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

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.

River profile response to normal fault growth and linkage: an example from the Hellenic forearc of south-central Crete, Greece

“…7). We again acknowledge that in transient landscapes rivers not only adjust to changes in rock uplift rate through river channel steepening but also through reduction in channel width in concert with spatial and temporal changes in sediment flux and grain size (Sklar and Dietrich, 2004;Finnegan et al, 2005Finnegan et al, , 2007Amos and Burbank, 2007;Whittaker et al, 2007;Cook et al, 2013;Attal et al, 2015;Shobe et al, 2016). However, it is beyond the scope of this study to assess the impact of this variable on river profile analysis.…”

“…Numerous studies emphasize the important role that sediment plays in river incision processes (Sklar and Dietrich, 2004;Finnegan et al, 2007;Whittaker et al, 2007;Cook et al, 2013;Attal et al, 2015;Shobe et al, 2016). Theoretical models developed to describe this process typically only describe the flux of sediment moving through the system, ignoring the potentially important effects of grain size on incision process (Tucker and Whipple, 2002;Tucker, 2004;Gasparini et al, 2006).…”

“…These studies illustrate a systematic and monotonic increase in erosion rate with metrics based on stream power or shear-stress models or river incision, but it is important to recognize that many different river incision models (detachment-limited, transport-limited, or sediment-fluxdependent) make similar predictions of river morphology and erosion rate at steady state (Tucker and Whipple, 2002;Tucker, 2004;Gasparini et al, 2006). However, studies of river incision in transient landscapes show a much more complicated story, where rivers exhibit changes in channel geometry, sediment flux, and grain size and the amount of bed cover and roughness that singularly or collectively may affect local rates of incision (Sklar and Dietrich, 2004;Finnegan et al, 2005Finnegan et al, , 2007Amos and Burbank, 2007;Whittaker et al, 2007;Cook et al, 2013;Attal et al, 2015;Shobe et al, 2016). These responses to transient river incision will affect the pace and style of landscape adjustment to a base-level fall event, as predicted by different classes of incision models (Tucker and Whipple, 2002;Tucker, 2004;Gasparini et al, 2006).…”

“…Several recent studies highlight observations of spatially heterogeneous changes in the flux and size of sediment delivered to the channel in transient landscapes that might be missed by simple expressions of sediment flux that are only scaled to local channel slope and drainage area, a proxy for discharge. Attal et al (2015) document that hillslope steepness and erosion rates are positively correlated to changes in sediment grain size on hillslopes and along the adjoining river channel bed in catchments undergoing transient adjustment. Shobe et al (2016) suggested that enhanced delivery of locally derived hillslope boulders to a channel can impact bed cover and shear stresses such that river incision can be slowed in steeper reaches, contrary to the assumptions of a straightforward stream power approach to determining incision rates.…”

Possible threshold controls on sediment grain properties of Peruvian coastal river basins

“…The size and shape of gravels bear crucial information about (i) the transport dynamics of mountain rivers (Hjulström, 1935;Shields, 1936;Blissenbach, 1952;Koiter et al, 2013;Duller et al, 2012;Attal et al, 2015), (ii) the mechanisms of sediment supply and provenance (Parker, 1991;Paola et al, 1992a, b;Attal and Lavé, 2006), and (iii) environmental conditions such as uplift and precipitation (Heller and Paola, 1992;Robinson and Slingerland, 1998;Foreman et al, 2012;Allen et al, 2013;Foreman, 2014). The mechanisms by which grain size and shape change from source to sink have often been studied with flume experiments (e.g.…”