302-2 Taking the Long View: Geomorphic Effects of Instream Wood 38 Years After the Great Yellowstone Fire

Abstract:

Wildfire can strongly influence the recruitment, mobility, and geomorphic function of large wood in mountain streams, yet long-term responses of instream wood following fire remain poorly constrained. Wildfires in the Yellowstone region during 1988 collectively formed the largest fire recorded in the history of the national park and changed the trajectory of fire management strategies. Jones and Crow Creek are two parallel catchments on the eastern boundary of the park with comparable catchment morphology and flow regimes. Jones Creek burned in the 1988 fires and Crow Creek did not. Michael Young measured large wood in both creeks during 1990. In 1999, Zelt and Wohl resampled large wood and channel morphology in these creeks. The unburned catchment had smaller channel width and more closely spaced, small accumulations of wood, suggesting greater wood mobility in the burned catchment. We repeated these surveys in summer 2026 to evaluate nearly four decades of post-fire recovery. We found minimal changes in bankfull channel width in the unburned catchment since 1999 and a slight increase in the burned catchment. Wood load in the unburned catchment increased slightly from 1990 to 1999 and then returned to 1990 values in 2026. Wood load in the burned catchment was consistently lower than the unburned catchment in all sample years, rising slightly in 1999 and then declining to half by 2026. Wood pieces in the burned catchment also create fewer geomorphic effects: about 15% of all pieces surveyed in both catchments had rootwads, but 38% of pieces in the unburned catchment had associated erosional or depositional features whereas only 17% of pieces in the burned catchment created geomorphic effects. Similarly, 11.6% of the wood pieces in the unburned catchment had associated fine sediment deposits in 1999, whereas only 7.5% of pieces in the burned catchment had deposits. Very slow forest regrowth in the burned catchment indicates minimal potential wood recruitment to the stream corridor for the next few decades, suggesting that recovery of instream wood loads and geomorphic effects will require more than a century.

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