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At geologic timescales, a significant proportion of the sediment eroded from continents is transferred from rivers to littoral cells, then through submarine canyons to deep ocean basins. Accumulation and occasional discharge of sediment from canyon headwall regions is a likely driver for the sediment gravity flows believed to be responsible for much of this mass transport, but the responsible mechanisms and event timescales in canyon heads are imprecisely defined, largely due to the drowning of canyon heads since the last glaciation and the resulting dearth of modern depositional environments to observe. We investigated a littoral cell adjacent to Mattole submarine canyon in Northern California to better understand the influence of bathymetry and coast shape on sediment trajectories and hypothesize that canyon heads promote littoral sediment accumulation due to bathymetric sheltering of the adjacent coast from waves and convergence of littoral cells exposed to varying wave direction. The uppermost gullies of Mattole Canyon extend to the littoral cell, and a train of sediment waves within the uppermost thalweg of Mattole Canyon suggests that gravity-driven flows of shore-derived sediment may have recently occurred. Mattole Canyon and nearby Mendocino Canyon flow into the deep Mendocino Channel, which has the highest observed Late Holocene sediment accumulation rates in Northern California. We documented the beach slope, grain size, and sediment provenance along 15 km of coast adjacent to Mattole Canyon and conducted numerical wave modeling to infer the dominant direction and magnitude of sediment transport, relative wave sheltering by bathymetry, and estimated sediment mobility in the vicinity of the canyon head over a multi-year period. South of the canyon headwall, in the vicinity of the Mattole River outlet, coarser sediment rich in metasandstone clasts and steeper beach slopes coincide with decadal-scale beach accretion. North of the canyon head, sediments are generally finer, beaches are flatter, clast lithologies include more quartz and mudstone grains, and there is decadal-scale net erosion. Wave modeling suggests the Mattole headwall region is subject to sustained sheltering from waves due to canyon bathymetry, littoral sediment convergence above the canyon head for much of a typical year, and occasional bed entrainment of sediment in the canyon's uppermost gullies by large waves. Larger winter waves from the northwest likely result in net southerly drift north of the canyon head, with a high likelihood of preferentially transporting fine-grained, more quartz-rich sediment towards the Mattole headwall. Smaller summer swells from the west and south likely results in net northerly transport of metasandstone clast-rich Mattole River-derived sediment towards the canyon head. The imbalance in seasonality in wave conditions, coast shape, and sediment delivery by the Mattole River and other coastal creeks is broadly consistent with the spatial patterns in grain size, mineralogy, slope, and beach width that we observe in the vicinity of the canyon, and the net delivery of sediment to the Mattole Canyon head. An approximate mass balance of sediment flux from coastal streams feeding Mattole River littoral cell suggests that sediment volumes likely accumulate in the canyon headwall region that could supply several density flow events in a typical decade. These findings highlight the importance of the connectivity of the canyon to the nearshore region for the processes and products in canyons and fans. We hypothesize that feedbacks between canyon bathymetry, sediment accumulation, and mass evacuating flows promotes the long-term persistence of canyon systems. [ABSTRACT FROM AUTHOR]

