport and optimize river maintenance. Earlier research mainly focused on river dune dynamics during high flows; therefore, low‐flow dune dynamics are hardly understood. In this study, we analyzed dune dynamics in a unique data set of 10 years of biweekly bed elevation profiles of the Rhine River, focusing on low‐flow conditions. Here, we show that dunes migrate and become lower and longer during low flows, while current literature states that dunes should become shorter. This dune migration and lengthening at low flows is a new finding and is linked to diffusive dune migration processes that become more relevant when sediment transport is just above incipient motion. Plain Language Summary: River dunes are wavelike features on sandy river beds. They are shaped by the interaction between the flowing water and the sand grains at the river bed and move slowly in the same direction as the river flow. Better understanding of the changes in shape, especially when water depths are small, can help to maintain the river bed since high dune peaks can hinder shipping. We studied the dune shape from bed elevation measurements of the Rhine River covering 10 years, focusing on a period with small water depths. We found that the dunes become lower in height at smaller water depths, which we expected based on earlier studies. We also found that the dunes become longer in length and keep moving, contradicting the current knowledge of dunes. Key Points: River dunes are mobile during low flowCrest‐to‐crest length of river dunes increases when sediment transport capacity approaches incipient motionDuring low flow dunes migrate diffusively [ABSTRACT FROM AUTHOR]

Previous modeling studies of hyporheic exchange induced by moving bedforms have used a moving reference frame, typically corresponding to an individual moving bedform. However, this approach is not suitable for simulating the exchange and accumulation of immobile fine particles beneath moving bedforms, which commonly occurs in sand‐bed streams, as both moving and stationary features must be considered. Here we present a novel simulation framework that may represent arbitrarily shaped, generally aperiodic mobile bedforms within a stationary reference frame. We combine this approach with particle tracking to successfully reproduce observations of clay deposition in sand beds, and the resulting development of a low‐conductivity layer near the scour zone. We find that increased bedform celerity and filtration both lead to a shallower depth of clay deposition and a more compact deposition layer. While increased filtration causes more clay to deposit, increased celerity reduces deposition by flattening hyporheic exchange flowpaths. Plain Language Summary: Stream water flows into and out of sand ripples along the stream bed. It is also common for streambed sand ripples to migrate downstream due to erosion and deposition of sediment. Mathematical models that simulate the flow of stream water into and out of streambed ripples have typically done so from the perspective of a viewer who moves downstream with the ripple. This approach can be useful but is less suitable for representing the accumulation of material deposited by the water flowing through the ripple. We present a novel mathematical model that represents moving sand ripples from the perspective of a viewer who is standing still and watching the ripples go by. Simulation results successfully reproduce experimental observations of the accumulation of material deposited by water flowing through the ripple. Ripples that move faster deposit material at shallower depths and deposit less of the material that flows through the ripple. Deposited particles with a higher tendency to become trapped between streambed sand grains will also deposit at shallower depths. This model will provide new insights into the transport and deposition of contaminants that enter streams and rivers. Key Points: A moving‐interface model for particle deposition in streambeds under moving bedform conditions is introducedModel reproduces experimental observations of the formation of a clay layer below the bedform scour zoneBedform celerity and particle filtration have interacting effects on the rate and location of particle deposition [ABSTRACT FROM AUTHOR]

Climate change is linked to changing precipitation, yet uncertainty remains as to whether and how hydroclimatic changes will influence flood frequency, magnitude, and timing. Part of this uncertainty is because few regional‐scale studies have analyzed changes in floods produced by different mechanisms. We identify six flood generating mechanisms (FGMs) based on prominent meteorological processes and moisture supply to the soil column in the Western U.S.; these include large‐scale frontal storms (atmospheric and non‐atmospheric rivers), monsoons, convective storms, snowmelt, and rain‐on‐snow. We assess trends in the frequency, magnitude, and timing of floods produced by different FGMs across 119 basins during 1960–2018. Overall, we find decreased frequency and magnitude of rain‐on‐snow‐driven floods, increased frequency and magnitude of convective‐storm‐driven floods, and a temporal shift to earlier in the year for snowmelt‐driven floods. The flood characteristics are generally stable for floods produced by other FGMs. Plain Language Summary: Floods are among the most common and costliest of natural disasters. Extreme rainfall is a major cause of flooding and has become more common as global temperatures have increased. However, many studies show that the number and size of flood events remain mostly unchanged. It is possible that climate change is affecting different types of floods in different ways, and so we separate floods into different categories and evaluate trends on a category‐by‐category basis. In particular, we group floods across the western U.S. into six categories, which we term flood generating mechanisms (FGMs). These include rain‐on‐snow, snowmelt, monsoons, convective storms, and two types of frontal storms. We then separately describe trends in the frequency, magnitude, and timing for each FGM from 1960 to 2018. We find that floods generated by convective storms have become more common and more extreme. On the other hand, rain‐on‐snow floods have become rarer and less extreme. Also, floods caused by snowmelt are occurring earlier within the year. Most of the other categories do not show significant changes in flood characteristics. These results give us important information on the types of flooding that may be most impacted by future climate change, which can aid future flood planning efforts. Key Points: The frequency and magnitude of rain‐on‐snow floods have decreased across the western U.S., and snowmelt floods are occurring earlierFloods generated by convective storms (primarily in the Southwest) have increased in frequency and magnitudeExcept for the above‐mentioned changes, the frequency, magnitude, and timing of floods have been relatively stable over time [ABSTRACT FROM AUTHOR]

