Fine Sediment Depositional Patterns and Fish Habitat Following Wildfire and Debris Flow

ABSTRACT The 2020 Cameron Peak wildfire burned part of the Poudre River catchment in Colorado, USA. The river flows through a high‐gradient canyon before entering a lower‐gradient transition zone with modified channel morphology and flow regime. Widespread post‐fire erosion introduced fine sediment (< 2 mm) to the river, causing sediment deposition and a major fish kill. Fine sediment deposition in the transition zone partially blocked or filled fish habitat in channel margin backwaters and side channels and filled interstices in fish‐spawning habitat along the cobble‐boulder bed channel. During summer 2023, we quantified fine sediment retention at 18 sites along the river by measuring fine sediment volumes and embeddedness. Our objectives were to (i) evaluate whether reach‐scale geomorphic attributes or river distance downstream from the sediment source correlate more strongly with reach‐scale fine sediment volume and embeddedness and (ii) use 2D hydraulic modeling and incipient motion equations to estimate the discharge necessary to mobilize the channel bed and remove excess fine sediment. Reach location (canyon vs. transition zone) best explained the volume of fine sediment and embeddedness, although the reach‐scale variables of gradient and cross‐sectional area were significant predictors of fine sediment retention. Hydraulic modeling indicated that a 2‐year return interval flow mobilizes a substantial portion of the bed at the canyon site, whereas bed mobilization requires a 5‐ to 10‐year flow at the transition zone sites. Management to mitigate post‐fire sedimentation and stressors to fishes may need to emphasize changes in river corridor geometry where consumptive water use limits flushing flows.