Effects of punctuated sediment supply on valley-floor landforms and sediment transport

Large, infrequent fluxes of sediment to streams by mass wasting are intrins ic to the erosion regime of mountain drainage basins. To elucidate the role of mass wasting in the construction and evolution of steep land channel en vironments, it is crucial that me identify the processes involved and recog nize their legacy on the valley floor. In the winter of 1996, nine storm-tr iggered debris flows carried similar to 18 000 m(3) of coarse debris into t he upper reaches of the South Fork of Gate Creek (Oregon Cascade Range) dur ing flood how. Analysis of resulting channel morphologies and bed textures shows that the sediment moved downstream as a wave-like pulse or pulses, ov erwhelming the channel and causing it to braid, with flooding and alluvial deposition over the valley poor. Downstream progression of the sediment wav e resulted in vertical accretion of the valley floor with sediment carried as bedload, the maximum depth of valley-poor burial being set by the amplit ude of the mwe. Passage of the wave left a channel incised to bedrock, inse t between coarse-grained alluvial terraces. This study examines the genesis of these features at Gate Creek and points to such terraces as held indica tors of massive episodic influxes of sediment and the associated formation of fluvially transported sediment waves.