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Alluvial rivers aggrade, incise, and adjust their sediment‐transport rates in response to changing sediment and water supply. Fluvial landforms, such as river terraces, and downstream stratigraphic archives may therefore record information about past environmental change. Using a physically based model describing sediment transport and long‐profile evolution of alluvial rivers, we explore how their responses to environmental change depend on distance downstream, forcing timescales, and whether sediment or water supply is varied. We show that amplitudes of aggradation and incision, and therefore the likelihood of terrace formation, are greater upstream and in shorter and/or wetter catchments. Aggradation and incision, and therefore terrace ages, may also lag behind environmental change. How sediment‐transport rates evolve depends strongly on whether water or sediment supply is varied. Diverse responses to environmental change could arise in natural alluvial valleys, controlled by their geometry and hydrology, with important implications for paleo‐environmental interpretations of fluvial archives. Plain Language Summary: Rivers carry sediment from upland regions, where it is produced by weathering and erosion, to low‐lying areas where it is deposited and stored. This process is sensitive to climate: for example, wetter conditions enhance rivers' capacity to transport sediment, causing rivers to cut into their beds, which in turn remobilizes sediment that is transported downstream. Changing climate may also lead to enhanced sediment production and supply to rivers, causing them to accumulate sediment in their beds. Climate cycles can therefore produce alternating periods of sediment accumulation and removal, resulting in ancient floodplains being abandoned and preserved alongside modern rivers ("terraces"), and in variations in the amounts of sediment delivered downstream. Terraces and patterns of sediment accumulation downstream could therefore provide valuable records of past climatic change. We develop a mathematical model that describes the effects of climatic change on river‐bed height and sediment‐transport rates. We show how the amount and timing of river in‐filling and down‐cutting, and the timescales over which they occur, depend on the rate of climatic change, on river length and associated drainage‐basin area, and on the amount of water it carries. These predictions will facilitate more detailed interpretations of terrace records along rivers and of the sedimentary deposits that they leave behind. Key Points: Alluvial rivers respond in diverse ways to cyclical climatic change depending on their geometry and hydrology and on the forcing frequencyTerrace formation is favored upstream and in shorter and wetter catchments and may lag appreciably behind environmental changeVariation in sediment output lags variation in sediment supply but can lead and be amplified with respect to variation in water supply [ABSTRACT FROM AUTHOR]

Detrital 10Be from continental river sands or submarine sediments has been extensively used to determine the average long-term denudation rates of terrestrial catchments, based on the assumption that the rate of cosmogenic nuclide production by the interaction of source rocks with cosmic radiation balances out the loss of these nuclides by surface denudation. However, the 10Be signal recorded in sediments may be affected at the source by the response time of mountainous catchments to high-frequency forcings. In addition, transient sediment storage in piedmonts, alluvial plains and lakes or near the coast may also induce a difference between the erosive signal and its record in the sedimentary sink. Consequently, a significant part of the signal recorded in shallow-water sediments can be lost, as deep marine sediments may simultaneously record a signal coming from newly eroded source rocks along with one coming from the destabilization of previously deposited sediments. In this paper, we use the landscape evolution model Badlands to simulate erosion, deposition and detrital 10Be transfer from a source-to-sink sedimentary system (the Var River catchment, southern French Alps) over the last 100 kyr. We first compare model-based denudation rates with the ones that would be extracted from the 10Be record of local continental sediments (equivalent to river sands) and from sediments deposited offshore over time in order to examine if this record provides an accurate estimate of continental denudation rates. Then, we examine which conditions (precipitation rate, flexure, ice cover) satisfy published measured river incision rates and 10Be concentration in submarine sediments. Our results, based on the Var catchment cosmic ray exposure dating and modelling indicate that, while river sands do accurately estimate the average denudation rate of continental catchments, this is much less the case for deep submarine sediments. We find that deep-sea sediments have a different and often much smoother 10Be signature than continental ones and record a significant time lag with respect to imposed precipitation rate changes, representing the geomorphological response of the margin. A model which allows us to fit both measured 10Be concentration in marine sediments and river incision rates on land involves an increase in precipitation rates from 0.3 to 0.7 m yr -1 after 20 ka, suggesting more intense precipitation starting at the end of the Last Glacial Maximum. [ABSTRACT FROM AUTHOR]

