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River profiles are shaped by climatic and tectonic history, lithology, and internal feedbacks between flow hydraulics, sediment transport and erosion. In steep channels, waterfalls may self‐form without changes in external forcing (i.e., autogenic formation) and erode at rates faster or slower than an equivalent channel without waterfalls. We use a 1‐D numerical model to investigate how self‐formed waterfalls alter the morphology of bedrock river longitudinal profiles. We modify the standard stream power model to include a slope threshold above which waterfalls spontaneously form and a rate constant allowing waterfalls to erode faster or slower than other fluvial processes. Using this model, we explore how waterfall formation alters both steady state and transient longitudinal profile forms. Our model predicts that fast waterfalls create km‐scale reaches in a dynamic equilibrium with channel slope held approximately constant at the threshold slope for waterfall formation, while slow waterfalls can create local channel slope maxima at the location of slow waterfall development. Furthermore, slow waterfall profiles integrate past base level histories, leading to multiple possible profile forms, even at steady‐state. Consistency between our model predictions and field observations of waterfall‐rich rivers in the Kings and Kaweah drainages in the southern Sierra Nevada, California, supports the hypothesis that waterfall formation can modulate river profiles in nature. Our findings may help identify how bedrock channels are influenced by waterfall erosion and aid in distinguishing between signatures of external and internal perturbations, thereby strengthening our ability to interpret past climate and tectonic changes from river longitudinal profiles. Plain Language Summary: Mountain rivers are shaped by tectonic and climatic forces; thus, the longitudinal profile of a river (a plot of river elevation vs. distance along the stream) can be used to investigate geologic history. Abrupt changes in river slope are often interpreted as signals of an external tectonic or climatic disruption. However, other factors may disrupt longitudinal profiles, including changes in rock type and differences in river erosion processes. Here, we investigate one such process: waterfalls formed by feedbacks between water flow, sediment transport and river‐bed erosion, and whether these waterfalls can alter longitudinal profiles without climatic or tectonic changes. We make a model to investigate what river profiles might look like if waterfalls naturally develop and alter erosion rates in all channels exceeding a specific slope threshold. Our model predicts that waterfalls generate distinct river profile features including a constant channel slope maintained over several kilometers where waterfalls occur and sharp increases in slope due to waterfall occurrence, both of which we have found in waterfall‐rich rivers in the southern Sierra Nevada, California. This research identifies similarities and differences between the impact of changes in tectonics and climate versus internal feedbacks such as self‐formed waterfalls on river profiles. Key Points: We investigate how erosion via self‐formed waterfalls alters river long profile form using a stream power model with a slope thresholdSelf‐formed waterfalls alter fluvial relief and concavity, create dynamic equilibriums, and, in cases, path‐dependent profile formsProfiles of rivers in the Sierra Nevada, California, with abundant waterfalls have similar features to our model [ABSTRACT FROM AUTHOR]

Riverine measurements of sediment and solute transport give empirical basin-scale estimates of bed-load, suspended-sediment, and silicate-solute fluxes for 100,000 km2 of northwestern California and western Oregon. This spatially explicit sediment budget shows the multifaceted control of geology and physiography on the rates and processes of fluvial denudation. Bed-load transport is greatest for steep basins, particularly in areas underlain by the accreted Klamath terrane. Bed-load flux commonly decreases downstream as clasts convert to suspended load by breakage and attrition, particularly for softer rock types. Suspended load correlates strongly with lithology, basin slope, precipitation, and wildfire disturbance. It is highest in steep regions of soft rocks, and our estimates suggest that much of the suspended load is derived from bed-load comminution. Dissolution, measured by basin-scale silicate-solute yield, constitutes a third of regional landscape denudation. Solute yield correlates with precipitation and is proportionally greatest in low-gradient and wet basins and for high parts of the Cascade Range, where undissected Quaternary volcanic rocks soak in 2-3 m of annual precipitation. Combined, these estimates provide basin-scale erosion rates ranging from ~50 t · km-2 · yr-1 (approximately equivalent to 0.02 mm · yr-1) for low-gradient basins such as the Willamette River to ~500 t · km-2 · yr-1 (~0.2 mm · yr-1) for steep coastal drainages. The denudation rates determined here from modern measurements are less than those estimated by longer-term geologic assessments, suggesting episodic disturbances such as fire, flood, seismic shaking, and climate change significantly add to long-term landscape denudation. [ABSTRACT FROM AUTHOR]

