DEBRIS FLOW OCCURRENCE IN THE IMMEDIATE POST-FIRE AND INTER-FIRE TIME PERIODS, AND THE ASSOCIATED EFFECTS ON CHANNEL AGGRADATION IN THE OREGON COAST RANGE, U.S.A

Dendrochronology was used to estimate the time since the previous debris flow and the last stand-replacement wildfire in unlogged basins in the central Coast Range of Oregon. Large influxes of sediment and wood delivered by debris flows to mainstem rivers and valley floor landforms were driven by the interplay between the timing of fires and high intensity rainstorms. Over 50% of the channels we investigated experienced a debris flow within 30 years of the last stand-replacement wildfire (151 years b.p.). In addition to this synchronous pulse of debris flow activity, large storm events sporadically trigged debris flows in the inter-fire time period resulting in a substantial background rate of debris flow activity. Debris flow deposits in mainstem river channels were reworked by subsequent high flow events, resulting in a sequence of channel bed adjustments. Channel aggradation downstream of debris flow inputs resulted in a deformable streambed of coarse gravel and the creation of deep pools; however, the bed was extremely porous and resulted in a dry channel because streamflow went subsurface during the summer dry season. Channels that were not aggraded formed smaller pools, which were limited by the depth to bedrock, but these pools had a greater likelihood of remaining wet throughout the summer because they intercepted subsurface flow that traveled parallel to the bedrock surface. Rates of sediment input and the associated effect on channel aggradation have important implications for the survival of threatened fish species that reside in mainstem river channels.