Predicting the spatial distribution of postfire debris flows and potential consequences to native trout in headwater stream networks
Sedell, Edwin Russell
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Native trout populations have declined over the last century because of habitat fragmentation and degradation, and the introduction/invasion of nonnative species. Now, many native species of salmonids exist in reduced ranges, primarily residing in headwater stream systems. Negative effects of predicted climate change, including increased water temperature, reduced precipitation, and increased risk of wildland fire may further jeopardize persistence of native trout populations. Headwater streams may be especially susceptible to disturbances such as debris flows. 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 may be prone to debris flows. Here I present a fine-scale spatial analysis of debris flow potential in 11 high-elevation stream networks of the Colorado Rocky Mountains occupied by isolated populations of Colorado River cutthroat trout. I identified stream channels at high risk of debris flow events using models based on storm and burn scenarios, and data from geographic information systems (GIS) describing topographic, soil, and vegetation 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 may provide sufficient refuge to prevent local extirpation. This method couples the spatially continuous distribution of fish and their habitat with a debris flow risk assessment model that accounts for the spatially variable properties within a watershed. Applying the risk assessment along the stream network, rather than to an entire watershed, provides a risk assessment that identifies the potential impacts of postfire debris flows within channels occupied by native fishes and other aquatic biota.