USING EVIDENCE OF FIRES IN ALLUVIAL FAN STRATIGRAPHY TO INTERPRET VARIATIONS IN HOLOCENE FIRE REGIMES IN CENTRAL IDAHO

In western U.S. ponderosa pine forests, tree-ring studies show that frequent low-intensity fires characterized fire regimes before European settlement, but these records extend only through the past 500 yr. Recent large, intense fires have resulted in major debris flows, suggesting to some that this massive sedimentation is unprecedented and primarily a result of fire suppression. To test this hypothesis, we interpret and radiocarbon-date Holocene fire-related sedimentation preserved in tributary alluvial fans in the Payette-Boise River region of the western Idaho batholith. Fire events are marked by burned soil surfaces, and where well-preserved by overlying sediments, suggest sedimentation in response to that fire. Fire-related sedimentation events are also indicated by debris-flow and flood deposits with abundant coarse, angular charcoal. An early Holocene alluvial fan shows frequent small fire-induced sheetflood events (RI 33-80 yr) and low average sediment yields from 7400-6600 cal yr BP. Events of similar frequency are recorded between 1630 and 1550 cal yr BP in a nearby fan. Prior work in Yellowstone indicates that these times had few large, intense fires and were effectively wetter. In contrast, two sites 30 km apart both record larger fire-induced debris flows at ~ 930 cal yr BP, corresponding to the Medieval Warm Period, a time of major fire-related debris-flow activity in Yellowstone. Estimates of Holocene-average sediment yields based on cosmogenic nuclides also indicate that episodic large debris-flow events must be an important mode of sediment flux from small Idaho batholith watersheds (Kirchner et al., 2001). We hypothesize that such events are commonly induced by large, intense fires during unusually warm and drought-prone periods, whereas cooler, effectively wetter periods produce frequent but low-intensity fires carried by dense grasses, as during the Little Ice Age in the tree-ring record. Pending dates will aid in evaluating this premise.