THE ROLE OF HOLOCENE CLIMATE CHANGE AND FIRE-RELATED DEBRIS FLOWS ON LONG-TERM (103-104) SEDIMENT YIELDS IN THE MIDDLE FORK SALMON RIVER WATERSHED, IN CENTRAL IDAHO

Changes in climate can dramatically influence landscape morphology by altering the frequency and severity of disturbance events such as wildfire and post-fire erosion. This study evaluates fire-related sedimentation along an ecosystem gradient to investigate 1) how climate influences the timing, frequency, and magnitude of fire over Holocene timescales and 2) the contribution of fire-related sedimentation from steep tributary basins to long-term (10³-10⁴) sediment yields in the Salmon River drainage. The Middle Fork Salmon River (MFSR) watershed is located in the Northern Rocky mountains of central Idaho and encompasses a range of ecosystems including subalpine pine and spruce forests (~3,000 -1,700 m), montane Douglas-fir and ponderosa pine-dominated forests (2650 - 1130 m) and sagebrush steppe (~ 1,800 - 900 m). Over 40% of the drainage area of the MFSR has burned in the last 30 years resulting in many large fire-related debris flows. Fire reconstructions from ¹⁴C dating of charcoal macrofossils in alluvial fan sediments indicate that times of synchronous fires across upper and lower ecosystems during the Holocene occurred ~700-1000, 1400-1600, 2800-3000, 6000-6800, 8000-9000 cal yr BP. On millennial timescales, dry conditions of the mid-Holocene are associated with relatively low fire frequency, few fire-related debris flows, and increased sheetflooding events potentially caused by decreased vegetation density and increased fire resistant species (e.g. Douglas fir). In contrast, millennial-scale cool/wet conditions of the late Holocene, combined with severe annual to decadal-scale droughts, are associated with increased fire frequency and large fire-related debris flows. During the last 2000 yrs, fire-related debris flows from eight sub-basins are estimated to contribute ~119 ± 21 T/km²/yr of sediment to the MFSR. This estimate accounts for ~50% of long-term (6500 yr) sediment yields (Kirchner et al. 2001). Fire-related deposits compose >70% of total alluvial fan thickness in cool/wet forested ecosystems and >30% of the total thickness in dry/warm less vegetated basins emphasizing the importance of vegetation density to fire-related sediment delivery.