EROSIONAL HISTORY OF THE NORTH PLATTE RIVER, CASPER, WYOMING

The North Platte River is a critically important resource of central Wyoming and its erosional history is linked to past regional climate conditions. Specific questions investigated in field work near Casper, Wyo., during summer 2012 were the locations and elevations of past channels, relations between river sediment and locally derived sediment, timing of erosion and deposition by the river in the study area, and change over time in river sediments. The river appears to have been consistently downcutting in the study area for at least 4500 years, based on one carbon date, but possibly not at a steady rate as suggested by bedrock truncation. Local streams have generally followed this trend with some minor episodes of aggradation. Optical stimulation luminescence dating of river and local sediment to better constrain the timeline is currently in progress at a USGS lab in Denver, Colo. Old river channels are present in the study area up to 150 feet above and 1 mile to the side of the modern channel. During the recent Quaternary time represented by these deposits, the size of particles transported has decreased, the number of metamorphic clasts have decreased while igneous and sedimentary percentages have remained stable, and angularity of the clasts has increased. These trends may be due to a past decrease in flow and to changing erosional patterns upstream.