The extreme heat from wildfire alters soil properties and incinerates vegetation, leading to changes in infiltration capacity, ground cover, soil erodibility, and rainfall interception. These changes promote elevated rates of runoff and sediment transport that increase the likelihood of runoff-generated debris flows. Debris flows are most common in the year immediately following wildfire, but temporal changes in the likelihood and magnitude of debris flows following wildfire are not well constrained. In this study, we combine measurements of soil-hydraulic properties with vegetation survey data and numerical modeling to understand how debris-flow threats are likely to change in steep, burned watersheds during the first 3 years of recovery. We focus on documenting recovery following the 2016 Fish Fire in the San Gabriel Mountains, California, and demonstrate how a numerical model can be used to predict temporal changes in debris-flow properties and initiation thresholds. Numerical modeling suggests that the 15-minute intensityduration (ID) threshold for debris flows in post-fire year 1 can vary from 15 to 30 mm/hr, depending on how rainfall is temporally distributed within a storm. Simulations further demonstrate that expected debris-flow volumes would be reduced by more than a factor of three following 1 year of recovery and that the 15-minute rainfall ID threshold would increase from 15 to 30 mm/hr to greater than 60 mm/hr by post-fire year 3. These results provide constraints on debris-flow thresholds within the San Gabriel Mountains and highlight the importance of considering local rainfall characteristics when using numerical models to assess debris-flow and flood potential. [ABSTRACT FROM AUTHOR]

We developed a technique for reconstructing annual gravel yields and generated a 55‐year record of gravel transport for the North Fork catchment of the Caspar Creek Experimental Watersheds in Northern California. The technique relies on field data collection including annual surveys of weir pond volumes and suspended sediment measurements, as well as an accounting for settling of suspended sediment and organic matter in the pond. We compared these annual yields to gravel yields predicted by the Wilcock two‐fraction bed load transport model, which we calibrated from measured values at Caspar Creek. We considered three velocity‐discharge relationships and found that values of hydraulic variables measured during storms produced the best fit between reconstructed and predicted annual gravel yields when years with large disturbances were excluded. We also compared predicted gravel transport rates to bed load transport rates measured from 1988 to 1995 with bed load pit samplers. We found that the calibrated model predictions agreed well with the field‐measured bed load transport rates. To investigate the role of supply and storage on gravel transport, we compared the reconstructed gravel yields to predicted gravel yields and found that increased occurrence of landslides and headcut erosion in the 1990s and early 2000s did not lead to an increase in gravel yields. Instead, input of large downed wood in the 1990s created storage space and decreased bed load delivery to the weir pond. Plain Language Summary: Bed load sediment yields are important for understanding catchment responses to natural and human caused disturbances, but are difficult to measure. We demonstrate a method that uses pond sediment accumulations to quantify annual bed load yields at Caspar Creek, CA. We also use these data and data collected during more focused storm‐based sampling to show that a common bed load model provides relatively accurate estimates of gravel transport at Caspar Creek. Comparing annual gravel yields to predicted gravel yields reveals that gravel transport decreased in the 1990s and 2000s after a large number of trees blew down and increased the wood content in the channel. The approach presented here can be applied to other sites where similar data are available, such as other ponds or small reservoirs. Key Points: We reconstructed a 55‐year record of annual gravel yields from weir pond deposits at Caspar Creek, CAReconstructed gravel yields compare well to modeled gravel yields and to a continuous record of bed load transport ratesComparing reconstructed yields to predictions reveals a decrease in transport associated with increased downed wood and sediment storage [ABSTRACT FROM AUTHOR]

