POSTFIRE SUSPENDED SEDIMENT FROM YELLOWSTONE-NATIONAL-PARK, WYOMING

Wildfires in 1988 burned over 2000 square miles of the greater Yellows tone area in Montana and Wyoming in the largest fires in the history o f Yellowstone National Park (YNP). A four-year postfire study to estim ate fire-related changes in suspended sediment transport on the Yellow stone River and its principal tributary in YNP, the Lamar River, benef itted from a recently completed three-year prefire baseline study. Bot h studies took daily depth-integrated samples from April through Septe mber. Fire-related changes in suspended sediment were distinguished fr om natural climatic variations by two methods: comparison of forecast postfire sediment loads estimated with prefire sediment-rating equatio ns to measured postfire loads; and by postfire changes in suspended se diment load expressed per unit volume runoff. Both methods indicated p ostfire sediment increases that varied according to season. The higher elevation Lamar River basin had little postfire increase in spring sn owmelt season sediment but large increases in summer sediment load. Th e Yellowstone River had postfire increases in sediment load for the sp ring but did not reflect the large summer increases of its upstream tr ibutary. The reasons for the difference in postfire snowmelt sediment response are unclear but may relate to basin elevation differences, th e effects of unburned watersheds, and cooler postfire springs. The few high streamflow snowmelt events in the postfire period mitigated post fire sediment increases.