A 14,000-Year Record of Wildfire and Alluvial Fan Deposition Reveals Relationships Among Fire, Climate, Vegetation, and Sediment Yields in the Middle Fork Salmon River, Idaho

Changes in climate influence vegetation distributions and the frequency and severity of fire and post-fire erosion. While the frequency of large fires has increased on all vegetated continents over the last decade, it remains unclear whether recent large fires are extraordinary over millennial timescales. In the Middle Fork Salmon River of central Idaho, over 40% of the watershed has burned in the last 30 years and the subsequent increase in erosion from severely burned hillslopes has produced many large fire-related debris flows. Fires have burned throughout the steep topographic gradient of the watershed that encompasses high elevation sub-alpine and mixed conifer forests and lower elevation rangeland ecosystems. This study uses 14C-dating of charcoal fragments from discrete alluvial fan deposits to reconstruct a 14,000 yr record of fire and fire-related sedimentation along an ecological gradient of the Middle Fork Salmon River. We investigate the role of climate on spatial and temporal variations in the timing, frequency, and severity of fire and the associated erosional response. We combine recent (1997-2008) fire-related debris flow sediment yields and reconstructed fire-related debris flow frequencies over the last 6 ka to quantify long-term (103 yr) sediment yields. Overall, fire-related deposits compose 74 ± 25% of total alluvial fan thickness in upper, wetter ecosystems versus 41 ± 33% recorded in lower, drier basins. Synchronous fires (multi-basin fires) burned 110 ±150, 450 ±120, 860 ±75, 1120 ±75, 1560 ±50, 1790 ± 70, 2070 ±70, 2240 ±100, 2780 ±60, 3730 ±100, and 6440 ±120 cal. yr BP. Fires burning during the early (14-8.5 ka) and late Holocene (4 ka – present) primarily produced debris flow deposits while fires burning during the mid-Holocene (8-5.5 ka) primarily produced sheetflood deposits. We hypothesize the shift in sediment delivery process was a consequence of decreased vegetation density and therefore fire severity driven by a stable, warm, dry mid-Holocene climate that resulted in more frequent and smaller magnitude depositional events (i.e., sheetfloods). Recent (1997-2008) debris flow events occurred following moderate to high severity fires that burned ecosystems containing soils composed of easily erodible Idaho Batholith granites and produced a range in sediment yields from ~1,450-34,550 Mgkm-2yr-1. Lower, drier basins produced significantly smaller single event debris flow yields than upper, wetter basins. Fire frequency, severity, and synchronicity have been high during the last 2 ka and fire-related debris flows have supplied 83-262 Mgkm-2yr-1 of sediment to the main-stem river. Increased fire and debris flow activity during the last 2 ka corresponds with the arrival of dense lodgepole pine (pinus contorta) forests at high elevations during the cool and effectively wetter late Holocene. Over the last 6 ka, we estimate fire-related debris flows have contributed ~30-101 Mg km-2 yr-1 of sediment to the Middle Fork Salmon River, which is lower than the last 2 ka because of decreased debris flow activity during the mid-Holocene. Our results demonstrate fire is a primary contol of millennial sediment yields in the Middle Fork Salmon River. We hypothesize that centennial scale intervals of increased fire frequency and severity occur during long-term intervals of relatively cool and (or) wet climate conditions and associated increased fuel production. Increased fuel loads, coupled with annual to decadal-scale droughts, produce widespread severe fire and debris flows, which contribute significantly to long-term sediment yields in the Middle Fork Salmon River