LINKING ANNUAL FLUX OF FLOODPLAIN SEDIMENT AND ORGANIC CARBON WITH DISCHARGE ALONG A MEANDERING MOUNTAIN RIVER

Linking floodplain response to variability in annual discharge provides insight into potential shifts in floodplain dynamics under increasing demand for freshwater and a changing climate. We use 13 historical aerial images spanning 45 years, a 0.5-m resolution aerial LIDAR dataset from 2015, field measurements of soil depth, soil samples, sediment aggradation tiles on the floodplain, and reconstructed hydrographs of the East River near Crested Butte, CO, USA to estimate net sediment and organic carbon (OC) flux in relation to discharge. Differences in river channel location are used to quantify bank erosion and lateral floodplain accretion rates within the period between 2 images. The area of deposition and vertical aggradation rate will be used to estimate the volume of accumulated sediment on the floodplain for each time interval. Net difference between volumes of eroded and deposited sediment will be used to estimate the flux of floodplain sediment within the period between aerial images. Preliminary results for the entire 13-km study reach indicate 22685 m² of lateral accretion and 25318 m² of erosion resulting in net erosion of 2633 m² between the late summer of 2013 and 2015. Using an accretion rate of 1 cm yr^-1, estimated from contemporary depths of fine sediment where gravel pointbars were present in 1990, and a mean measured depth of fine sediment of 57 cm along a subreach ~4 km long, we estimate that 455 m³of deposition and 14378 m³ of erosion resulted in a net efflux of 13923 m³ of sediment across the entire floodplain. Using a measured mean soil bulk density of ~0.86 g/cm³ and soil organic carbon content of 3.5% and assuming these values represent sediment both imported and exported to the floodplain, this equates to a net efflux of carbon of 419 Mg and 1.44 Mg C ha^-1 y^-1. Flows during 2014 and 2015 were relatively high compared to bankfull discharge with approximately 5- and 3-year recurrence intervals, respectively, but analysis was completed only point bars and did not account for vertical accretion across the entire floodplain. Furthermore, meander cutoffs and infilling of oxbow lakes, likely store a significant amount of organic carbon.