HOW DOES HOLOCENE CLIMATE CHANGE DRIVE WILDFIRES, SEDIMENT DELIVERY FROM HILLSLOPES, AND MAIN CHANNEL RESPONSE?

Changes in Holocene climate drive changes in the frequency and severity of wildfires. Fires, in turn, alter the magnitude and frequency of sediment and wood delivery from hillslopes to mainstem rivers. While sediment yields from tributary debris flows following fires exceed ‘background’ erosion rates by 2-3 orders of magnitude, the effect of tributary fire-related debris flows and sediment-charged floods on main channels varies with valley confinement, stream power, and sediment size. We compare records of Holocene fire-related sedimentation events along an elevational gradient in central Idaho, from low elevation rangeland, mid-elevation ponderosa-pine, and high elevation lodgepole-pine dominated ecosystems.

The smaller (~ 26 km²) lower elevation rangeland site of Wood Creek preserves records of main channel flood events and tributary debris flows in response to fire. A ~4 km reach of Wood Creek shows 1-6 m of channel incision over the past ~200 yr. Over the past ~2000 yr most fire-related flood events occurred during wetter intervals (e.g. the Little Ice Age (LIA)). Records indicate switching between ‘fuel-limited’ and ‘moisture-limited’ fire regimes with changes in vegetation and climate. In the South Fork Payette (SFP) ponderosa-pine dominated ecosystem the peak in frequent, likely small fire-related events occurred during the LIA; however large, widespread fire-related debris flows during prolonged Medieval droughts ~900 cal yr BP account for over 25% of overall fan thickness and likely contributed large amounts of sediment to the main channel. Sediment volumes from single recent fire-related debris flows on the Payette exceed 14,000 m³, with over 10,000 m³ deposited into the main channel (Meyer et al., 2001). OSL and ¹⁴C dating of terraces of the SFP indicate ~5 m of mainstem aggradation on the SFP ~7-6 ka, corresponding with insolation-induced aridity. Fire records during the mid-Holocene are scarce, limiting comparison between hillslope sediment inputs and main channel response.

High elevation sites in the Sawtooth Mountains show good correspondence with fire records in the nearby SFP, with peak delivery of fire-related sediment to channels during the LIA, MCA and ~1400 cal yr BP. Ongoing work examines modern sediment yields to and the role of fire in contributing organic matter to the M.F. Salmon River.