The northern California littoral cell of the Klamath River, which is a mixed rocky and sandy system with significant shoreline curvature, was investigated by examining ∼40 yr of satellite‐derived shoreline positions and historical records. We find that an accretion wave of sediment was initiated near the Klamath River mouth in the late 1980s and translated downcoast over the subsequent decades. The wave passed rapidly (∼2,500 m/yr) through a rocky coastal reach with more oblique wave directions and slowly through a sandy reach (∼200 m/yr) where wave crests approach at more normal angles. Within the sandy reach, the accretion wave extended over 200 m offshore, was ∼10 km long, incorporated 20 ± 6 million m3 of sediment, and averaged 1.3 ± 0.4 million m3/yr of longshore sediment transport over a 20‐yr interval. Diffusion of the accretion wave was observed, but the diffusivity coefficient (εobs ∼0.01 m2/s) was lower than values predicted by theory, which we attribute to net sediment transport convergence in the study area caused by the curvature of the shoreline. Examining historical records, we find that increased sediment discharge in the Klamath River occurred during the 20th century from industrial‐scale logging and climatic extremes. Thus, we hypothesize that increased river sediment discharge introduced new sediment to the littoral cell that initiated the observed accretion wave. These hypotheses can be tested with stratigraphic and mineralogic investigations of the broad study area beach that has formed during the past 150 years. Plain Language Summary: The existence of beaches along the world's coasts is dependent on the availability of sand or other sediment sources that build up these landforms. As sediment sources come and go over time, beaches may accrete or erode in response to these changes. These kinds of shoreline change patterns can be measured using new satellite‐based techniques that are especially valuable for coasts with limited ground‐based surveys. We applied these satellite‐based shoreline techniques to a portion of the northern California coast, which revealed that a massive body of sediment appeared on this coast in the 1980s, and that this sediment moved downcoast over the subsequent decades. The sediment stayed in a relatively continuous body—termed a coastal accretion wave—that extended 100's of meters offshore and several kilometers alongshore. The satellite measurements also allowed us to track the evolution of the shape and speed of this accretion wave and compare these results with physical conditions and sediment movement theory. Comparing with historical measurements, maps, and accounts, we hypothesize that sediment from the Klamath River has been a primary source of sediment to this coast, including the formation of the accretion wave and study area's broad beach. Key Points: An accretion wave of ∼20 million m3 of littoral sediment formed in the late 1980s and translated ∼15‐km downcoast over the next 30+ yearsAlongshore diffusion of the wave was less than predicted by theory, which is attributed to transport convergence from the shoreline curvatureIncreases in river sediment discharge from human impacts and climate extremes may have initiated the accretion wave [ABSTRACT FROM AUTHOR]

Wildfire risk has increased in recent decades over many regions, due to warming climate and other factors. Increased sediment export from recently burned landscapes can jeopardize downstream infrastructure and water resources, but physical landscape response to fire has not been quantified for some at‐risk areas, including much of northern California, USA. We measured sediment yield from three watersheds (13–29 km2) that drain to Whiskeytown Lake, California, within the area burned by the 2018 Carr Fire. Structure‐from‐Motion photogrammetry on aerial images combined with sonar bathymetric mapping of submerged areas indicated first‐year post‐fire sediment yields of 4,080 ± 598 t/km2 (Brandy Creek), 2,700 ± 527 t/km2 (Boulder Creek), and 305 ± 58.0 t/km2 (Whiskey Creek)—some of the first post‐fire yields measured in northern California and 64, 42, and 4.8 times greater than pre‐fire yields, respectively. These were measured during a wet year and resulted largely from rilling erosion and fluvial sediment transport, without post‐fire debris flows. Rilling preferentially developed in contact with dirt roads, aided by thin soils and exposed bedrock, and on slopes vegetated by chaparral pre‐fire. The second post‐fire year (a dry year) was characterized by fluvial reworking and delta progradation of the first‐year deposits and relatively little new sediment export. First‐year sedimentation of 111,000 m3 represented minor loss of storage capacity in Whiskeytown Lake but would be detrimental to smaller reservoirs; in general, increased sediment yields from western US watersheds as fire and extreme rainfall increase will likely pose risks to water quality and storage. Plain Language Summary: Climate change is increasing wildfires across many regions, including California. Burned landscapes typically produce large amounts of sediment after a fire, which reduces water quality, decreases space for water storage in reservoirs, and sometimes produces hazardous debris flows. We investigated landscape response after the 2018 Carr Fire in northern California. Sediment yield measured in the first year after the fire (a wet year) varied among three study watersheds, ranging from one to two orders of magnitude greater than before the fire. The primary means of sediment mobilization was by water flowing over the land surface, instead of by landslides. These results will aid prediction of future fire response elsewhere in northern California, a growing research need given the increasing tendency for large fires to affect northern California. Key Points: Aerial photogrammetric and sonar bathymetric mapping quantify sediment volume change for 2 years after the major 2018 Carr Fire, CaliforniaFirst‐year post‐fire sediment yields in three watersheds were 64, 42, and 4.8 times greater than long‐term yields, some of the first post‐fire data from northern CaliforniaHillslope erosion was dominated by rilling rather than landslides; rilling commonly occurred downslope of dirt roads [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]

