Abstract

We document initiation and flow processes, deposit facies, and geomorphic effects of forest-fire-related sedimentation on small alluvial fans in Yellowstone National Park. Brief, intense convective-storm precipitation on steep basins burned in the 1988 fires produced sedimentation events involving a variety of depositional processes on fans. Over the course of all documented events, flows on fans progressed from higher to lower sediment concentration. Events were often dominated by either debris flows or relatively sediment-poor streamflow processes; in some events, however, flows ranging a over wide spectrum of sediment concentration produced significant fan deposits. Debris flows were generated by progressive sediment bulking involving pervasive surface runoff and rill erosion on steep upper basin slopes, followed by deep incision as flows progressed down channels. Debris-flow deposits show a marked decline in thickness and coarse gravel content downfan, often with extensive distal gravel-poor facies. We recognized a relatively minor percentage of noncohesive debris-flow and hyperconcentrated-flow facies, with sorting and stratification intermediate between muddy debris-flow and streamflow facies; these were deposited where dilute flows bulked with coarse sediment by eroding channel alluvium or earlier deposits of the event. Below incised fan channels, streamflows expanded as sheetfloods, which prograded lower fans with distally fining deposits. Basins > 3 km 2 typically produced streamflow events on fans, but sediment texture and availability on slopes and in channels are primary factors determining flow processes on fans of smaller basins. Burned soil surfaces provided abundant silt and clay for debris-flow generation, but because soil surface sediment was stripped and/or compacted over time, the lack of available fines resulted in dominance of streamflow processes in later events.

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