Fire, storms, and erosional events in the Idaho batholith

In late December 1996, the South Fork Payette River basin in west-central Idaho experienced a prolonged storm that culminated on January 1. 1997, with intense rain on melting snow that triggered slide failures, producing debris flows and sediment-charged floods. Failures occurred in saturated, cohesionless. grussy colluvium derived from Weathered Idaho batholith granitic rocks. Many failures along the South Fork Payette River originated in ponderosa pine forests burned in the 1989 stand-replacing Lowman fire. An example is the 0.49 km(2) 'Jughead' Creek basin. where a single large colluvial failure produced almost 40% of the total volume eroded from the basin and generated a massive and rapid debris flow. Failures also occurred in steep, unburned. and unforested drainages such as Hopkins Creek. In this south-facing 0.58 km(2) basin, 15 colluvial hollows failed. but no single failure produced more than 10% of the total eroded volume. Sediment transport in Hopkins Creek occurred by prolonged sediment-charged sheetflooding. Despite vegetation differences, sediment yields from the geomorphically similar Hopkins Creek (similar to 42 000 Mg km(-2)) and Jughead Creek (similar to 44 000 Mg km(-2)) basins were quite similar. These 1997 erosion events are equivalent to several thousand years of sediment yield at low rates (2.7-30 Mg km(2) year(-1)) measured by short-term sediment trapping and gauging in Idaho batholith watersheds. If similar large events were solely responsible for sediment export. recurrence intervals (Rls) of several hundred years would account for higher sediment yields averaged over similar to 10(4) year from Idaho batholith watersheds. Dating of small fire-induced sheetflooding events in an early Holocene tributary junction fan of Jughead Creek indicates that frequent small sedimentation events (RI approximate to 33-80 year) occurred between 7400 and 6600 cal year BP. with an average yield not greatly exceeding 16 Mg km(-2) year(-1). Compared with the Holocene average, erosion rates during that 800 year period were unusually low, suggesting that sediment yields have not been constant over time, and that climatic variations and related fire regime changes may exert a strong influence on the probability of major erosional events, Copyright (C) 2001 John Wiley & Sons. Ltd.