The drainage evolution in eastern Tibet during the India‐Eurasia collision has attracted considerable attention because of its critical link to topographic development and plateau growth, but the discussion still persists. In the present study, new and published detrital zircon U‐Pb ages are integrated from Pliocene‐Early Pleistocene Xigeda Formation (XGD) and modern river sands to decipher the paleo‐drainage evolution in eastern Tibet. The results confirm the eastward flow of the Jinsha River before the Pliocene, but the drainage patterns of its major tributaries (Dadu, Yalong, and Anning) in the Pliocene, differ from the modern ones. In conjunction with the previously documented fluvial incision in eastern Tibet, we interpreted that the excavation of the Paleo‐Xigeda lake in Early Pleistocene time initiated and accentuated incision of the Jinsha River and its major tributaries. This led to drainage reorganization and the modern river pattern establishment in eastern Tibet. Plain Language Summary: The developmental history of the Jinsha River and its major tributaries (Dadu, Yalong, and Anning) in eastern Tibet are considered to be closely associated with the topographic evolution during the Cenozoic plateau growth. However, the formation timing and mechanism of the current configuration of these large rivers remains debated. Here, we used detrital zircon U‐Pb age as "provenance tracer" to identify source signatures of the Pliocene‐Early Pleistocene Xigeda Formation, a sequence of lacustrine deposits sporadically distributed along the Middle Jinsha River and its large tributaries. Our provenance results demonstrate that current east‐flowing Jinsha River has been established before Pliocene time, while the present‐day pattern of its major tributaries, including the Yalong, Dadu, and Aning rivers, was achieved in the late Early Pleistocene. Combined with previous thermochronological and provenance studies, we infer that the spillover of dammed lake initiated the fluvial incision that led to drainage reorganization and, consequently, the formation of the modern river system. This study not only challenge existing models of Plio‐Pleistocene drainage pattern of eastern Tibet that flowed southward to the South China Sea, but also illustrates how river reorganization alone can dramatically influence landscape development even in the absence of significant tectonic and climatic forces. Key Points: The Jinsha River has flowed eastward before Pliocene timeThe present pattern of the Dadu, Yalong, and Anning Rivers achieved in the Early PleistoceneThe spillover of the Paleo‐Xigeda lake during the Early Pleistocene triggered incision of the Jinsha River and its major tributaries [ABSTRACT FROM AUTHOR]

Deltas have long been considered a constraining element to reconstruct the water level of an ancient ocean that may have once occupied the northern lowlands of Mars, and recently this hypothesis started to be challenged. We investigate this hypothesis and present a global inventory of fan‐shaped features showing typical deltaic traits across the entire Martian surface. For each element, we provide descriptive details and classifications based on morphology, location, and relation with characterizing environmental features. In this catalog of 161 deltas, we identified only six having high potential to constrain an oceanic paleoshoreline. Nonetheless, age and location of these candidates display discrepancies with what was previously suggested from independent data sets about shoreline age and locations. Our analyses hence indicate that deltas alone are insufficient to delineate a globally consistent ancient oceanic shoreline, but they have the potential to locally constrain the water level both in space and time. Plain Language Summary: Deltas are sedimentary features generated by the deposition of fluid‐carried sediments as a river flow enters a larger water body. These landforms have been observed on Mars and may have recorded the water level of the basin where they formed. In this study, we present a global distribution of deltas on Mars that we have characterized in terms of their geomorphology, location, and relation to their surroundings. A total of 161 deltas are included in this global catalog. Among them, only six deltas can be associated with the shoreline of a past Martian ocean. However, their estimated age and location are difficult to reconcile with previously proposed shorelines. This indicates that a global ocean level on Mars is difficult to constrain from the information provided by deltas alone. Instead, these features are more likely to indicate local‐scale water levels during the Martian past. Key Points: We present an updated global catalog of deltas on Mars and characterize them in terms of their geomorphologyOut of 161 deltas, 29 are located at the dichotomy boundary and only six can be used to investigate the presence of an oceanic shorelineAge and location of the six deltas are at odds with proposed shorelines; hence, deltas alone do not define a globally consistent water level [ABSTRACT FROM AUTHOR]

