Predicting the Spatial Distribution of Postfire Debris Flows and Potential Consequences to Native Trout in Headwater Streams

Many native salmonids exist in reduced ranges, primarily residing in headwater stream systems.  Headwater streams in the intermountain west are susceptible to disturbances such as post wildfire debris flow torrents.  Because the probability of debris flow increases in landscapes that have recently burned, identifying susceptible areas before the occurrence of wildfire may provide information necessary to protect remnant headwater populations.  Predicting the timing, extent, and severity of wildfires and subsequent precipitation and runoff events is difficult; however, it is possible to identify channels in stream networks that are prone to debris flows.  We conducted fine-scale spatial analyses of debris flow potential in 11 high-elevation stream networks of the Colorado Rocky Mountains.  We identified at-risk channels using models based on storm and burn scenarios, and data from geographic information systems (GIS) describing topographic characteristics, and assessed the potential for catastrophic population disturbance given a variety of wildfire and post-wildfire storm scenarios.  Results from GIS models suggest that populations in many of the study watersheds occupy areas with a high probability of experiencing post-wildfire debris flows, but the extent of their distribution and location within the stream network provide sufficient refuge to prevent watershed-scale extirpation of fishes.  These models yield a decision making tool that is intended only to provide qualitative assessment of debris flow potential in stream reaches within a basin context.  A design criterion for the reach-scale model was made for simplicity and flexibility; thus, they can be used to develop comprehensive management strategies for restoration, protection, and post-disturbance remediation of headwater stream networks that support remnant populations of native fishes.