Vegetative and Climatic Controls on Holocene Wildfire and Erosion Recorded in Alluvial Fans of the Middle Fork Salmon River, Idaho

The Middle Fork Salmon River watershed spans high-elevation mixed-conifer forests to lowerelevation shrub-steppe. In recent decades, runoff from severely burned hillslopes has generated large debris flows in steep tributary drainages. These flows incised alluvial fans along the mainstem river, where charcoal-rich debris-flow and sheetflood deposits preserve a record of latest Pleistocene to Holocene fires and geomorphic response. Through deposit sedimentology and 14C dating of charcoal, we evaluate the processes and timing of fire-related sedimentation and the role of climate and vegetation change. Fire-related deposits compose ~66% of the total measured fan deposit thickness in more densely forested upper basins versus ~33% in shrubsteppe-dominated lower basins. Fires during the middle Holocene (~8000 5000 cal yr BP) mostly resulted in sheetflood deposition, similar to modern events in lower basins. Decreased vegetation density during this generally warmer and drier period likely resulted in lowerseverity fires and more frequent but smaller fire-related sedimentation events. In contrast, thick fire-related debris-flow deposits of latest Pleistocene-early Holocene (~13,500-8000 cal yr BP) and late Holocene (< 4000 cal yr BP) age are inferred to represent higher-severity fires, though data in the former period are limited. Widespread fires occurred in both upper and lower basins within the Medieval Climatic Anomaly (1050-650 cal yr BP) and the early Little Ice Age ca. 550 cal yr BP. We conclude that a generally cooler late Holocene climate and a shift to denser lodgepole pine forests in upper basins by ~2500 cal yr BP provided fuel for severe fires during episodic droughts.