QUANTIFYING GEOMORPHIC CHANGE OVER MULTIPLE TIME SCALES ALONG PLEASANT CREEK, CAPITOL REEF NATIONAL PARK, UTAH

The North American monsoon and flash floods are important factors in annual sediment budgets and geomorphic change along streams in the southwestern United States. In 2013 we initiated a project to investigate paleo, historic and present day patterns of fluvial geomorphic change within the lowlands of the Water Pocket Fold along a ~500 m reach of Pleasant Creek (PC) in Capitol Reef National Park (38.18025°, -111.18064°). The goal of this investigation is to track reach-scale geomorphic change through a broad range of seasonal hydrologic patterns and to contextualize this with historic records and prehistoric trends. Along this reach of PC we identified four fluvial terraces. Outcrops and refraction seismology show that all terraces are straths, with < 4 meters of fill mantling each surface. The oldest terrace is ~50 m above the active channel and appears to contain a stage III calcic soil horizon, indicating the fluvial system has been down-cutting for ~0.5 Mya, likely in response to regional Colorado Plateau uplift. RTK-GPS and Structure from Motion surveys combined with Geomorphic Change Detection software have allowed us to quantify surface change over the past two years. These surveys highlight the impact that flash floods have on sediment transport and the geomorphic characteristics of PC. Between September 2014 and April 2015 significant flash flooding occurred, resulting in detectable change across 63% of the study area. The majority of the reach experienced aggradation, up to 40 cm thick on the floodplain; whereas only 14% of the area experienced detectable erosion. This sedimentation suggests that during flash floods sediment stored upstream is mobilized and then deposited within this reach due to a decrease in channel slope. We have tracked historical changes in sinuosity at PC using air photos and discharge data from the nearby gaged Fremont River. We find that high discharge years (68% > than mean annual discharge) are followed by sinuosity increases on the order of 3% over the next few years. We infer that such sinuosity increases are due to the stream meandering through recent flash flood deposits. Thus, recent records imply that flash floods along Pleasant Creek tend to result in aggradation over this reach, contrasting with the long term trend of river incision in response to regional uplift.