RESPONSE OF RIPARIAN CORRIDORS AND ALLUVIAL FANS TO WILDFIRES IN PERENNIAL WATERSHEDS IN RANGES OF THE GREAT BASIN

When wildfires occur in the Great Basin, the geomorphic responses of the watersheds impacted determine the potential recovery for the riparian ecosystems. Riparian ecosystems occupy only 1 percent of the Great basin but provide many great ecosystem services. In this study we aimed to better understand and predict the geomorphic responses and recovery of watersheds to wildfire disturbances in the Great Basin.

Since 1984, over 500 wildfires have burned well over 1 million hectares throughout the study area. Using time-series aerial images of 28 study watersheds within the Great basin, we collected data assessing the impacts of wildfires on the geomorphic characteristics of the watershed. This was done using quantitative measurements of the valley floor and alluvial fans. The data collected shows that the geomorphic response was highly variable within the 28 basins. 7 of the 28 basins displayed extensive change. The majority of channels in these watersheds were either infilled or destroyed post fire. In some of the most impacted watersheds, the channel type changed from a single channel system to an aggregational multi-channel system. 14 of the basins were minimally affected; these basins displayed no changes within the channel. The remaining 7 watersheds displayed moderate impacts. We recognized two major trends in our data. As alluvial fan density increased within a basin, the amount of sediment reworking within the basin increased. Also, we saw that as post-fire sediment from alluvial fans increased, so did the amount of change in the valley floor. In the future we plan to look for more trends in the data between alluvial fans, valley floor, and impacts on the basins as a whole based on further analysis of time-series images and field work. We also plan to quantitatively assess how these different basins recover through time post wildfire.

Geomorphic response to fires is controlled by pre-burn vegetative cover on the hillslopes, burn frequency, and post-fire hydrologic events, but also by basin sensitivity to disturbance. Catchments that have an abundance of fine-grained sediments and side-valley fans are more likely to have larger responses to wildfire that can result in century-scale geomorphic change and riparian ecosystem shifts.