Studies have quantified the contribution of tropical cyclones (TCs) toward seasonal precipitation, but limited analysis is available on TC contribution toward seasonal streamflow over the southeastern and southcentral (SESC) United States (U.S.). Using an extensive network of hydroclimatic data that consists of 233 TC tracks and daily precipitation and streamflow, we estimate TC contribution toward precipitation and streamflow during the hurricane season over the SESC U.S. We found that TCs account for 12% of seasonal streamflow and 6% of seasonal precipitation over the region. Florida, North Carolina, and Louisiana have the highest fractional occurrence of TC‐generated annual maximum precipitation (∼20%–32%) and streamflow (∼15%–27%). We also found the fractional occurrence of TCs associated with peak‐over threshold precipitation (streamflow) events ranges from 5% to 8% in coastal regions (10%–20% over FL and 5%–10% over coastal NC). Increased antecedent moisture results in increased TCs contribution to streamflow leading to different land‐surface responses for similar hurricane events. Plain Language Summary: While tropical cyclones cause major flooding over coastal areas of the eastern United States (U.S.), they are also an important source of moisture supply. Our analysis shows, on average, TCs contribute 6% of precipitation and 12% of streamflow during the hurricane season over the southeastern and southcentral (SESC) U.S. Our findings also show that TCs play a key role in generating annual extreme precipitation and streamflow over the SESC U.S. Key Points: Tropical cyclones account for 6% of the total seasonal precipitation and 12% of the total seasonal streamflow in the southeastern and southcentral United StatesFlorida, North Carolina, and Louisiana have the highest fractional occurrence of tropical cyclone (TC)‐generated annual maximum precipitation (∼20%–32%) and streamflow (∼15%–27%)Increased antecedent moisture leads to increased TC contribution resulting in increased land‐surface runoff for similar hurricane events [ABSTRACT FROM AUTHOR]

The Tien Shan and Pamir mountains host over 28,000 glaciers providing essential water resources for increasing water demand in Central Asia. A disequilibrium between glaciers and climate affects meltwater release to Central Asian rivers, challenging the region's water availability. Previous research has neglected temporal variability. We present glacier mass balance estimates based on transient snowline and geodetic surveys with unprecedented spatiotemporal resolution from 1999/00 to 2017/18. Our results reveal spatiotemporal heterogeneity characterized by two mass balance clusters: (a) positive, low variability, and (b) negative, high variability. This translates into variable glacial meltwater release (≈1-16%) of annual river runoff for two watersheds. Our study reveals more complex climate forcingrunoff responses and importance of glacial meltwater variability for the region than suggested previously. Plain Language Summary Glaciers in Central Asia act as water towers for millions of people by storing and releasing water in response to climate. Monitoring glaciers is difficult due to their often very remote locations. Satellite remote sensing has emerged as a powerful method but a drawback is their (semi-)decadal resolution for glacier mass change surveys. We present a methodology, combining multiyear elevation change maps with frequent snowline observations to estimate mass changes and variability at annual scale, which allows us identifying so far unrecognized regions of contrasting trends for the Tien Shan and Pamir mountains. These "hot spots" reveal a far more complex climate-glacier interplay than previously known. The additional meltwater released from the retreating glaciers varies considerably and contributes to the river flow for warm dry years by twice as much as for cold wet years. Our findings will help to better understand the impact of climate change on Central Asian glaciers and their meltwater release. [ABSTRACT FROM AUTHOR]

The spatial and temporal span of hydrological impact of the "Little Ice Age" (LIA) in the north‐western Himalaya is not well constrained due to data limitation. We evaluated a network of tree‐ring chronologies from moisture‐stressed ecological settings in Jammu and Kashmir to identify the impact of LIA over the western Himalaya. Our study reflects three centennial scale phases; the middle phase (1650s–1850s CE) with stable precipitation and in accordance with other hydrological records clearly demarcates the LIA impact over the Himalaya. However, high‐magnitude droughts recorded in early (1383–1650s CE) and late phases (1850s–2017) underpin that the north‐western Himalaya did not witness any LIA influence before 1650s. Plain Language Summary: Here, we present a 635‐year long annual (prior year October to current year September) precipitation reconstruction from Kishtwar, Jammu and Kashmir. A network of 16 tree‐ring chronologies of Himalayan cedar and neoza pine was used in the study. The reconstruction captured three distinct phases in precipitation regime from 1383–1650s, 1650s–1850s, and 1850s–2017 CE and among these, the middle phase (1650s–1850s) showed relatively prolonged pluvial phase with consistently stable precipitation advocating for the active LIA over the western Himalaya. The other two phases reflect high‐amplitude droughts with intervening wet periods contrasting the existence of the LIA‐induced wetting. To validate the reconstruction skill and the LIA effect over the western Himalaya and Central Asia, the present record was compared with the other available precipitation, river flow and hydrological proxy records which support the LIA influenced precipitation during 1650s–1850s CE. Key Points: Three distinct centennial scale phases have been identified in 635 years long precipitation record developed from the northwest HimalayaStable precipitation during 1650s–1850s CE reveals clear evidence of Little Ice Age (LIA) for the north‐western HimalayaHigh‐magnitude droughts or low precipitation limit the LIA influence before 1650s CE [ABSTRACT FROM AUTHOR]

