Abstract

We have investigated factors controlling the surface morphology of debris-flow fans comprising the bajada along the western slope of Owens Valley, California. These fans have average slopes of 4°; an extensive network of abandoned, boulder-lined channels; rough, undulatory surfaces near the range front; and smooth distal surfaces. Field relationships and mechanical considerations indicate that the channels of the bajada are products of fluvial incision and not debris-flow scour. This is significant because detailed geomorphic maps indicate that the channels strongly influence the pattern of debris-flow deposition. The locus of debris-flow deposition on a channelized fan surface is set by the interaction of debris flows with the channel system and is controlled by channel size, channel gradient, flow volume, flow hydrograph, and flow rheology. Debris-flow behavior is most directly controlled by variations in bulk-sediment concentration and its influence on flow rheology. Whereas low-sediment-concentration debris flows tend to smooth the surface of the lower fan, spreading into thin sheets and filling channels and surface undulations, repeated deposition of high-sediment-concentration debris flows produces the rugged topography of the upper fan. The texture of the fan surface, rough or smooth, is determined by the relative volumetric importance of these two types of debris flow. In addition, channel avulsions and the associated long-term shifting of depositional loci are driven by in-channel deposition of debris flows with the highest sediment concentrations. These debris flows, therefore, play a critical role in determining both the structure of the channel network and the long-term pattern of deposition on the fan surface as a whole. We infer that the frequency distribution of debris-flow rheologies, set by source-terrain geology and hydrology, is an important control on fan morphology.

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