Intense transport of sediments occurs along the eastern coast of the Gulf of California, interacting with several coastal lagoons like Santa María-La Reforma. This sediment dynamics plays an important role in the conservation issue of marine species. The effect of tidal currents on the bed load sediment transport has been investigated for this lagoon, emphasizing the mechanisms of the geomorphologic development of the sea bottom. The flow dynamics has been studied applying the vertically integrated shallow water equations through a modified two dimensional non-linear finite difference model. A parameterized formula for the bed load sediment transport and a sediment conservation equation were applied to investigate the morphodynamics of the sea bottom of the lagoon. The predicted currents pattern allowed identifying a zone where tidal waves, entering into the lagoon throughout the northern and southern openings, meet. This process had consequences on the bed load transport of sediment. The results revealed areas where the currents induce noticeable erosion and accretion processes. The complex geometry of the lagoon seems to determine the accretion and erosion areas. Important changes of the order of 0.04 m per year were predicted nearby the Northern inlet and around the Talchichitle Island. The lagoon system exports sediment through the two inlets. This research work contributes to the understanding of the sediment dynamics in an extensive region with a series of coastal lagoons located on the eastern side of the Gulf of California. [ABSTRACT FROM AUTHOR]

Abstract: An autonomous underwater vehicle (AUV) carrying a multibeam sonar and a chirp profiler was used to map sections of the seafloor within the La Jolla Canyon, offshore southern California, at sub-meter scales. Close-up observations and sampling were conducted during remotely operated vehicle (ROV) dives. Minisparker seismic-reflection profiles from a surface ship help to define the overall geometry of the La Jolla Canyon especially with respect to the pre-canyon host sediments. The floor of the axial channel is covered with unconsolidated sand similar to the sand on the shelf near the canyon head, lacks outcrops of the pre-canyon host strata, has an almost constant slope of 1.0° and is covered with trains of crescent shaped bedforms. The presence of modern plant material entombed within these sands confirms that the axial channel is presently active. The sand on the canyon floor liquefied during vibracore collection and flowed downslope, illustrating that the sediment filling the channel can easily fail even on this gentle slope. Data from the canyon walls help constrain the age of the canyon and extent of incision. Horizontal beds of moderately cohesive fine-grained sediments exposed on the steep canyon walls are consistently less than 1.232million years old. The lateral continuity of seismic reflectors in minisparker profiles indicate that pre-canyon host strata extend uninterrupted from outside the canyon underneath some terraces within the canyon. Evidence of abandoned channels and point bar-like deposits are noticeably absent on the inside bend of channel meanders and in the subsurface of the terraces. While vibracores from the surface of terraces contain thin (<10cm) turbidites, they are inferred to be part of a veneer of recent sediment covering pre-canyon host sediments that underpin the terraces. The combined use of state of the art seafloor mapping and exploration tools provides a uniquely detailed view of the morphology within an active submarine canyon. [Copyright &y& Elsevier]

The quantity of suspended sediment in an estuary is regulated either by transport, where energy or time needed to suspend sediment is limiting, or by supply, where the quantity of erodible sediment is limiting. This paper presents a hypothesis that suspended-sediment concentration (SSC) in estuaries can suddenly decrease when the threshold from transport to supply regulation is crossed as an erodible sediment pool is depleted. This study was motivated by a statistically significant 36% step decrease in SSC in San Francisco Bay from water years 1991-1998 to 1999-2007. A quantitative conceptual model of an estuary with an erodible sediment pool and transport or supply regulation of sediment transport is developed. Model results confirm that, if the regulation threshold was crossed in 1999, SSC would decrease rapidly after water year 1999 as observed. Estuaries with a similar history of a depositional sediment pulse followed by erosion may experience sudden clearing. [ABSTRACT FROM AUTHOR]