Changes in climate are expected to influence discharge of the lower Mississippi River, but projections disagree on whether discharge will increase or decrease over the coming century. Using a reconstructed median peak annual flow for the past 1,500 years based on geomorphic scaling laws, we show that discharge on the lower Mississippi River decreased during the Medieval era (c. 1000–1200 CE)—a period of regionally warm and dry conditions that serves as a partial analog for projected warming. These changes in discharge inferred from channel morphology track discharge simulated in the Community Earth System Model Last Millennium Ensemble. Simulations show that decreased Medieval era discharge is driven primarily by regionally enhanced evapotranspiration. Our findings are consistent with 21st century projections of decreased discharge on the lower Mississippi River under moderate greenhouse forcing scenarios, and demonstrate consistency between reconstructed and simulated discharge over the last millennium. Plain Language Summary: River flow is sensitive to changes in climate, but it is unclear how flow on the lower Mississippi River will change over the next century in response to regional warming. The lower Mississippi River is economically important, so it is critical to understand how it may change for infrastructure planning and water resources management. Here, we use river properties from historical maps and output from a global climate model to estimate how flow of the lower Mississippi River changed over the last 1,500 years, including through a period of moderate warming ∼1,000 years ago. Our reconstruction shows that the relatively warm Medieval era (ca. 1000–1200 CE) was a time of reduced discharge on the lower Mississippi River, mostly due to increased evapotranspiration. These findings imply that the lower Mississippi River's flows will decrease under a moderate warming scenario. Key Points: We use channel morphology to infer changes in lower Mississippi River discharge over the last 1,600 yearsMedian peak annual discharge was reduced during the Medieval era (1000–1200 CE)Climate model simulations indicate that enhanced evapotranspiration primarily reduced Medieval era discharge [ABSTRACT FROM AUTHOR]

Large rivers play crucial roles in determining locations of civilization, biodiversity, and efflux to the oceans. The paths they take across Earth's surface vary with scale. At long‐wavelengths rivers can have simple flow paths. At smaller scales, in meanders for example, their paths change rapidly as a consequence of lithology, biota, and other environmental variables. It is not straightforward to identify the scales at which river planforms are set. We overcome these issues by developing a spectral (wavelet) methodology to map flow‐directions as a function of distance and scale. This methodology allows short‐wavelength features (e.g., meanders) to be filtered from river flow‐paths. With short‐wavelength structure removed, the flow‐directions of rivers in Western USA correlate with long‐wavelength gravity anomalies suggesting control by subplate support. This relationship is replicated by an ensemble of landscape evolution models. These results combined suggest that drainage at large scales, O(103) km, is set by subplate support. Plain Language Summary: Rivers play important roles in developing civilization, biodiversity, and the supply of sediment and nutrients to the oceans. Their position on Earth's surface depends on geologic, climatic, hydraulic, and biologic processes, which operate at a large range of spatial and temporal scales. Rivers obviously flow from high to low topography; however, their paths vary at a range of scales. They can be determined by, for example, rock fractures, biota, and antecedence (preexisting drainage patterns). It is generally unclear what processes determine the flow directions of rivers across the scales of interest. Here, a methodology is developed to map the scale‐dependent shapes of rivers, which can be compared to independent observations at appropriate scales. This spectral approach is used to quantify how rivers' flow directions change as a function of scale. We show that it can be used to remove small‐scale variations in flow directions (e.g., meanders) leaving only the larger‐scale changes in flow direction of western North American rivers. The flow direction of these rivers at large scales run broadly parallel to the line‐of‐descent of Earth's gravity field and away from areas where the lithosphere is thin. This result is consistent with western North American drainage patterns being principally controlled by mantle convection. Key Points: River flow directions are determined as a function of position and scale by wavelet transformsSpectra show importance of long wavelength (>1,000 km) processes for determining flow directionsLandscape modeling indicates river planforms can be predicted using proxies for subplate support [ABSTRACT FROM AUTHOR]

