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Fluvial response a decade after wildfire in the northern Yellowstone ecosystem: A spatially explicit analysis

Abstract:

Forest fire is a vital ecological process capable of inducing complex fluvial response, but the integration of these effects across entire watersheds remains poorly understood. We collected downstream cross-sectional and geomorphic data, acquired geographic information on land cover and forest fire, and performed spatially explicit statistical analyses to examine fire-related impacts in catchments burned to varying degrees. Generalized least squares (GLS) regression models suggested that channels with a greater percentage of burned drainage area were associated with markedly higher cross-sectional stream power, relatively smaller width/depth ratios, and lower bank failure rates 12 to 13 years after the fires. These results implied that streams became more powerful in the aftermath of forest fire and that net incision had been the primary response in second- to fourth-order channels since the 1988 Yellowstone fires. The extensive geographic coverage of our data, spanning multiple basins with measurements spaced every 100 m, allowed us to hypothesize a process-response model based on these results. We suggest that a wave of fire-related sediment propagates through burned catchments. High runoff events or even moderate flows provide sufficient energy to evacuate the finer-grained material delivered from burned hillslopes to the channel network over a period of 5-10 years. The combination of elevated post-fire discharges and decreased sediment supply then induces an episode of incision. Site-specific channel changes are highly variable because streams can accommodate post-fire increases in energy and sediment supply through multiple modes of adjustment. Characterizing the spatial distribution of stream power would provide a valuable management tool because this variable is strongly associated with percent-burned drainage area and integrates several elements of complex fluvial response. Future research focused on the channel substrate and its evolution through time is needed, but our results indicate a fundamental linkage between fire and fluvial processes. © 2002 Elsevier Science B.V. All rights reserved.