Understanding the Effects of the Twitchell Canyon Wildfire on Stream Geomorphology and Fish Habitat

Impacts of wildfire are highly variable; some areas experience only modest changes and are quick to recover while other areas incur profound changes to aquatic biota, in-stream habitat, water quality, watershed hydrology, hillslope erosion, and sediment transport. In some cases, these impacts only affect areas within or near the burned area, while in other cases the impacts fare propagated far downstream. At present we have very limited ability to predict which parts of the landscape, and thus which populations of fish, are most likely to be negatively affected by fire. Similarly, we have little basis for projecting ecosystem recovery and prioritizing areas for fish populations. The Twitchell Canyon fire burned 45,000 acres near Beaver, UT in July 2010. Over 30% of the area burned at high severity, which included two major headwater streams that sustained a trout population. In summer 2011, monsoonal thunderstorms caused massive debris flows and sheet-flow erosion that altered channel morphology and aquatic habitat in the burned area. A previously robust, non-native trout fishery was nearly extirpated as a result of the geomorphic response to the wildfire. Extensive field observations were correlated with predictive models for post-wildfire debris flows and characteristics of the topography and topology of the watersheds. Watershed characteristics that appear to preclude negative impacts to fish habitat are slope and valley width. Where slopes flatten and valleys widen, sediments are deposited in a way that high quality fish habitat is buried. Radiocarbon dating of burned material and field observation were used to determine the frequency of wildfire and its synchronicity over the Holocene. Our initial sampling campaign appears to document 10-15 individual wildfires and with age estimate spanning the recent half of the Holocene (~8,000 years)