Rivers are among the most imperiled ecosystems globally, yet we do not have broad‐scale understanding of their changing ecology because most are rarely sampled. Water color, as perceived by the human eye, is an integrative measure of water quality directly observed by satellites. We examined patterns in river color between 1984 and 2018 by building a remote sensing database of surface reflectance, RiverSR, extracted from 234,727 Landsat images covering 108,000 kilometers of rivers > 60 m wide in the contiguous USA. We found 1) broad regional patterns in river color, with 56% of observations dominantly yellow and 38% dominantly green; 2) river color has three distinct seasonal patterns that were synchronous with flow regimes; 3) one third of rivers had significant color shifts over the last 35 years. RiverSR provides the first map of river color and new insights into macrosystems ecology of rivers. Plain Language Summary: Rivers can appear different colors such as blues, greens, browns, and yellows. Water color is linked to water quality and can be related to the amount of sediment, algae, and dissolved organic carbon in water. Humans can therefore discern waters' suitability for use with our eyes. While we know many rivers are impaired globally, often due to poor water quality, the color of rivers has not been widely measured to investigate changes through space and time. Satellites act as "eyes in the sky" and regularly observe earth's large rivers. Using satellite remote sensing records from 1984 to 2018, we measured the color of rivers across the USA. We found that large rivers have distinct seasonal patterns in color that change with river flow, and that the dominant color in one third of rivers has significantly changed. Observations of water color can pinpoint rivers undergoing rapid environmental change and work toward continental‐scale understanding of rivers. Key Points: ∼15.9 million satellite observations over 35 years show USA rivers (>60 m wide) are dominantly yellow and green in colorRiver color has three distinct seasonal patterns that are synchronous with flow regimesRiver color significantly changed over the last three decades in one third of large US rivers [ABSTRACT FROM AUTHOR]

The North American Monsoon occurs July–September in the central Rocky Mountains bringing significant rainfall to Colorado River headwater basins. This rain may buffer streamflow deficiencies caused by reductions in snow accumulation. Using a data‐modeling framework, we explore the importance of monsoon rain in streamflow generation over historical conditions in an alpine basin. Annually, monsoon rain contributes 18 ± 7% water inputs and generates 10 ± 6% streamflow. The bulk of rain supports evapotranspiration in lower subalpine forests. However, rains have the potential to produce appreciable streamflow at higher elevations where soil moisture storage, forest cover, and aridity are low and rebound late season streamflow 64 ± 13% from simulated reductions in spring snowpack as a function of monsoon strength. Interannual variability in monsoon efficiency to generate streamflow declines with low snowpack and high aridity, implying the ability of monsoons to replenish streamflow in a warmer future with less snow accumulation will diminish. Plain Language Summary: Monsoon rains bring much needed summer moisture to the southwestern United States, but it remains unclear whether rains have a significant effect on streamflow in the snow‐dominated headwaters of the Colorado River. Lack of understanding is largely due to the difficulty in measuring rain and snowfall in steep, mountainous basins, and the effect both have on seasonal plant consumption of water. Using a hydrological model populated with ground, airborne, and synthesized climate data, we compare relative efficiency of monsoon rain to generate stream water over multiple decades in an alpine basin. Monsoon rains deliver one fifth of the basin's water and produce 10% the annual streamflow, with additions largely confined to the upper elevations of the watershed where soils are thin, water is plentiful, and forests are less abundant. In contrast, lower elevations contain dense aspen and conifer forests that consume monsoon rain and limit streamflow response. Subsequently, even strong monsoon events cannot fully replenish lost snow. Summer rains produce more streamflow during cooler years with large snow accumulation. This hints that streamflow from summer rain may diminish in a warmer future with less snow. Key Points: Monsoon rains generate 10 ± 6% of annual streamflow while late spring snowfall delivers twice as much for the same water inputThe influence of monsoon rain on streamflow is half of late spring snow due to evapotranspiration in the lower subalpine forestMonsoon efficiency in generating streamflow decreases in warm years with low snow accumulation [ABSTRACT FROM AUTHOR]

The drastic decline in sediment discharge experienced by large rivers in recent years might trigger erosion thus increasing the vulnerability of their extensive deltas. However, scarce information is available on the erosion patterns in mega‐deltas and associated physical drivers. Here a series of bathymetries in the South Passage, Changjiang Delta, were analyzed to identify morphodynamic variations during high riverine flow and tropical cyclones (TCs). Results indicate that high river flow during flood season triggers large‐scale net erosion along the inner estuary, generating elongated erosion‐deposition patches. Erosion magnitude gradually weakens moving seaward with few localized bottom variations in the offshore area. TCs transport sediment landward and are accompanied by an overall weak erosion, with a less organized spatial pattern of erosion‐deposition. TCs can therefore significantly alleviate erosion, reducing the sediment loss induced by riverine flows by over 50%. These results highlight the role of TCs on the sediment dynamics of mega‐deltas. Plain Language Summary: In recent years, climate change and reduced riverine sediment inputs have significantly affected the morphology of large deltas. However, it is unknown whether typhoons cause erosion or deposition in the submerged part of a delta, due to challenge in capturing infrequent events. Here we study the impact of high river flow and typhoons on the South Passage, Changjiang Delta. Results show that high riverine flows can cause large‐scale erosion. Typhoons reduce fluvial erosion by transporting offshore sediments in the delta. The relative accretion caused by a typhoon is more than half the net erosion induced by riverine floods. This research sheds light on the sediment budget of large deltas and inform policy measures that can mitigate the effect of increasing anthropogenic pressure, extreme events, and sea level rise on delta morphology. Key Points: High riverine flow during the flood season triggers massive erosion in the Changjiang DeltaTropical cyclones reduce the overall fluvial erosion during the flood seasonDeposition triggered by typhoons can offset 50% of the estuarine erosion caused by high riverine flow [ABSTRACT FROM AUTHOR]

