Sedimentary Characteristics of a Course Gravel-Bed River Choked with Fine Sediment, West Walker River, California

Systematic bed material measurements of 15 macro-bedforms in the West Walker River in eastern California demonstrate that the channel is enriched with fine sediment (\u3c 2 mm). Along 15 km of the stream system, bed material varies from sand to boulders in the upstream canyon exit from the Sierra Nevada, fines downstream in Antelope Valley to cobbles and fines. Fines become the dominant sedimentary fraction downstream to Smith and Mason Valleys. The stream network is heavily impacted by irrigation diversions and off-channel storage into Topaz Lake reservoir, and the manipulated hydrology affects the bed material distribution by reducing the river\u27s competence to transport coarser fractions, especially during high flows. As a result, the intervening agricultural valleys are aggradational. The river bed in Antelope Valley is heavily armored (1 or more orders of magnitude) in the macro-bedforms (bars, riffles and pools), but the abundance of fines does not only appear in the subsurface layers but also in forced and free patches within the surface layer. Furthermore, the New Year\u27s flood of 1997, the largest flood on the modern gauged record that occurred in the basin (estimated RI between 100-400 year) induced large amounts of fine sediment still deposited along the floodplains and large overbank bars. These deposits are only activated during bankfull and overbank flows and contribute additional fines to the stream system. Based on these results, we conclude the river bed is highly unstable and very active. While this is partially a result of a natural sedimentation process, the intensive anthropogenic water allocation does not allow the river to efficiently evacuate and propagate the sediment downstream, recover from the large-scale disturbance, and return to a quasi-equilibrium geomorphic state. Even in the minor human disturbance part of the upper basin, the river\u27s background amounts of fines are very large with repercussions for any decisions about river restoration of the lowland valleys