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

M Attal,S M Mudd,K Yoo,M Naylor,B Weinman,M D Hurst

doi:10.5194/esurf-3-201-2015

M Attal, S M Mudd + Show 4 more

Open Access

https://doi.org/10.5194/esurf-3-201-2015

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Abstract

Abstract. The characteristics of the sediment transported by rivers (e.g. sediment flux, grain size distribution – GSD) dictate whether rivers aggrade or erode their substrate. They also condition the architecture and properties of sedimentary successions in basins. In this study, we investigate the relationship between landscape steepness and the grain size of hillslope and fluvial sediments. The study area is located within the Feather River basin in northern California, and studied basins are underlain exclusively by tonalite lithology. Erosion rates in the study area vary over an order of magnitude, from >250 mm ka−1 in the Feather River canyon to <15 mm ka−1 on an adjacent low-relief plateau. We find that the coarseness of hillslope sediment increases with increasing hillslope steepness and erosion rates. We hypothesise that, in our soil samples, the measured 10-fold increase in D50 and doubling of the amount of fragments larger than 1 mm when slope increases from 0.38 to 0.83 m m−1 is due to a decrease in the residence time of rock fragments, causing particles to be exposed for shorter periods of time to processes that can reduce grain size. For slopes in excess of 0.7 m m−1, landslides and scree cones supply much coarser sediment to rivers, with D50 and D84 more than one order of magnitude larger than in soils. In the tributary basins of the Feather River, a prominent break in slope developed in response to the rapid incision of the Feather River. Downstream of the break in slope, fluvial sediment grain size increases, due to an increase in flow competence (mostly driven by channel steepening) as well as a change in sediment source and in sediment dynamics: on the plateau upstream of the break in slope, rivers transport easily mobilised fine-grained sediment derived exclusively from soils. Downstream of the break in slope, mass wasting processes supply a wide range of grain sizes that rivers entrain selectively, depending on the competence of their flow. Our results also suggest that, in this study site, hillslopes respond rapidly to an increase in the rate of base-level lowering compared to rivers.

Highlights

In the rock cycle, clastic sediment is produced in upland mountainous areas
Attal et al.: Impact of change in erosion rate and landscape steepness on hillslope in the Feather River basin ment flux and grain size distribution (GSD) affect the ability of rivers to erode their substrate in two ways: they control (i) the availability and effectiveness of tools for bedrock erosion and (ii) the extent of the protective alluvial cover that the rivers need to mobilise during floods for erosion to happen (e.g. Gilbert, 1877; Sklar and Dietrich, 2004; Cowie et al, 2008; Hobley et al, 2011)
We have quantified the grain size distribution of sediment in both source areas and channels in a mountainous landscape where the underlying lithology is exclusively tonalite and where erosion rates vary over an order of magnitude (Riebe et al, 2000; Hurst et al, 2012)

Summary

Introduction

Clastic sediment is produced in upland mountainous areas. The type of sediment delivered from hillslopes to the fluvial system conditions the characteristics of the sediment that is transported by rivers and exported from mountain ranges to sedimentary basins (Knighton, 1982; Parker, 1991; Heller et al, 2001; Attal andLavé, 2006; Sklar et al, 2006; Chatanantavet et al, 2010; Whittaker et al, 2010; Bennett et al, 2014; Michael et al, 2014). Gilbert, 1877; Sklar and Dietrich, 2004; Cowie et al, 2008; Hobley et al, 2011) They control the architecture and properties of the stratigraphic successions in sedimentary basins, because the distance travelled by sediment particles before being deposited is dictated primarily by their grain size Fluvial sediment flux and GSD condition whether a river aggrades or incises, both in upland areas and throughout sedimentary basins This point is of particular relevance when considering the impact of climate change and land use on river dynamics and human infrastructures within river basins, since both changing climate and land use modify sediment and water fluxes from hillslopes to rivers, with a potentially negative impact on the capacity of rivers to hold water within their channels (Lane et al, 2007)

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