RECORDS OF FIRE AND GEOMORPHIC RESPONSE IN ALLUVIAL SEDIMENTS

MEYER, G.A. (1) AND PIERCE, J.L. (2).

(1) Department of Earth and Planetary Science, University of New Mexico, Albuquerque, New Mexico 87131, (2) Department of Geosciences, Idaho State University Pocatello, Idaho 83209.

Fire can dramatically increase runoff in mountain landscapes, as well as destabilize slope sediments by loss of root strength, such that debris flows and sediment-charged floods commonly result. Small alluvial fans at tributary junctions and along broad valley floors receive charcoal-laden deposits, providing a datable stratigraphic record of fire and geomorphic response in basins of up to ~1000 ha. Valley-fill alluvium proximal to burned slopes also preserves interpretable records. A fire-related sedimentation record involves the combined probabilities of fire extent and severity with precipitation event probability (most importantly intensity). In general, the greater the fire severity, the more likely the response, and the larger the magnitude of erosion and deposition. Charcoal often remains suspended in high-energy streamflows while higher density particles drop out, so that resulting fan deposits are not identifiable as fire-related. Commonly only part of a fan experiences deposition in a single event. Together, these processes imply that no single stratigraphic section or fan provides a complete record of fire in a small basin, and that low-severity fires in particular will be incompletely represented. A large sample of events (50-100+), however, provides a statistical basis for interpreting the timing, relative frequency, and general severity of fires in an area (but also requires considerable field work and 14C-dating funds). Modern events provide analogous sediments for interpreting past fire-related deposits, and verification of more recent events may be possible via tree-ring fire records, although large uncertainties in 14C calibration over the last ~450 yr are problematic. Overall, alluvial sediments provide valuable multi-millennial archives of fire, especially in landscapes where lake sediment charcoal records are absent.