More than half of the world's large rivers flow towards the ocean crossing passive continental margins. Here using an analytical solution and numerical models, we demonstrate that on passive margins, river basins may be integrated by major margin‐parallel channels, which form as a flexural isostatic response of the lithosphere to mechanical/erosional unloading along the margin. We analyzed the downstream courses of large rivers flowing across the passive margins and find that the majority of them (31 of 36) have major margin‐parallel channels. Occurrences of these channels are generally consistent with the model predictions, although the exact locations and geometry of these rivers may also be controlled/changed by other factors. Our results suggest that the lithosphere strength has an important control on the geometry of large river systems on passive margins, linking the evolution and routing of the Earth's freshwater systems to its deep interior dynamics. Plain Language Summary: Tectonically stable continents are often drained by large rivers, which flow to the ocean through a relatively narrow main stem. We investigated why these drainage patterns develop and specifically, why these rivers do not flow straightly to their outlets. Along passive margins, we find that most of the world's biggest rivers develop channels parallel to the margin before they enter the ocean and the occurrences of these channels are responsible for the formation of large drainage basins. The locations of these channels indicate that they were created and maintained by isostatic uplift in response to the erosional removal of the surface rocks. This uplift along the margin creates a depression further inland that causes the rivers to flow parallel to the margin and increases the size of the river basin. Our proposed mechanism demonstrates how the structure and dynamics of the Earth's interior could impact the cycles in the hydrosphere and thus biosphere at the surface. Key Points: Lithosphere rigidity has a control on the geometry of large rivers near passive marginsFlexure is responsible for the formation of axial rivers running parallel to marginsThis explains why continental interiors drain through narrow, widely spaced outlets into oceans [ABSTRACT FROM AUTHOR]

Lowland rivers regularly flood and create complex inundation patterns where energy and matter are exchanged between landscape patches over a dynamic network of surface‐water connections. Scale‐freeness of networks for phenomena in many disciplines have been studied with mixed results. Here we present the first documented example of a (roughly) scale‐free network of surface‐water connections within a river‐floodplain landscape. We accomplish this by simulating 23 inundation maps across the historical range of flows for the Mission River in Texas. We then analyze the topology of the surface‐water connections between the river and two habitat patch types. Results show that surface‐water connectivity is scale‐free for ≥64% of simulated flows (≥70% for flows with floodplain inundation). Moreover, the dynamic surface‐water connections meet five of the six conceptual criteria of scale‐free networks. Our findings indicate that river‐floodplain landscapes are self‐organizing toward scale‐free surface‐water connections among patches that optimizes energy and matter exchange. Plain Language Summary: Rivers that flow over relatively flat landscapes that are formed by their own sediment deposits flood in a manner where river water escapes the confines of the river channel and spills onto the adjacent floodplain. This flooding connects the river to a variety of habitats on the floodplain with the number of connections changing with the size of the river flow. The scale‐free network conceptual model has been used to represent connections and interactions among phenomena within numerous disciplines that include the geophysical, physical, biological, mathematical, engineering, and social sciences. Here we present the first documented example of a pattern of connections between a river and surrounding habitats induced by flooding that resembles the scale‐free network model. We accomplish this by modeling various river flows for Mission River in Texas using specialized computer programs. We then analyze the pattern of connections induced by flooding using specialized statistical methods and computer codes. Our results indicate that most of our modeled river flows create connections that meet five of the six conceptual criteria for scale‐free networks. This indicates that the parts of the river landscape are organizing themselves in a manner that attempts to optimize exchange of energy and matter. Key Points: Backwater inundation is a major component in the structure and function of river‐floodplain interactionsSurface‐water connectivity creates a dynamic network with a size, configuration, and function indicative of a scale‐free networkChannel and channel‐like features in the river landscape act as hubs that hold together the structure of surface‐water connectivity [ABSTRACT FROM AUTHOR]