Waterfall plunge pools experience cycles of sediment aggradation and scour that modulate bedrock erosion, habitat availability, and hazard potential. We calculate sediment flux divergence to evaluate the conditions under which pools deposit and scour sediment by comparing the sediment transport capacities of waterfall plunge pools (Qsc_pool) and their adjacent river reaches (Qsc_river). Results show that pools fill with sediment at low river discharge because the waterfall jet is not strong enough to transport the supplied sediment load out of the pool. As discharge increases, the waterfall jet strengthens, allowing pools to transport sediment at greater rates than in adjacent river reaches. This causes sediment scour from pools and bar building at the downstream pool boundary. While pools may be partially emptied of sediment at modest discharge, floods with recurrence intervals >10 yr are typically required for pools to scour to bedrock. These results allow new constraints on paleodischarge estimates made from sediment deposited in plunge pool bars and suggest that bedrock erosion at waterfalls with plunge pools occurs during larger floods than in river reaches lacking waterfalls. [ABSTRACT FROM AUTHOR]

High-resolution topographic surveys using LiDAR and multibeam sonar can be used to characterize and quantify fluvial change. This study used repeat surveys to explore how topographic change, fluvial processes, sediment budgets, and aggradation and incision rates vary across spatial scales and across two contrasting decadal flow regimes in a regulated gravel/cobble river. A novel method for quantifying digital elevation model uncertainty was developed and applied to a topographic change detection analysis from 2006/2008 to 2014. During this period, which had four modest ~ 3–5 year floods, most sediment was laterally redistributed through bank erosion and channel migration. Erosion primarily occurred in the floodplain (97,000 m 3 ), terraces (80,000 m 3 ), and lateral bars (58,000 m 3 ); while deposition occurred in the adjacent pools (73,000 m 3 ), fast glides (48,000 m 3 ), and runs (36,000 m 3 ). In contrast, significantly higher magnitude and longer duration floods from 1999 to 2006/2008 caused sediment to be displaced longitudinally, with the upstream reaches exporting sediment and the downstream reaches aggrading. The river maintained floodplain connectivity during both periods, despite different processes dominating the type of connectivity. Larger floods promoted overbank scour and avulsion, while smaller floods promoted bank erosion and lateral migration. This study explores and illustrates how the geomorphic response to contrasting flood regimes in a nonuniform river is highly dependent on which landforms are controlling hydraulics. [ABSTRACT FROM AUTHOR]

This paper develops a numerical model for the analysis of sediment transport in irrigated channels. The overland flow equation considers infiltration into the subsurface. The model solves Saint Venant and sediment continuity equations to analyze overland flow and sediment transport, while Richard's equation is used to analyze subsurface flow. Saint Venant and sediment continuity equations are solved using MacCormack-scheme-based finite-difference method and Richard's equation is solved using a mass conservative fully implicit finite-difference method. The model is validated by comparing model-predicted aggradation with experimental observations. The model is used to study the effect of infiltration on aggradation of a channel bed during sediment-laden flows. It was observed that infiltration significantly affects and alters the aggradation phenomenon and aggradation considering infiltration is found to be higher than that without considering infiltration. Among the different unsaturated soil parameters, the saturated hydraulic conductivity and initial dryness of the soil were found to be the main factors affecting the sediment transport in irrigated channels. [ABSTRACT FROM AUTHOR]

In mountain environments, the coupling of hillslopes processes with the channel network during extreme events is of great importance for rivers dynamics, as debris flows and landslides are among the most important sources of sediments. The Stolla Creek (40 km2 drainage area, South Tyrol, Italy) is a confined and partly confined mountain channel that was affected by an extreme flood in August 2017, followed by a smaller event in August 2020. The geomorphic effects of the two floods were investigated both in the main channel and over the entire basin with the aim to assess the impacts of the lateral sediment connectivity to the channel response and to the event-scale sediment export. An integrated approach was applied, including radar rainfall estimation, hydrologic-hydraulic analysis, analysis of morphological changes and sediment delivery to the stream network. Hillslope and channel processes were mapped and characterized by using geomorphological analysis of multitemporal orthophotos and Digital Terrain Models. Debris-flow connectivity to the main channel was derived by combining field evidence and GIS-based analysis. The 2017 flood was caused by rainfall with a short duration (6 h) and a rainfall intensity exceeding 45 mm/h. More than 600 debris flows were triggered in the Stolla basin, and the main channel experienced widening (width ratio between 1.3 and 4.9) through bank erosion and overbank deposition. Widening was accompanied by aggradation in the river corridor up to 1.2 m or incision down to −2.2 m. The 2020 flood was characterized by lower rainfall intensity (max 17 mm/h) and a longer duration (48 h), and debris flows were not triggered. The moderate magnitude of the 2020 flood peak did not lead to channel widening, but marked bed incision (up to −1.4 m) occurred in the reaches where aggradation took place during the 2017 event. In both flood events, limited volumes of sediments were exported from the catchment outlet. Overall, our results highlight how structural connectivity at the basin scale determines the potential sediment cascades linking hillslopes to channels but time-varying functional connectivity – driven by hydrological drivers as rainfall intensities and durations – eventually control the actual sediment transport effectiveness both on hillslopes and along the channel. • An integrated geomorphological and hydrological approach is applied to two floods • Basin structural connectivity well describes potential sediment delivery to the main channel • Functional connectivity determined by rainfall characteristics controlled actual sediment supply and transport [ABSTRACT FROM AUTHOR]