Landscapes following wildfire commonly have significant increases in sediment yield and debris flows that pose major hazards and are difficult to predict. Ultimately, post-wildfire sediment yield is governed by processes that deliver sediment from hillslopes to channels, but it is commonly unclear the degree to which hillslope sediment delivery is driven by wet versus dry processes, which limits the ability to predict debris-flow occurrence and response to climate change. Here we use repeat airborne lidar topography to track sediment movement following the 2009 CE Station Fire in southern California, USA, and show that post-wildfire debris flows initiated in channels filled by dry sediment transport, rather than on hillsides during rainfall as typically assumed. We found widespread patterns of 1-3 m of dry sediment loading in headwater channels immediately following wildfire and before rainfall, followed by sediment excavation during subsequent storms. In catchments where post-wildfire dry sediment loading was absent, possibly due to differences in lithology, channel scour during storms did not occur. Our results support a fire-flood model in bedrock landscapes whereby debris-flow occurrence depends on dry sediment loading rather than hillslope-runoff erosion, shallow landslides, or burn severity, indicating that sediment supply can limit debris-flow occurrence in bedrock landscapes with more-frequent fires. [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]

Particle size spectra of suspended sediment eroded from the Santa Ana River, a human-impacted urban river in southern California are investigated to consider fractal behaviors in suspended sediment transport. Widespread urbanization has fueled hydrological change of the watershed over four decades, showing significant increases in storm water runoff with decreases in instantaneous suspended sediment concentration. In-site observation of particle size distributions (PSDs) during three storm studies reveals two transport regimes (flow-controlled or bedcontrolled) depending on whether average particle size increase (flow-controlled) or decrease (bed-controlled) with flow rate. Despite their complexity, the observed PSDs exhibit power-law mass-size statistics, and satisfy fractal (power-law) scaling in a very robust manner. The evolution of PSD to a stable form with asymptotic behavior in suspended sediments suggests that shedding of suspended sediments in urban watershed environment system has its own selforganized characteristics wherever the sediment source come from. [ABSTRACT FROM AUTHOR]

Abstract: The fate and transport of mercury are of critical concern in lowland floodplains and wetlands worldwide, especially those with a history of upstream mining that increases the mobility of both dissolved and sediment-bound Hg in watersheds. A mass budget of total mercury (THg) quantifies sources and storage for particular areas — knowledge that is required for understanding of management options in lowland floodplains. In order to assess contaminant risk in the largest flood-control bypass, prime wetland, and restoration target in the Sacramento River basin, we estimated empirical relationships between THg, suspended sediment concentration (SSC), and streamflow (Q) for each of the major inputs and outputs using data from various publicly available sources. These relationships were improved by incorporating statistical representations of the dynamics of seasonal and intra-flood exhaustion (hysteresis) of sediment and mercury. Using continuous records of Q to estimate SSC suspended sediment flux and SSC to estimate THg flux, we computed the net transfer of sediment-adsorbed mercury through the Yolo Bypass over a decade, 1993–2003. Flood control weirs spilling Sacramento River floodwaters into the bypass deliver ~75% of the water and ~50% of the river''s suspended sediment load, while one Coast Range tributary of the bypass, Cache Creek, contributes twice the THg load of the mainstem Sacramento. Although estimated sediment flux entering Yolo Bypass is balanced by efflux to the Sacramento/San Francisco Bay-Delta, there is much evidence of deposition and remobilization of sediment in Yolo Bypass during flooding. These factors point to the importance of the bypass as sedimentary reservoir and as an evolving substrate for biogeochemical processing of heavy metals. The estimates of mercury flux suggest net deposition of ~500kg in the 24,000ha floodway over a decade, dominated by two large floods, representing a storage reservoir for this important contaminant. [Copyright &y& Elsevier]

