Spatial and temporal variations in rainfall are hypothesized to influence landscape evolution through erosion and sediment transport by rivers. However, determining the relation between rainfall and river dynamics requires a greater understanding of the feedbacks between flooding and a river's capacity to transport sediment. We analyzed channel geometry and stream-flow records from 186 coarse-grained rivers across the United States. We found that channels adjust their shape so that floods slightly exceed the critical shear velocity needed to transport bed sediment, independently of climatic, tectonic, and bedrock controls. The distribution of fluid shear velocity associated with floods is universal, indicating that self-organization of near-critical channels filters the climate signal evident in discharge. This effect blunts the impact of extreme rainfall events on landscape evolution.
[1] Sediment transport is an intrinsically stochastic process, and measurement of bed load in the environment is further complicated by the unsteady nature of river flooding. Here we present a methodology for analyzing sediment tracer data with unsteady forcing. We define a dimensionless impulse by integrating the cumulative excess shear velocity for the duration of measurement, normalized by grain size. We analyze the dispersion of a plume of cobble tracers in a very flashy stream over two years. The mean and variance of transport distance collapse onto well-defined linear and power-law relations, respectively, when plotted against cumulative dimensionless impulse. Data suggest that the asymptotic limit of bed load tracer dispersion is superdiffusive, in line with a broad class of geophysical flows exhibiting strong directional asymmetry (advection), thin-tailed step lengths and heavy-tailed waiting times. The impulse framework justifies the use of quasi-steady flow approximations for long-term river evolution modeling.
Fine particle deposition and streambed clogging affect many ecological and biogeochemical processes, but little is known about the effects of groundwater flow into and out of rivers on clogging. We evaluated the effects of losing and gaining flow on the deposition of suspended kaolinite clay particles in a sand streambed and the resulting changes in rates and patterns of hyporheic exchange flux (HEF). Observations of clay deposition from the water column, clay accumulation in the streambed sediments, and water exchange with the bed demonstrated that clay deposition in the bed substantially reduced both HEF and the size of the hyporheic zone. Clay deposition and HEF were strongly coupled, leading to rapid clogging in areas of water and clay influx into the bed. Local clogging diverted exchanged water laterally, producing clay deposit layers that reduced vertical hyporheic flow and favored horizontal flow. Under gaining conditions, HEF was spatially constrained by upwelling water, which focused clay deposition in a small region on the upstream side of each bed form. Because the area of inflow into the bed was smallest under gaining conditions, local clogging required less clay mass under gaining conditions than neutral or losing conditions. These results indicate that losing and gaining flow conditions need to be considered in assessments of hyporheic exchange, fine particle dynamics in streams, and streambed clogging and restoration. Plain Language Summary Deposition and accumulation of excessive amounts of clay and silt isone of the common causes of degradation of river ecosystems. We conducted experiments to evaluate the effects of flow from the stream into the groundwater (losing stream) and from the groundwater into the stream (gaining stream) on the deposition of clay particles in a sand bed and the resulting changes in water exchange between the stream and the subsurface. We found that clay deposition substantially reduced water exchange due to clogging. Computer simulations of this process revealed that the locations of clogging are closely related to locations where water exchange occurred, but these patterns differed in losing and gaining streams. This type of clay accumulation influences water budgets in streams and reduces connectivity between streams, floodplains, and the underlying aquifers. Such a reduction in connectivity may negatively affect water resources, ecosystem functions, and river resilience. These results indicate that losing and gaining flow conditions in streams need to be considered in assessments of streambed clogging and river restoration.FOX ET AL. 4077
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