FIRE, CLIMATE, AND FANS

Small valley-side alluvial fans are ubiquitous in mountain environments and provide valuable event-stratigraphic records of erosion. Where vegetation is both dense enough to control erodibility and prone to burning, fire is a major catalyst of such events that is strongly linked to climate. Convective storms produce postfire debris flow-flood events due to widespread surface runoff, smooth flow paths, and erodible soil surfaces, thus rill and gully erosion and sediment bulking of flows. Sediment concentration and mass transport depends in part on the degree and spatial pattern of combustion of vegetation and the litter layer. Therefore, relatively thick charcoal-rich debris-flow deposits suggest high-severity stand-replacing fire, whereas thin charcoal-bearing sheetflood deposits are typical of mixed- to low-severity fires, albeit with large uncertainty for individual deposits. Postfire erosion also stems from loss of root strength leading to large colluvial failures and debris flows, which may dominate response where storm precipitation is less intense and infiltration rates are higher. The larger the drainage basin, the more likely that sediment and charcoal are reworked from older deposits, and the less likely that small fire-related flows will reach the fan.

We reconstructed Holocene fire and geomorphic history from alluvial fan deposits in high-elevation Yellowstone NP, drier ponderosa pine forests in central Idaho, and ponderosa-mixed-conifer forests in the Sacramento Mountains, southern New Mexico, where precipitation falls mostly during the summer monsoon. All of these environments saw rapid fan aggradation during Medieval time 1050-650 cal yr BP, but activity peaked during different multidecadal drought episodes. In central Idaho, fire-related debris flow deposits from 1000-800 cal yr BP constitute ~25% of the sampled fan thickness in the last 4000 yr. In both Yellowstone and the Sacramento Mountains, however, fans built most rapidly ~5500-4000 cal yr BP, concurrent with large paleofloods in the SW USA (Ely 1997). These and other records imply increased climatic variability in that interval, where anomalously wet decades allowed forest stands to become denser and more susceptible to severe fires in subsequent droughts. Increased storm intensity could also promote postfire and overall sediment transport to fans.