DROUGHT, FOREST FIRES, AND DEBRIS FLOWS: MODERN AND 800-1000 CAL YR BP

Recent stand-replacing fires in many forested regions of the western US correspond with a 20 ^th century increase in temperatures and drought severity. In addition to hazards presented by fires themselves, post-fire debris flows and floods pose significant hazards to people, structures, water supplies, and ecosystems. On intensely burned slopes, fire-related soil hydrophobicity and surface sealing reduce infiltration and increase overland flow during moderate storm events such as brief, intense summer thunderstorms. Runoff-generated debris flows and flash floods often ensue, and decay of tree roots may also lead to mass failures in burned areas. Warmer climates and associated droughts may also act to increase thunderstorm activity through amplified surface heating and convection. Thunderstorms provide both a source of ignition for fire from lightning strikes, and precipitation that generates hazardous postfire flows. These climate-driven relationships among drought, fire and geomorphic response exist on seasonal to multi-millennial timescales. Postfire deposits are preserved in alluvial fan stratigraphic sequences, which contain a record of the timing and magnitude of fire-related debris flows and floods over Holocene timescales. Radiocarbon-dated records of fire-related debris flows from central Idaho and Yellowstone National Park show a peak in fire-related debris flows between ~1000 and 800 cal yr BP. This interval corresponds with the Medieval Climatic Anomaly (~1050-650 cal yr BP), a time characterized by drought conditions throughout much of the western US. Comprehensive drought area indices for the western US indicate that ~810-775 cal yr BP is the most extreme period of multidecadal drought in the last 1200 years, with other significant (p < 0.05) multidecadal droughts centered on 1014, 916, and 697 cal yr BP (Cook et al., 2004). In central Idaho, a few massive debris flows between ~1000 and 800 cal yr BP account for over 25% of the total dated fan thickness. Over millennial timescales, large fire-related debris flows in central Idaho and Yellowstone are concentrated in periods of at least regional-scale warmth. With probable future warming and increasing drought, along with increased forest densities from fire suppression, fire-related geomorphic hazards in the West will likely also increase.