Plant response to elevated CO2 concentration is known to increase leaf‐level water‐use efficiency through a reduction in stomatal opening. Recent studies have emphasized that increased plant water‐use efficiency can ameliorate the impact of drought due to climate change. However, there is a potentially counterbalancing impact due to the increased leaf area. We investigate long‐term trends (1951 to 2015) of observed streamflow in the Southeastern United States (SE US) and quantify the contribution of major drivers of streamflow changes using single factor climate modeling experiments from Community Land Model Version 5 (CLM5). The SE US streamflow observations do not exhibit a trend, which is in agreement with the CLM5 control experiment. Using the factorial set of CLM5 experiments, we find that increased leaf area under elevated CO2 leads to decreased runoff and completely counteracts increased runoff due to water‐use efficiency gains under elevated CO2 and land‐use change. Plain Language Summary: Understanding drivers of hydrological changes are societally important. Many studies have investigated the effects of climate change and land‐use change on the regional water cycle. However, the effect of plant response under elevated CO2 concentration is less known. Plant response can play two primary roles: (1) It can increase water availability due to increased water‐use efficiency, and (2) it can decrease water availability due to increased plant growth. The observations show no significant changes in water availability from 1951 to 2015 in the Southeastern United States (SE US). Climate modeling experiments show that plant growth under elevated CO2 concentration has negated the effect of increased water‐use efficiency on water availability in the SE US. Hence, we conclude that the plant growth, this seemingly less known and slowly moving driver, can play an important role in the water cycle changes at the regional scale. Key Points: Long‐term streamflow observations (1951 to 2015) do not exhibit a significant trend in the Southeastern United StatesIncreased leaf area or plant growth under elevated CO2 decreases runoff or streamflowPlant growth completely counteracts increased runoff due to water‐use efficiency gains under elevated CO2 [ABSTRACT FROM AUTHOR]

Hydrological drought quantification using surface water information has been lacking at a large scale due to limited in situ data. We introduce a framework for monitoring hydrological droughts using a global, long‐term, monthly, remotely sensed reservoir surface area dataset. At regional scales, a new index—the reservoir area drought index (RADI)—is defined as the monthly normalized reservoir area time series. RADI was validated using an in situ reservoir storage based index (average R2 = 0.83). RADI not only helps to characterize drought propagation from meteorological and agricultural droughts to hydrological droughts but also fills the information gap between streamflow/runoff based and groundwater based drought indices. The surface area dataset was further used to characterize the recovery rate (and to estimate the recovery time) at the individual reservoir scale during droughts. This across‐scale drought monitoring framework can help mitigate drought impacts and increase water use efficiency among multi‐reservoir systems. Plain language summary: Drought is a global phenomenon that can have substantial impacts on regional economies and ecosystems. Quantification of droughts is of great importance for mitigating losses. Leveraging an existing remotely sensed global reservoir area dataset, we introduce a new hydrological drought index–the reservoir area drought index, RADI—that can serve as an indicator of surface water availability at a regional scale. Nine drought‐prone regions were chosen to evaluate the performance of RADI. Results suggest that RADI can contribute new information to hydrological drought monitoring and can complement other indices to fully depict the propagation of different kinds of droughts. In addition, reservoir area time series can also be used for quantifying the responses of individual reservoirs to droughts. Key Points: A new hydrological drought index was developed using remotely sensed reservoir surface area time seriesThe drought evaluation framework permits drought quantification at both regional and local scalesThe new index is informative about reservoir water availability and contributes to the evaluation of drought propagation. [ABSTRACT FROM AUTHOR]

Modes of climate variability are known to influence rainy season onset, but there is less understanding of how they impact flood timing. We use streamflow reanalysis and gauged observation data sets to examine the influence of the Indian Ocean Dipole and El Niño–Southern Oscillation across sub‐Saharan Africa. We find significant changes in flood timing between positive and negative phases of both Indian Ocean Dipole and El Niño–Southern Oscillation; in some cases the difference in the timing of annual flood events is more than three months. Sensitivity to one or other mode of variability differs regionally. Changes in flood timing are larger than variability in rainy season onset reported in the literature, highlighting the need to understand how the hydrological system alters climate variability signals seen in rainy season onset, length, and rainfall totals. Our insights into flood timing could support communities who rely on flood‐based farming systems to adapt to climate variability. Plain Language Summary: Patterns of climate variability such as the El Niño–Southern Oscillation or the Indian Ocean Dipole affect the timing of the start of the rainy season, but little is known about how this translates into changes in the timing of the annual flood in rivers. We use computer model reconstructions and observed records of river flows in Africa to understand how these patterns of climate variability are changing the timing of river floods. In eastern and southern Africa, in particular, the differences in flood timing can be more than three months. This information could help farmers in African floodplains to adapt their water management, planting, and cropping practices to these patterns of climate variability. Key Points: Modes of climate variability in the Pacific and Indian Oceans drive significant changes in flood timing across sub‐Saharan AfricaThe hydrological system enhances and alters climate variability signals seen in rainy season onset, duration, and total rainfallUnderstanding of how climate variability changes the timing of floods can support flood‐based farming systems [ABSTRACT FROM AUTHOR]

