GEOLOGIC CONTROLS ON POST-FIRE CHANNEL RESPONSE, MARKAGUNT PLATEAU, UTAH

The Brian Head fire burned approximately 30,000 hectares of the Markagunt Plateau in southern Utah in the summer of 2017. Within a month of fire containment, intense thunderstorms initiated large scale flooding, channel erosion, and channel aggradation in the major basins that burned during the fire. Here, we explore the geologic controls on differences in channel response between basins draining differing proportions of distinct lithologies in a tectonically active area. In the study watersheds, volcanic rocks composed primarily of Oligocene to Miocene ash-flow tuffs are more exposed in the headwater areas that were heavily burned. Quartzite conglomerate, sandstone, and lacustrine sediments deposited prior to the ignimbrite flare-up are exposed in areas less affected by the fire. We use point counts and XRF analysis of these lithologic differences to untangle sediment connectivity to sources in the burned versus unburned areas. We then relate sediment source areas to spatial patterns of erosion and deposition using post-fire lidar, repeat cross sections, and field observations. In all of the study basins, widespread channel change was induced by coarse sediment pulses, although some reaches remained relatively stable. This is observed in the Center Creek basin, as the incision of a headwater tributary basin within the burned area likely led to a debris flood that propagated down the channel, inducing sequences of aggradation and degradation in downstream channel. In the adjacent Bowery Creek basin, however, sediment from the burned area was likely disconnected to downstream reaches due to localized deposition. Instead, flood induced channel widening and erosion caused similar sequences of aggradation and degradation downstream from the burned area. The Bunker Creek watershed only drains volcanic rocks and exhibits less topographic relief than the other watersheds. This watershed provides insight into the channel’s response in a simpler geologic setting, but where the entire watershed was affected by fire, and observations show increased bank instability, erosion and channel migration. Post-fire channel response in these basins differed in the degree of sediment connectivity to fire proximal sources, but resulted in large scale channel change both within and outside of the burned area.