Channel Characteristics and Large Organic Debris in Adjacent Burned and Unburned Watersheds

Ronald B. Zelt (rbzelt@usgs.gov), U.S. Geological Survey, Cheyenne, Wyo. (307) 778-2931

Fire affects not only vegetation communities, but also hydrologic and geomorphic processes. Fire-caused changes in vegetation and soils can impact stream channels through altered hydrology, sediment inputs, and riparian disruption, and produce effects on channel erosion, sediment storage and transport, and large organic debris (LOD). Many fire effects are immediate or short-term, while other impacts are delayed or long-term effects.

Late in the 1988 wildfire season, the Clover-Mist Fire spread eastward from Yellowstone National Park and burned nearly all of the 6680-ha Jones Creek watershed of the North Fork Shoshone River basin. However, less than 8 percent of the adjacent, 4950-ha watershed of Crow Creek was burned. These two watersheds are very similar in topographic, geologic, and vegetation characteristics, except for recent fire-disturbance level and the modest size difference. During 1998-1999, ten reaches of each stream were studied to quantify differences in width, substrate, residual pool volume and percentage of that volume occupied by fine sediment (V*), and LOD load and characteristics, that may be attributed to wildfire disturbance.

A multiple-regression model explained 87 percent of the variability in reach-average bankfull width as a function of drainage area, number of large LOD jams, and a dummy-variable for the difference between streams. Average width of Jones Creek was 6.6 percent greater than that of Crow Creek. Bankfull widths of pools and riffles also were analyzed separately, and for riffles, the difference between streams was smaller: average width for Jones Creek was 5.4 percent greater than that of Crow Creek. A multiple-regression model explained 69 percent of the variability in median particle diameter of substrate on riffles as a function of reach gradient and the dummy-variable for difference between streams. Average d₅₀ of Jones Creek riffles (58 mm) was 16 mm, or 28 percent, coarser than that of Crow Creek. Increases in channel width and riffle substrate coarseness are consistent with theoretically predicted channel responses to increased runoff following fire.

Residual pool volume and reach-average V* (V*_w) did not significantly differ between streams. Median V*_w was 17 percent for Crow Creek and 23 percent for Jones Creek overall. Results from regression models indicate that for both streams, deeper pools, larger pools, and more closely spaced pools are all associated with greater filling by fine sediment. Multiple regression results for pool-fines size distribution indicate that reach-average gradient explains 64 percent of the variability in median diameter, with no significant difference between the streams.

Smaller size of LOD pieces in Jones Creek (mean diameter 10 percent less, and mean volume 16 percent less) reflects increased loading from younger, fire-killed trees. LOD was better anchored and more frequently occurred in contact with other LOD in Crow Creek. LOD jams were larger and a greater percentage of the debris present occurred in large jams (accumulations of 10 or more pieces) in Jones Creek. Although mobility was not measured directly in 1998-99, these differences suggest that LOD mobility continues to be greater in Jones Creek, as was reported by M.K. Young (USDA Forest Service) from a study during 1990-1991.