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Effects of sediment pulses on channel morphology in a gravel-bed river

Daniel F. Hoffman^,1 and Emmanuel J. Gabet^,1

¹ Department of Geosciences, University of Montana, Missoula, Montana 59802, USA

Sediment delivery to stream channels in mountainous basins is strongly episodic, with large pulses of sediment typically delivered by infrequent landslides and debris flows. Identifying the role of large but rare sediment delivery events in the evolution of channel morphology and fluvial sediment transport is crucial to an understanding of the development of mountain basins. In July 2001, intense rainfall triggered numerous debris flows in a severely burnt watershed in the Sapphire Mountains of Montana. Ten large debris flow fans were deposited on the valley floor, and investigations focused on the channel response to these sediment pulses. The channel has aggraded immediately upstream of each fan, and braided in reaches immediately downstream. Channel incision through the fans has created sets of coarse-grained terraces. The deposition upstream of the pulses consists almost exclusively of fine material, resulting in a median bed material size (D₅₀) 1–2 orders of magnitude lower than the ambient channel material. The volume of sand being transported is so great that these aggrading reaches can extend hundreds of meters upstream of the fans, with 1–2 m of sand deposited across the entire valley floor. Along a 10 km study reach, cross section surveys, longitudinal profiles, and pebble counts chronicle channel response to a punctuated increase in sediment supply and provide insight on the processes of sediment wave dispersal.

Key Words: fluvial geomorphology • sediment • debris flows • bed load • fire