The Jinsha River, the upper reaches of the Yangtze River, winds and incises through the eastern margin of the Tibetan Plateau, where Asian monsoon climate prevails. Along the Jinsha River, extensive and thick Quaternary valley fillings and multiple river terraces have been developed. However, their specific relations to climate change and tectonics remain elusive. In this study, we conducted field investigation, chronological studies and numerical modeling in its lower reaches. Our study indicates that the extensive valley fillings in the Wudongde-Menggu segment were formed in cold-arid climate with Optically Stimulated Luminescence (OSL) ages of ~23–19 ka B.P., coinciding with the timing of the Last Glacial Maximum (LGM). We suggest that the post-LGM warming and humidifying, by restrained sediment supply while enhanced discharge, switched the valley from prevailing aggradation into incision. Multiple subsequent fill-cut terraces, postdating the LGM, were formed and nested in the valley fillings. OSL data indicate that sediments of three terraces (T1–T3) were formed at ~10 ka B.P., ~11.5–12.5 ka B.P. and ~13.5 ka B.P., respectively. Our study suggests that these terraces are sensitive records of millennial and sub-millennial cold-arid climate perturbations superimposed over the post-LGM deglaciation. Our modeling highlights a high sensitivity of valley aggradation and incision to cold-arid and warm-wet climate change, as well as a fast valley landscape response in the Jinsha River characterized by high sediment transport influx. We propose that valley landscape and terraces serve as a timely record for climate change in certain rivers with short response time threshold characterized by high sediment transport influx. • Valley filling was driven by the cold-arid climate in the LGM. • Fill-cut terraces record sub-millennial cold-arid climate perturbations. • Modeling supports a sensitive and fast response of Jinsha River to climate change. • The present valley landscape remains in a transient adjustment since the LGM. [ABSTRACT FROM AUTHOR]

It has been observed that the relation between sediment storage and sediment transport rate in gravel-bed rivers exhibits counter-clockwise hysteresis, or aggradation-degradation cycles. Previous work has attributed these cycles to external changes to the sediment supply rate. We present a set of nine flume experiments that test the hypothesis that aggradation-degradation cycles can occur even under steady sediment supply conditions when the sediment transport rate temporarily nears the sediment supply rate. The experiments are designed to examine the transport-storage relations under a range of conditions with different initial bed condition, sediment supply rate, discharge, and supply material texture. The sediment transport efficiency of the experimental channel was strongly influenced by the existing bed morphology inherited from historical flows. The discharge and grain size distribution of the supply material played a secondary role, with higher discharge conditions and finer sediment feed material producing increased transport efficiencies. The transport-storage relations revealed that the transport rate of the channel changed with the volume of sediment stored and was, therefore, mediated by the channel morphology. Aggradation-degradation cycles were observed even under steady sediment supply and discharge conditions and were mediated by morphologic adjustments within the channel that temporarily led the transport rate to be near equal to the sediment supply rate. In other words, the probability of hysteresis in the transport-storage relation increases when the channel is near a transport equilibrium. The findings demonstrate the complexity associated with developing a unique relation between sediment transport and storage in gravel-bed rivers, and highlights the importance of considering the morphologic evolution and historical bed conditions. • Flume experiments conducted to investigate cycles of aggradation-degradation. • Sediment transport rate variability linked to bed morphology and texture changes. • Transport rate adjusted according to volume of sediment stored. • Aggradation-degradation cycles tend to occur when channel is near equilibrium. [ABSTRACT FROM AUTHOR]

