THE NATURE OF FLOW AND SEDIMENT MOVEMENT FOLLOWING WILDFIRE IN LITTLE GRANITE CREEK NEAR BONDURANT, WY

Wildfire not only consumes vegetation, but can also alter the hydrologic and sedimentation characteristics of the burned watershed. Increased erosion and flooding are among the most visible and dramatic post-fire impacts and can affect channels and aquatic habitat for considerable distances from the burned site. However, fire related sedimentation can be highly variable, depending on fire severity, topography, underlying geology and soils, and runoff patterns, making the potential impact of wildfire on streams difficult to predict. To further complicate the issue, there is often little baseline data against which the impacts of fire-related disturbance may be evaluated.

In this presentation, we report results from the first year monitoring of post-fire impacts on stream sedimentation in Little Granite Creek near Bondurant, WY. In August 2000, the Boulder Creek wildfire burned about one-third of the forest vegetation in the Little Granite Creek watershed. Researchers from the US Geological Survey and US Forest Service had previously collected data on rates of sediment transport during 13 runoff seasons between 1982 and 1997 at the mouth of Little Granite (Ryan and Emmett, 2002). Hence, the Boulder Creek burn within the watershed presents a unique opportunity for evaluating local and downstream effects of fire-related sedimentation relative to the baseline established previously.

The primary post-fire impact observed to date is elevated rates of suspended sediment associated with thunderstorms that generate shallow mudflows from burned areas. Baseline rates of suspended sediment transport in Little Granite Creek were already quite high due to the nature of the underlying geology, including marine sedimentary formations from Cretaceous and Tertiary periods (between 100 and 1000 mg/L at highest measured discharges). Measured rates following thunderstorms of moderate intensity in the summer of 2001 were on the order of 20,000 to 50,000 mg/L, representing a 3-4 order of magnitude increase for comparable levels of flow. While these events were short-lived and rates of suspended sediment returned to baseline values within a few days, the stream bed became increasingly clogged with fine sediment following each event. Hence, even moderate runoff events can produce substantial changes in aquatic habitat in areas burned by wildfire.