ABSTRACT The dynamics of sediment transport capacity in gravel-bed rivers is critical to understanding the formation and preservation of fluvial landforms and formulating sediment-routing models in drainage systems. We examine transport-storage relations during cycles of aggradation and degradation by augmenting observations of three events of channel aggradation and degradation in Cuneo Creek, a steep (3%) gravel-bed channel in northern California, with measurements from a series of flume runs modeling those events. An armored, single-thread channel was formed before feed rates were increased in each aggradation run. Output rates increased as the channel became finer and later widened, steepened, and braided. After feed rates were cut, output rates remained high or increased in early stages of degradation as the incising channel remained fine-grained, and later decreased as armoring intensified. If equilibrium was not reached before sediment feed rate was cut, then a rapid transition from a braided channel to a single-thread channel caused output rates for a given storage volume to be higher during degradation than during aggradation. Variations in channel morphology, and surface bed texture during runs that modeled the three cycles of aggradation and degradation were similar to those observed in Cuneo Creek and provide confidence in interpretations of the history of change: Cuneo Creek aggraded rapidly as it widened, shallowed, and braided, then degraded rapidly before armoring stabilized the channel. Such morphology-driven changes in transport capacity may explain the formation of flood terraces in proximal channels. Transport-storage relations can be expected to vary between aggradation and degradation and be influenced by channel conditions at the onset of changes in sediment supply. Published in 2011. This article is a US Government work and is in the public domain in the USA. [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]

Pool-riffle dynamics is governed by complex time and spatial interactions between water and sediment flows. In the last few decades, significant advances have been made in characterizing and modeling the hydrodynamics of pool-riffle sequences, and this information has been extensively used as the basis of conceptual models to describe or infer pool-riffle morphodynamics. A lot less attention, however, has been paid to the coupled dynamics of flow and sediment, which is essential to fully understand these complex geomorphic systems. This paper uses an unsteady 1-D flow-morphology and bed-sorting model to analyze pool-riffle dynamics. The model is first applied to a pool-riffle sequence on a 1.1 km reach of the lower Bear Creek, Arkansas, United States. After showing the model's ability to describe the general reach hydrodynamics and morphological evolution over 1 year, the detailed sediment and flow information is used to investigate pool-riffle dynamics in terms of self-maintenance mechanisms. Two effects that have been only marginally explored in the past, i.e., bed sediment sorting and downstream riffle control, are explained and quantified with the help of the model's outputs. The results show that self-maintenance occurs more frequently than previously thought as a result of grain sorting and that erosion or deposition of contiguous riffles also constitutes a self-maintenance mechanism. These findings provide the support for a physically based, integral description of pool-riffle morphodynamics and highlight the importance of flow and sediment variability on pool-riffle self-maintenance. The morphodynamic analysis bridges the gap between observations and current theories based mainly on hydrodynamic information. [ABSTRACT FROM AUTHOR]

Abstract: Vertical sediment fluxes and their dominant controlling processes in Hueneme and Mugu submarine canyons off south-central California were assessed using data from sediment traps and current meters on two moorings that were deployed for 6months during the winter of 2007. The maxima of total particulate flux, which reached as high as 300+g/m2/day in Hueneme Canyon, were recorded during winter storm events when high waves and river floods often coincided. During these winter storms, wave-induced resuspension of shelf sediment was a major source for the elevated sediment fluxes. Canyon rim morphology, rather than physical proximity to an adjacent river mouth, appeared to control the magnitude of sediment fluxes in these two submarine canyon systems. Episodic turbidity currents and internal bores enhanced sediment fluxes, particularly in the lower sediment traps positioned 30m above the canyon floor. Lower excess 210Pb activities measured in the sediment samples collected during periods of peak total particulate flux further substantiate that reworked shelf-, rather than newly introduced river-borne, sediments supply most of the material entering these canyons during storms. [Copyright &y& Elsevier]

