Impacts of Wildfire on Runoff and Sediment Loads at Little Granite Creek near Bondurant, Wyoming

There are a variety of mechanisms by which sediment is transferred from burned hillslopes to stream channels following wildfire. Post-fire erodability is related to a number of factors, including fire severity, topography, underlying geology, and nature of the soils, making the potential sedimentation impacts difficult to predict. Large scale disturbances, such as increased landsliding and debris flows, present dramatic responses that have long-lasting influences on channel form and process. However, not all burned areas undergo increased landsliding following wildfire. Perhaps a more common watershed response is the post-fire delivery of fine sediment that is carried in suspension during moderate to high flows. In this presentation we describe changes in suspended sediment loads observed in a burned watershed in NW Wyoming from the years 2001-03 and later in 2008. In August 2000, wildfire burned portions of the Little Granite Creek watershed near Bondurant, Wyoming where bedload and suspended sediment measurements had been collected during 13 previous runoff seasons. This presented an opportunity to quantify increases in sediment loads associated with wildfire. The first 3 years post-fire were warm and dry, with low snowpacks and few significant summer storms. Despite relatively low flows during the first runoff season, the estimated sediment load was about 4 times that predicted had there been no fire. This was attributed to increases in sediment loading during the rising limb and snowmelt peak (54%) and during the few summer storms (44%). In later years, loads decreased, indicating a return to baseline levels. The results from our sediment monitoring lacked some of the more dramatic responses that have been observed following fire. In other environments, high intensity rainstorms caused progressively bulking flows that triggered debris flows though channel erosion. In Little Granite Creek, there were a few substantial storms, but they did not trigger this type of response. Instead, blackwater and hyper-concentrated flows were observed with no physical evidence of in-channel debris flows. The geomorphic consequences of blackwater and hyper-concentrated flows include the infiltration of fines into stream beds, filling of pools, and during larger events, movement and breakup of large wood and jams. These impacts tend to be transient and the effects removed during subsequent flows. The sedimentation pattern and geomorphic response in Little Granite Creek may be fairly typical of stream responses to wildfire during times of continued drought and in the absence of widespread, significant rainfall.