Increasing availability of high-resolution (1m) topography data and enhanced computational processing power present new opportunities to study landscape organization at a detail not possible before. Here we propose the use of "directed distance from the divide" as the scale parameter (instead of Horton's stream order or upstream contributing area) for performing detailed probabilistic analysis of landscapes over a broad range of scales. This scale parameter offers several advantages for applications in hydrology, geomorphology, and ecology in that it can be directly related to length-scale dependent processes, it can be applied seamlessly across the hillslope and fluvial regimes, and it is a continuous parameter allowing accurate statistical characterization (higher-order statistical moments) across scales. Application of this scaling formalism to three basins in California demonstrates the emergence of three distinct geomorphic regimes of divergent, highly convergent, and moderately convergent fluvial pathways, with notable differences in their scaling relationships and in the variability, or spatial heterogeneity, of topographic attributes in each regime. We show that topographic attributes, such as slopes and curvatures, conditional on directed distance from the divide exhibit less variability than those same attributes conditional on upstream contributing area, thus affording a sharper identification of regime transitions and increased accuracy in the scaling analysis. [ABSTRACT FROM AUTHOR]

Abstract: Simulations of estuarine bathymetric change over decadal timescales require methods for idealization and reduction of forcing data and boundary conditions. Continuous simulations are hampered by computational and data limitations and results are rarely evaluated with observed bathymetric change data. Bathymetric change data for Suisun Bay, California span the 1867–1990 period with five bathymetric surveys during that period. The four periods of bathymetric change were modeled using a coupled hydrodynamic-sediment transport model operated at the tidal-timescale. The efficacy of idealization techniques was investigated by discontinuously simulating the four periods. The 1867–1887 period, used for calibration of wave energy and sediment parameters, was modeled with an average error of 37% while the remaining periods were modeled with error ranging from 23% to 121%. Variation in post-calibration performance is attributed to temporally variable sediment parameters and lack of bathymetric and configuration data for portions of Suisun Bay and the Delta. Modifying seaward sediment delivery and bed composition resulted in large performance increases for post-calibration periods suggesting that continuous simulation with constant parameters is unrealistic. Idealization techniques which accelerate morphological change should therefore be used with caution in estuaries where parameters may change on sub-decadal timescales. This study highlights the utility and shortcomings of estuarine geomorphic models for estimating past changes in forcing mechanisms such as sediment supply and bed composition. The results further stress the inherent difficulty of simulating estuarine changes over decadal timescales due to changes in configuration, benthic composition, and anthropogenic forcing such as dredging and channelization. [Copyright &y& Elsevier]

pplications of process-based morphodynamic models are often constrained by limited availability of data on bed composition, which may have a considerable impact on the modeled morphodynamic development. One may even distinguish a period of 'morphodynamic spin-up' in which the model generates the bed level according to some ill-defined initial bed composition rather than describing the realistic behavior of the system. The present paper proposes a methodology to generate bed composition of multiple sand and/or mud fractions that can act as the initial condition for the process-based numerical model Delft3D. The bed composition generation (BCG) run does not include bed level changes, but does permit the redistribution of multiple sediment fractions over the modeled domain. The model applies the concept of an active layer that may differ in sediment composition above an underlayer with fixed composition. In the case of a BCG run, the bed level is kept constant, whereas the bed composition can change. The approach is applied to San Pablo Bay in California, USA. Model results show that the BCG run reallocates sand and mud fractions over the model domain. Initially, a major sediment reallocation takes place, but development rates decrease in the longer term. Runs that take the outcome of a BCG run as a starting point lead to more gradual morphodynamic development. Sensitivity analysis shows the impact of variations in the morphological factor, the active layer thickness, and wind waves. An important but difficult to characterize criterion for a successful application of a BCG run is that it should not lead to a bed composition that fixes the bed so that it dominates the 'natural' morphodynamic development of the system. Future research will focus on a decadal morphodynamic hindcast and comparison with measured bathymetries in San Pablo Bay so that the proposed methodology can be tested and optimized. [ABSTRACT FROM AUTHOR]

Presents a model study that evaluated sediment transport in a geomorphic channel proposed for restoration and flood damage reduction of an 11-kilometer tidally influenced reach of the Napa River located in California. Objective of the model study; Description on the hydrodynamic and sediment transport models; Model calibration; Conclusions.