A newly developed watershed environmental hydrology (WEHY) model is presented as a state-of-the-art nonpoint source (NPS) model. The model consists of hydrologic and environmental modules, and describes environmentally relevant hydrologic processes based upon physically based governing equations to model the fate of pollutants such as sediment and phosphorus in the watershed. Unlike other physically based NPS models, the WEHY model is unique in its upscaling approach to the governing equations of hydrologic and environmental processes, which results in the governing equations that are compatible with the computational grid resolution while accounting for subgrid heterogeneities through upscaled model parameters. Upscaling was performed by means of a technique called ensemble averaging. The model was tested at the Ward Creek Watershed in Lake Tahoe Basin for its performance in a subalpine watershed setting. Comparisons of predicted and observed values were in good agreement and showed good promise of the approach used in the development of the model. Because of the physical basis of the WEHY model and its use of upscaled conservation equations, the model has the advantage of being applicable to ungauged basins and to large watersheds. [ABSTRACT FROM AUTHOR]

Detailed sampling of the axis and flanks of upper Monterey Canyon (water depths of «1500 m) was undertaken using a remotely operated vehicle (ROV)-deployed vibracoring system. The objective was to document the characteristics and distribution of sediment within the canyon and to elucidate the sources and processes by which materials move into and through the canyon. Detailed multibeam bathymetric surveys guided the sampling. The combination of core data, multi-beam bathymetry, and multi-beam reflectivity reveals for the first time the facies and facies distribution patterns associated with the axial channel of an active submarine canyon. Coarse-grained deposits form a narrow trail of material that is tightly restricted to the axial channel floor, indicating that upper Monterey Canyon is an active submarine system closely coupled to sand transport along the shoreline. Sand from the modern beach and nearshore environment captured by the canyon head is moving down the axial channel of the canyon, while fine-grained sediments are accumulating on the flanks of the canyon. Our observations suggest that other submarine canyons in close proximity to the shoreline are presently active and that a modern supply of sand-sized material is moving down these canyon systems. [ABSTRACT FROM AUTHOR]

ABSTRACT The vertical and horizontal distributions of aeolian mass flux were measured at Oceano Dunes, California, and these data were used to evaluate a numerical model of saltation. Grain-size analyses showed that the distributions of the modal sediment size class corresponded closely to those of the total sediment population, and modelling thus focused on replicating the distributions of the mean grain size. Although much previous work has assumed that the mean launch speed of saltating particles varies in proportion to shear velocity, simulations using a constant mean launch speed were found to yield the closest approximations to the mass flux distributions observed in the field. Both exponential and gamma distributions of launch velocity produced realistic simulations, although the latter approach required the inclusion of an additional reptation component to achieve good results. A range of mean launch angles and an equivalent sphere correction were also found to generate comparable results, providing the other input parameters could be varied freely. All the modelling approaches overestimated the proportion of mass flux occurring at the bottom of the vertical distributions, and underestimated the proportion occurring at the upwind end of the horizontal distributions. No theoretical shortcoming that would account for these small, but systematic, discrepancies could be identified, and experimental error thus represents a more plausible explanation. The conclusion that mean grain launch speeds are essentially constant and independent of shear velocity suggests that the additional kinetic energy extracted by grains under more energetic wind conditions is largely transferred to the bed, and that increases in the transport rate are therefore driven primarily by the ejection of additional grains. It is suggested that the kinetic energy of rebounding grains is constrained by the ability of the bed to resist deformation, equivalent to a plastic limit. Hence, grains of larger mass (diameter) rebound from the bed at lower speeds, and follow shorter, lower trajectories, as has been widely observed previously. [ABSTRACT FROM AUTHOR]

An anomalous increase of pore volume with increasing depth in the range 0-1,900 ft. Occurs in fine-grained sediments along the east side of the San Joaquin Valley of California. Several possible causes for the anomaly were inferred from a literature search and from study of the core samples. Statistical analyses of the core sample data suggest the principle causes to be variations in particle size, the diatom-skeleton content, and the type of exchangeable cation adsorbed by the clay-mineral constituents of the sediments. [ABSTRACT FROM AUTHOR]