FIRE FREQUENCY AND FIRE-RELATED DEPOSITION DURING THE HOLOCENE: A STUDY OF ALLUVIAL FANS IN THE MIDDLE FORK SALMON RIVER WATERSHED, IDAHO

Changes in climate influence the frequency and severity of wildfire and post-fire erosion, which have considerable effects on landscape morphology and aquatic habitat. The Middle Fork Salmon River (MFSR) Idaho encompasses a range of ecosystems including high elevation (~3,000 -1,700 m) subalpine pine and spruce forests, mid-elevation (2650 - 1130 m) montane Douglas-fir and ponderosa pine-dominated forests, and lower elevation (~1,800 - 900 m) sagebrush steppe. >40% of the watershed has burned in the last 30 years producing many large fire-related debris flows emanating from small tributary basins. During the last century, the two largest fire years in the MFSR occurred during significant droughts and resulted in 8 of 15 documented channel changing fire-related debris flows. However, longer-term (e.g. 10³-10⁴) relationships among fire, climate, and geomorphic response are poorly understood. The purpose of this study is to evaluate the natural range of variability of fire regimes and geomorphic response to fire across ecosystems of the MFSR. This study uses ¹⁴C dating of charcoal fragments and field stratigraphic relationships to reconstruct 1) fire histories and fire-related sedimentation events, and 2) variations in fire/climate interactions along ecological gradients. Deep (>2 m) recent incision into alluvial fans reveals debris flow deposits comprise >60% of the thickness of alluvial fans. ~75% of all sedimentation in the upper basin is fire related, while ~40% of deposits in the lower basin are fire related. Depositional processes in lower basin fans are difficult to link to fire due to decreased fuel diameter and lower vegetation density. Fire-related deposits span ~8,000 cal yr BP. Preliminary results indicate variations in fire-related sedimentation with ecosystem type. Lower basins burned ~6500, ~900, and ~400 cal yr BP. Upper basins burned ~8200, 5400, 3800, 1300, and 500 cal yr BP. These preliminary results suggest different ecosystems respond to different climate drivers. Understanding past connections among climate, fire, and geomorphic response will help predict future climate-driven changes in wildfire frequency, magnitude, and severity and help define the natural range of variability of fire and fire-related sedimentation in the MRSR Idaho.