Abstract We used lidar differencing and field observations to map volumes, and interpret the origins of, sediment mobilized from mountain canyons by large post‐wildfire debris flows near Montecito, CA, USA in 2018. The debris flows progressively entrained and partially redeposited 550,000 m 3 of previously stored channel sediments throughout the canyon networks. The observations that scour depths and volumes were highest where the largest volumes of bouldery colluvium and debris‐flow deposits had accumulated, and that scour persisted beyond the mountain front, indicates that debris‐flow volumes in this extreme event were ultimately controlled by the coarse sediment reservoir available for scour. Because the volumes of available stored sediment result from the stochastic interaction of colluvial mass wasting, the magnitude and frequency of previous debris flows, and the accommodation space provided by valley morphology, the study reinforces the importance of estimating stored sediment volumes when developing debris‐flow hazard assessments.
Temporary storage of sediment within alluvial valley floors modulates the long‐term transport of sediment through landscapes. The fate of weathering minerals or sediment‐bound constituents in fluvial environments depends on the relative time scales of constituent degradation and particle residence time within valleys. Particles follow a set of trajectories through valley floors: some particles pass directly through the channel, reaching the basin outlet rapidly after being introduced to the fluvial system; others remain for long periods in deposits such as flood plains. Traditional sediment routing theory, based on the principle of sediment mass conservation along reaches of channel, does not account for exchanges of sediment with temporary sediment storage reservoirs outside the channel, such as flood plains, deltas, and alluvial fans. This article formalizes a theory that incorporates the role of such exchanges in the migration of sediment through river systems, by computing the probabilistic structure of particle trajectories through alluvial valley floors. Equations are developed for computing these trajectories from the sediment budget of a valley floor in steady state. Mathematical strategies for using such relationships to model transient storage conditions are proposed, and other potential model enhancements are discussed. The approach is illustrated using a hypothetical valley floor as an example. The theory can be used to examine rates of sediment overturn in valleys, map particle residence times, and account for the redistribution and decomposition of weathering minerals and particle‐bound constituents. The theory has numerous potential management applications, some of which are discussed herein. The hypothetical example demonstrates that the probability distribution of particle residence times in the valleys of most alluvial rivers should be strongly right skewed.
The boundary shear stress pattern and the superelevation of the water surface in a meander on a small stream are predicted from two simple equations representing a frictionally dominated force balance. This comes about even though inertial forces due to local bed topography delay crossing of the boundary shear stress maximum to a position farther downstream than would otherwise be expected. Measured bedform migration rates reflect the boundary shear stress field, and in this river, bedform crest orientations respond to gradients in the shear stress field by becoming oblique to the general flow direction. When so aligned the bedforms interact with the helical flow in the curved channel and induce a near bottom secondary current which, in the upstream part of the bend, is outward above the crests and inward along the troughs. At the downstream end of the bend this pattern is reversed and significant quantities of sediment are transported along troughs from one point bar to the next. This near bottom flow pattern imposes a cross-isobath, zig-zag trajectory on the sediment grains and sorts the bed material. Further, it provides a realizable mechanism for maintenance of equilibrium channel geometry in streams with bedforms.
Increased energy demand has led to plans for building many new dams in the western Amazon, mostly in the Andean region. Historical data and mechanistic scenarios are used to examine potential impacts above and below six of the largest dams planned for the region, including reductions in downstream sediment and nutrient supplies, changes in downstream flood pulse, changes in upstream and downstream fish yields, reservoir siltation, greenhouse gas emissions and mercury contamination. Together, these six dams are predicted to reduce the supply of sediments, phosphorus and nitrogen from the Andean region by 69, 67 and 57% and to the entire Amazon basin by 64, 51 and 23%, respectively. These large reductions in sediment and nutrient supplies will have major impacts on channel geomorphology, floodplain fertility and aquatic productivity. These effects will be greatest near the dams and extend to the lowland floodplains. Attenuation of the downstream flood pulse is expected to alter the survival, phenology and growth of floodplain vegetation and reduce fish yields below the dams. Reservoir filling times due to siltation are predicted to vary from 106–6240 years, affecting the storage performance of some dams. Total CO2 equivalent carbon emission from 4 Andean dams was expected to average 10 Tg y-1 during the first 30 years of operation, resulting in a MegaWatt weighted Carbon Emission Factor of 0.139 tons C MWhr-1. Mercury contamination in fish and local human populations is expected to increase both above and below the dams creating significant health risks. Reservoir fish yields will compensate some downstream losses, but increased mercury contamination could offset these benefits.
Abstract Geomorphic and hydraulic processes, which form gravel bars in large lowland rivers, have distinctive characteristics that control the magnitude and spatial patterns of infiltration and exfiltration between rivers and their immediate subsurface environments. We present a bedform‐infiltration relation together with a set of field measurements along two reaches of the San Joaquin River, CA to illustrate the conditions required for infiltration and exfiltration of flow between a stream and its undulating bed, and a numerical model to investigate the factors that affect paths and residence times of flow through barforms at different discharges. It is shown that asymmetry of bar morphology is a first‐order control on the extent and location of infiltration, which would otherwise produce equal areas of infiltration and exfiltration under the assumption of sinusoidal bedforms. Hydraulic conductivity varies by orders of magnitude due to fine sediment accumulation and downstream coarsening related to the process of bar evolution. This systematic variability not only controls the magnitude of infiltration, but also the residence time of flow through the bed. The lowest hydraulic conductivity along the reach occurred where the difference between the topographic gradient and the water‐surface gradient is at a maximum and thus where infiltration would be greatest into a homogeneous bar, indicating the importance of managing sand supply to maintain the ventilation and flow through salmon spawning riffles. Numerical simulations corroborate our interpretation that infiltration patterns and rates are controlled by distinctive features of bar morphology.
The sediment budget and geomorphological evolution of large rivers are affected by continental-scale crustal dynamics and climate acting over long time scales through their effect on hydrology, hydraulics and sediment transport. Unraveling these effects requires tools and information from a number of geosciences. The Amazon River system receives 2-3 billion tonnes of sediment per year from the Andes and disperses it into three large depocenters: the foreland basin east of the range; the central Amazon trough; and the Atlantic coastal estuary and continental shelf. The processes of sediment transport and channel-floodplain exchange vary with discharge and along the valley as channel gradient and confinement change in response to recent crustal deformation. We have quantified the processes involved in this dispersal and the resulting floodplain geomorphology along several reaches from the mountain range to the estuary through several periods of the basin's geologic history. There appears to have been a puzzling severalfold increase in sediment supply from the Andes during recent millennia.
