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

¹ Earth Systems Institute, 1314 North East 43rd Street, Seattle, Washington 98105, USA

Large, infrequent fluxes of sediment to streams by mass wasting are intrinsic to the erosion regime of mountain drainage basins. To elucidate the role of mass wasting in the construction and evolution of steep land channel environments, it is crucial that we identify the processes involved and recognize their legacy on the valley floor. In the winter of 1996, nine storm-triggered debris flows carried 18 000 m³ of coarse debris into the upper reaches of the South Fork of Gate Creek (Oregon Cascade Range) during flood flow. Analysis of resulting channel morphologies and bed textures shows that the sediment moved downstream as a wave-like pulse or pulses, overwhelming the channel and causing it to braid, with flooding and alluvial deposition over the valley floor. Downstream progression of the sediment wave resulted in vertical accretion of the valley floor with sediment carried as bedload, the maximum depth of valley-floor burial being set by the amplitude of the wave. Passage of the wave left a channel incised to bedrock, inset between coarse-grained alluvial terraces. This study examines the genesis of these features at Gate Creek and points to such terraces as field indicators of massive episodic influxes of sediment and the associated formation of fluvially transported sediment waves.

Key Words: debris flow • fluvial transport • sediment supply • sediment transport • terraces

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