Impact of change in erosion rate and landscape steepness on hillslope and fluvial sediments grain size in the Feather River basin (Sierra Nevada, California)

摘要

The characteristics of the sediment transported by rivers (e.g. sedimentflux, grain size distribution – GSD) dictate whether rivers aggrade or erodetheir substrate. They also condition the architecture and properties ofsedimentary successions in basins. In this study, we investigate therelationship between landscape steepness and the grain size of hillslope andfluvial sediments. The study area is located within the Feather River basinin northern California, and studied basins are underlain exclusively bytonalite lithology. Erosion rates in the study area vary over an order ofmagnitude, from >250 mm ka?1 in the Feather River canyon to<15 mm ka?1 on an adjacent low-relief plateau. We find that thecoarseness of hillslope sediment increases with increasing hillslopesteepness and erosion rates. We hypothesise that, in our soil samples, themeasured 10-fold increase in D₅₀ and doubling of the amount offragments larger than 1 mm when slope increases from 0.38 to 0.83 m m?1 is dueto a decrease in the residence time of rock fragments, causing particles tobe exposed for shorter periods of time to processes that can reduce grainsize. For slopes in excess of 0.7 m m?1, landslides and scree cones supplymuch coarser sediment to rivers, with D₅₀ and D₈₄ more than oneorder of magnitude larger than in soils. In the tributary basins of theFeather River, a prominent break in slope developed in response to the rapidincision of the Feather River. Downstream of the break in slope, fluvialsediment grain size increases, due to an increase in flow competence (mostlydriven by channel steepening) as well as a change in sediment source and insediment dynamics: on the plateau upstream of the break in slope, riverstransport easily mobilised fine-grained sediment derived exclusively fromsoils. Downstream of the break in slope, mass wasting processes supply awide range of grain sizes that rivers entrain selectively, depending on thecompetence of their flow. Our results also suggest that, in this study site,hillslopes respond rapidly to an increase in the rate of base-level loweringcompared to rivers.