HYDROLOGIC DRIVERS OF FLOODPLAIN EROSION AND DEPOSITION ALONG THE MEANDERING EAST RIVER, CO

Alteration of flow regimes in fluvial systems resulting from climate change and flow regulation is likely to impact channel migration, erosion, and sedimentation of rivers. To understand and prepare for potential changes in hydrogeomorphic dynamics of river systems, it is important to understand historical responses to past hydrologic conditions. We used repeat remotely sensed imagery from 1955 to 2015 to quantify river migration rates, lateral accretion, and erosion along 9 valley segments of the East River near Crested Butte, CO. Historical records from a nearby USGS stream gage predating 1950 correlates with discharge measurements at the study reach (r² = 0.98). A reconstructed hydrograph for the 50-yr duration was used to calculate mean values of flow conditions including timing, magnitude, and duration of flows for six time intervals between seven images. Flow conditions and channel characteristics for each time interval along the nine sub-reaches were used in stepwise multiple linear regression to determine predictors for channel accretion and erosion. Lateral accretion was controlled by channel sinuosity, mean channel width, and the magnitude of peak flow. Bank erosion was best predicted by sinuosity, the number of peaks exceeding bankfull flow, and the slope of the recession of the annual hydrograph peak. Aerial lidar from 2015, measured deposition along feldspar marker horizons, and 254 measurements of soil depth along a single sub-reach provide estimates of sediment volume and annual fluxes. Results suggest an efflux of sediment from the floodplain over the last 50 years in what appears to be a river in quasi-steady state equilibrium. Observed overbank flows from 2017 are being used to inform modeling of overbanks flows and aggradation needed to close the floodplain sediment budget. We use IPCC future climate predictions to model future hydrologic response of the East River using the variable infiltration capacity (VIC) model and estimate potential changes in floodplain erosion under changing hydrologic regimes.