Many world's large rivers have experienced significant sediment reduction in the past century. However, channel aggradation near the mouth of these rivers may be still occurring or even accelerating, as evidenced by a recent study on the Lowermost Mississippi River. This study aims to investigate whether such a trend exists near the mouth of another large lowland alluvial river, the Atchafalaya River, which enters the Gulf of Mexico via two outlets — the natural mainstem and a man-made straight channel that is shorter and has a much greater slope. The overarching hypothesis is that, despite the reduction in riverine sediment delivery, lowland alluvial rivers such as the Lower Atchafalaya River in their backwater zone aggrade. We collected bathymetric survey records in 1977 and 2006 to analyze riverbed deformation. We utilized stage records at several gauging stations to determine the change in the hydraulic head over time. Geospatial techniques were used to develop channel bed Digital Elevation Models which were used to identify deformation of the 174-km channel bed of the Lower Atchafalaya River. We found a cumulative erosion of 6.34 × 107 m3 and a cumulative deposition of 8.79 × 107 m3, resulting in a net deposition of 2.45 × 107 m3 during the study period. While much of the river including the man-made channel was in a degrading state, the last 35 km of the river's natural mainstem experienced substantial aggradation. On average, riverbed elevation of the Lower Atchafalaya decreased by 1.47 m in the upper 98 km, but increased by 0.96 m in the final 35 km of the river's mainstem to the Gulf of Mexico. The channel erosion in the upstream reach will likely continue with the river discharge projected to increase, and the channel deposition in the downstream mainstem may accelerate because of sea-level rise. • Assessment of channel bed dynamics of a large lowland alluvial river on the Mississippi River Delta • Changes in volume and mass of the channel beds are quantified, and factors affecting them are analyzed. • Channel degradation occurs in the upper reach of the Lower Atchafalaya, mainly affected by flood frequency. • Channel aggradation mainly occurs in the river's final 35 km despite of reduced sediment delivery. [ABSTRACT FROM AUTHOR]

The supply of sediment from hillslopes to channels is rarely constant, with discrete events (e.g., landslides) known to transfer large volumes of sediment in geologically-short periods of time, especially in tectonically active areas. Understanding the rates and patterns of subsequent sediment evacuation is important for understanding variability of landscape evolution, as well as mitigating the risk of geohazards associated with bed aggradation and the loss of channel capacity to convey flood waters. Here, we performed a series of controlled laboratory flume experiments to explore the controls on sediment transport after a single sediment input in scenarios with (i) different initial input volumes (4–25 kg) under constant flow conditions (40 l/s), (ii) a constant initial input volume (12 kg) under different flow magnitudes (5–60 l/s), and (iii) a repeat of the input sediment volumes in scenario (i) but with the same volume of water delivered using a ramped hydrograph (0–60–0 l/s). We find the presence of sediment piles impacts the flow hydraulics, with a backwater effect developing upstream of the pile that causes a flow acceleration around the location of the pile. For a given discharge, larger sediment piles have a greater impact on the flow hydraulics, which in turn induces higher rates of sediment transport and erosion of the pile. In all cases, sediment remains at the initial pile input location for the duration of the sediment evacuation, acting to protect the bed from erosion. We highlight the role of geomorphic-hydraulic interactions in controlling the sediment evacuation, and suggest there is an optimal combination of pile size and flow conditions (flow magnitude and hydrograph shape) for accelerated rates of sediment transport, which are important for the short-term and long-term channel dynamics and the landscape evolution variability. • Experiments documenting sediment transport processes following landslide inputs. • Identified the co-evolving interactions between hydraulic and geomorphic processes • Backwater effect around sediment pile induces accelerated transport rates. • Highest sediment transport rates associated with larger piles due to backwater effect. [ABSTRACT FROM AUTHOR]

