EPISODIC EROSION FORCED BY FIRE AND CLIMATE IN THE WESTERN UNITED STATES

Many studies have documented extreme transient increases in erosion rates following severe forest fires in mountain regions. Most often, erosion results from dramatic increases in surface runoff generation and soil erodibility, combined with smooth overland flow paths. With intense but not necessarily extreme rainfall, widespread runoff leads to rill and gully incision, sediment bulking and debris-flow transport. Postfire erosion also stems from loss of root strength leading to large colluvial failures and other mass movements such as dry ravel, depending on the climatic and geomorphic setting. Postfire mass failures in central Idaho yielded up to 44,000 Mg km^-2 from small basins, equivalent to several thousand years of sediment yield at low rates measured over a few decades in unburned Idaho watersheds (Kirchner et al. 2001).

Erosion driven by storm events is often modeled as a stochastic process, but drought and fire are not random; like storms, they are often strongly clustered in time. Despite contrasts in climate, forest types, and characteristic fire regimes, severe multidecadal droughts during the Medieval period 1050-650 cal yr BP (Cook et al. 2004) produced large fire-related debris flows in subalpine Yellowstone, drier ponderosa-mixed conifer forests in central Idaho, and mixed-conifer forests in the monsoonal climate of the Sacramento Mountains, New Mexico. In central Idaho, fire-related debris flow deposits from this episode make up ~25% of the sampled fan deposit thickness over the last 4000 yr. In all of these areas, very rapid fan aggradation ~5500-4000 cal yr BP implies the highest slope erosion rates following the Pleistocene-Holocene transition - but not entirely due to fire. A number of proxy records worldwide indicate severe droughts and rapid environmental change within the 5-4 ka period. By conservative estimate, fire-induced deposits make up ~30% of late-Holocene fan alluvium in Yellowstone, where extremely steep and erodible unvegetated volcaniclastic bedrock also contributes, and ~50% in central Idaho, where fire is key to mobilizing grussy colluvium. An unanswered question is whether long-term, quasi-permanent changes in fire regime (linked to climate change) also alter overall denudation rates (e.g., via debris-flow bedrock erosion), in addition to episodic short-term increases in stripping of weathered material.