CONTROLS ON GEOMORPHIC RESPONSES TO WILDFIRES IN RIPARIAN CORRIDORS OF MONTANE WATERSHEDS IN THE GREAT BASIN

Geomorphic response to wildfires in montane watersheds of the Great Basin determine the recovery potential of riparian ecosystems. Riparian ecosystems comprise only about 1 percent of the Great Basin, but are critical to the region’s biodiversity, agriculture, and recreation. Our interdisciplinary research group is expanding on earlier work to better understand and predict the geomorphic response of watersheds to wildfires in the Great Basin and its effects on riparian ecosystems.

From 1984 to 2015, some 500 wildfires burned an area of about 1 million hectares in study watersheds (n=~1500). Preliminary analysis of time-series aerial imagery from 32 catchments from three ecoregions in the central Great Basin shows that geomorphic response to wildfires is highly variable. Most basins (n=26) had negligible geomorphic change. Five basins had moderate change: some incision of axial channel and/or reworking of parts of valley floor, with or without slope activity (none significant). Two basins had extensive change: dozens of debris flows, incision of side-valley alluvial fans, pervasive reworking of the valley floor with complete loss of pre-fire channel form.

Basins with extensive change exhibited a complex suite of responses resulting from massive inputs of sediment to the valley floor from debris flows and entrenchment of side-valley fans. Axial valleys exhibited a post-fire history of aggradation and incision resulting from alterations in flow with high and low sediment concentrations, respectively. As time since burn increased, new sediment inputs decreased and stream processes were dominated by incision. Post-fire channel incision locally connected portions of the watershed that previously were not connected, thereby increasing surface flow and sediment inputs to the axial valley and decreasing groundwater recharge.

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, both common to basins with limestone bedrock, are more likely to have larger responses to wildfire that can result in century-scale geomorphic change and riparian ecosystem shifts.