Rock-avalanches (RAs) are large erosional events which, in a matter of minutes, can deliver acatastrophic volume of debris (≥ 1×106m3) directly to a fluvial system, often dammingmountainous rivers. RAs have a boulder carapace protecting readily transportable sedimentsizes, differing from landslides. Evidence suggests that the point-load sediment delivery ofrock-avalanche deposits (RADs) on fluvial systems can force rivers into a chronicallydisturbed state for up to 104 years. Fieldwork and micro-scale modelling are used to trace RAsediment dispersion in Ram Creek, New Zealand. The data forms a new conceptual model ofriver erosion and depositional response, changes to river planform and sedimentyield, and an effective model timescale of perturbation for both RA and landslidedams.Fieldwork investigation shows large scale aggradation from RA outburst flood sedimentdispersion, buffering vertical bedrock incision. The system is unable to transport the quantityof dam-derived sediments, and will not attain equilibrium before the next major event; it is ina state of persistent disturbance where localised reworking dominates. Lateral bevelling in theslot-type bedrock gorge topography appears to be the dominant form of erosion promotinggorge widening of the weak bedrock. It is theorised that aggradation of RA debris and lateralbevelling may result in the formation of a strath terrace if sediment cover persists,independent of external tectonic and climatic forcing. Normalised steepness and concavityindices of the Ram Creek river profile were compared to four neighbouring streams. TheRAD perturbation to profile metrics is not substantial enough to be discerniblefrom neighbouring streams containing large tectonically induced knickpoints. Thissuggests RA frequency estimates may be underrepresented in landscape evolutionmodels.Micro-scale modelling of the responses to a catastrophic sediment input into Ram Creek;either a RA (including carapace) or a landslide, each show different geomorphic responses.The RA model shows an un-breached carapace acts to trap fluvial sediments, resulting inpartial burial. Further downstream, initial sediment starved incision is followed by a slowerrecovery to pre-RA bed elevations, and continued aggradation of fluvial sediment tovertically displace the profile up towards the RA. In the landslide model, immediate breachchannel formation results in a sediment pulse downstream. Dispersed sediment forcesaggradation, controlling fluvial geomorphology.Fieldwork and modelling both show catastrophic sediment inputs control fluvialplanform, sediment yield, calibre, and storage, and geomorphic forms throughout theaffected fluvial profile. This contradicts the view that rivers in the Southern Alps, areunder-loaded and transport all sediment that is supplied to them. The inability of theriver to remove both the RA and landslide deposits suggests that the river will notrecover before the return period for the next large sediment input, and is a scenariowitnessed across numerous other feeder catchments to larger rivers in New Zealand. Itis suggested here that RA and landslide sediment storage from multiple eventscontrols the geomorphic evolution of smaller mountainous catchments, and RAforced dis-equilibrium may actually be the 'normal’ state in many mountainousstreams. [ABSTRACT FROM AUTHOR]

Sediment production and transport along sediment-routing systems are thought to vary with prevailing climatic and tectonic conditions. As such, the environmental conditions (climate, tectonics) of the past can potentially be reconstructed from sedimentary deposits. Reconstructions, however, must account for sedimentary signal modification through temporary storage and remobilization along the sediment-routing system. Fluvial fill terraces – the product of alternating phases of channel-bed aggradation and incision – are geomorphic features that result from temporary sediment storage and release. Although a range of field studies, physical experiments, and numerical models have investigated alluvial-channel responses and fill-terrace formation due to changes in environmental boundary conditions, we lack a systematic comparison of differences in channel morphology and terrace morphology formed by different types of perturbations.Here, we show results from seven physical experiments, in which sediment supply, water discharge and base-level elevation were regulated individually. We investigated how different perturbations affected channel morphology, fill-terrace formation, and the transient response of sediment discharge. We found that either a decrease in water discharge or an increase in sediment supply resulted in channel-bed aggradation and channel steepening. In contrast, an increase in water discharge, a decrease in sediment supply, or a drop in base-level triggered channel incision and terrace abandonment. The lag-time between perturbation onset and terrace cutting defines how well channel morphology prior to the perturbation is preserved in the terrace morphology. Lag-times were found to depend on the local channel incision rate and on the synchronicity of incision along the channel. An increase in water discharge caused faster channel incision (and shorter lag-times) compared to a decrease in sediment supply. Whereas incision driven by water- and sediment-discharge changes were initiated nearly simultaneously along the entire channel reach, incision related to base-level drop caused a wave of upstream knickzone migration and diffusion, forming terraces with decreasing ages in the upstream direction. We found that channel conditions prior to perturbation were best preserved by terraces formed following increases in discharge.Morphological adjustments of the channel in response to changes in incoming water or sediment supply also affected sediment discharge at the basin outlet. An increase or decrease in water discharge triggered channel incision or aggradation, recognizable as a temporary increase or decrease, respectively, in sediment discharge at the outlet. Permanent changes in upstream sediment supply were not recognizable at the outlet during the channel adjustment phase due to a negative feedback between the amount of sediment supply and the channel gradient. Changes in upstream sediment supply are thought to become measurable at the outlet only once the channel has adjusted to its new boundary conditions. [ABSTRACT FROM AUTHOR]