The transport of freshwater from continents to oceans through rivers has traditionally been estimated by routing runoff from land surface models within river models to obtain discharge. This paradigm imposes that errors are transferred from runoff to discharge, yet the analytical propagation of uncertainty from runoff to discharge has never been derived. Here we apply statistics to the continuity equation within a river network to derive two equations that propagate the mean and variance/covariance of runoff errors independently. We validate these equations in a case study of the rivers in the western United States and, for the first time, invert observed discharge errors for spatially distributed runoff errors. Our results suggest that the largest discharge error source is the joint variability of runoff errors across space, not the mean or amplitude of individual errors. Our findings significantly advance the science of error quantification in model‐based estimates of river discharge. Plain Language Summary: Computer programs are routinely used to simulate the movement of water on Earth, including the flow of freshwater in rivers. A common approach for calculating river flow is to run a first computer program that estimates how much of the rain is partitioned into runoff, evaporation, or storage, then a second that transports the runoff through rivers. These computer programs are based on a simplified representation of nature and are imperfect. Until now, it has been unclear how the imperfections are transferred from one program to the next, and therefore challenging to understand the sources of errors. In this study, we present a methodology that can be used to backtrack errors in runoff, an otherwise unknown quantity, from observed errors in river flow. Our results reveal a surprising error source and can help diagnose limitations in simulations of the Earth's water cycle. Key Points: We present the first analytical equations for propagating runoff uncertainty into discharge uncertainty through a river networkOur methodology allows the inverse inference of runoff errors from observed discharge errors at continental scalesThe largest discharge error source is the joint variability of runoff errors across space, not the mean or amplitude of individual errors [ABSTRACT FROM AUTHOR]

The isotopic composition of surface and groundwater is impacted by many hydrologic processes. The long‐term stable water isotopic response of systems to hydrologic change is critical for interpreting isotopic information for streamflow generation, stream‐aquifer‐coupling, and recharge processes. To evaluate the response of stream‐aquifer systems to extreme precipitation events we use 743 surface and groundwater isotopes with drainage areas ranging from 0.1 to >800 km2. Results show multiannual trends from high to low isotopic compositions associated with increases in the composition of shallow groundwater. The year 2011 was one of the wettest years and the months of August and September were the wettest consecutive 2‐month period in the 123‐year record. This increase in the isotopic composition has long‐term impact on the isotopic composition of surface and groundwater highlighting the importance of groundwater sources of baseflow to streams and the transient storage and release mechanisms of groundwater at the catchment scale. Plain Language Summary: We present and analyze surface and groundwater samples collected from late 2011–2016 across a large region of western Massachusetts. The data together show multiyear trends in the stable water isotope compositions, a tracer of water source and hydrological processes. These trends are not reflective of typical seasonal behavior and point to a change in the isotopic composition due to a unique hydrologic event. The year of 2011 was the wettest calendar year on record for western Massachusetts and the months of August and September of 2011 were the wettest consecutive 2‐month period in the 123‐year precipitation record. This wet period saturated the ground with excess water leading to large increases in the regional water table and high infiltration rates. The isotope data suggest that 4–5 years later we still observe this water draining from the groundwater system to surface streams. These results point to the important role of extreme precipitation events to surface and groundwater catchments, even in regions with high annual precipitation rates. The inclusion of groundwater processes (recharge, storage, and discharge) are important controls on surface water isotopic composition. Key Points: Stable isotopic composition of surface waters show a persistent increase due to summertime precipitation in the year 2011Length of isotopic recession of stream water is a strong function of watershed areaStream water isotopes consistent with a large pulse of water being stored and released from groundwater [ABSTRACT FROM AUTHOR]

Abstract: Discharge projections into the Hudson Bay Complex to 2070 are investigated for global mean temperature warming levels of 1.5 and 2.0 °C. Median precipitation increases from 1986–2005, ranging from 2% during summer to 19% during winter, are projected to increase discharge in all seasons except summer. The rise in discharge is greatest furthest north, into Foxe Basin, Ungava Bay, and Hudson Strait, exceeding 10% above historical annual means. A 2.0 °C warming results in higher discharge than 1.5 °C warming owing to greater precipitation (e.g., 6.5% greater spring discharge increase); however, summer discharge for 2.0 °C warming is lower due to enhanced evaporation and lower precipitation increase from historical (4.0% lower summer discharge increase). Extreme daily high flows are projected to be greater than historical, more so for 2.0 °C warming than 1.5 °C warming, and this is greatest in the eastern and northern regions. These projections suggest continued increasing river discharge into pan‐Arctic coastal oceans. [ABSTRACT FROM AUTHOR]