Post-fire Sediment Research Articles

Postfire Sediment Deposition in Geographically Restricted Steelhead Habitat

In November 1993, fire burned the southeast portion of the Malibu Creek watershed, Los Angeles County, California. There was concern that sediment produced during the first postfire wet season would degrade pool habitat, thereby adversely affecting the population of steelhead Oncorhynchus mykiss confined to the 4.8-km reach between Rindge Dam and the Pacific Ocean. We tested the null hypothesis that first-year postfire sediment deposition had no effect on pool channel characteristics (depth, area, shape, and surficial sediment particle types) by monitoring pools after the fire but before and after the first postfire wet season. We added field observations of channel morphology to extend the results of the pool measurements. Our findings did not support a suggestion that pool habitat would degrade following the first-year wet season because pool-channel characteristics were generally unaffected by postfire sediment deposition. Consequently, no management action was needed to mitigate the effect of sediment deposition on pool habitat. The amount of soil erosion and sediment deposition that could have been produced during the first postfire wet season may have been reduced by various factors including the burn characteristics, below-average precipitation, and earthquake-induced recruitment of cobble and gravel.

POSTFIRE SUSPENDED SEDIMENT FROM YELLOWSTONE NATIONAL PARK, WYOMING1

ABSTRACT: Wildfires in 1988 burned over 2000 square miles of the greater Yellowstone area in Montana and Wyoming in the largest fires in the history of Yellowstone National Park (YNP). A four‐year postfire study to estimate fire‐related changes in suspended sediment transport on the Yellowstone River and its principal tributary in YNP, the Lamar River, benefitted from a recently completed three‐year prefire baseline study. Both studies took daily depth‐integrated samples from April through September. Fire‐related changes in suspended sediment were distinguished from natural climatic variations by two methods: comparison of forecast postfire sediment loads estimated with prefire sediment‐rating equations to measured postfire loads; and by postfire changes in suspended sediment load expressed per unit volume runoff. Both methods indicated postfire sediment increases that varied according to season. The higher elevation Lamar River basin had little postfire increase in spring snowmelt season sediment but large increases in summer sediment load. The Yellowstone River had postfire increases in sediment load for the spring but did not reflect the large summer increases of its upstream tributary. The reasons for the difference in postfire snowmelt sediment response are unclear but may relate to basin elevation differences, the effects of unburned watersheds, and cooler postfire springs. The few high streamflow snowmelt events in the postfire period mitigated postfire sediment increases.