CHARACTERIZING THE GRAIN SIZE DISTRIBUTION OF ALLUVIAL DEPOSITS IN THE LOWER YAMPA RIVER TO LINK A PARTITIONED FLUX-BASED SEDIMENT BUDGET TO CHANNEL CHANGE

The forms of channels and floodplains are largely determined by the stream-flow regime and the watershed sediment supply. When the supply of water and sediment to a river change, the river may respond by accumulating or evacuating sediment that in turn changes the attributes and availability of aquatic habitat. Continuous measurements of sediment transport at reach-bracketing gaging stations allow for the construction of mass-balance sediment budgets that identify periods of sediment loss or accumulation; however, such measurements cannot identify where channel change has occurred. Thus, it is important to link reach-scale changes in sediment mass balance to field measurements of channel change. In this study, we use a continuous mass-balance sediment budget and geomorphic analysis of channel change to evaluate the links among sediment flux, streamflow, and changes in channel morphology on the lower Yampa River, Colorado. Flux-based measurements showed that the efflux of sand exceeded the influx in every year since 2012, resulting in a sand deficit of 470,000 ± 320,000 metric tons, which corresponds to between 0.04-0.20 m of erosion averaged over a channel length of 12.5 km. When the mass of erosion was partitioned by sand size classes, fine, medium, coarse, and very coarse sand was found to have been continuously in deficit since 2012, but very fine sand had accumulated in 2014 and since 2016. Thus, any net channel change must primarily involve those geomorphic units where fine to coarse sand was eroded and very fine sand was deposited. To determine where different grain sizes were stored across the river landscape, we comprehensively mapped the grain size of alluvial surfaces using a combination of field samples with lab analysis and a method to derive the grain size distribution from field photographs. Results show that alluvial deposits create regions composed of different sand-size classes and that alluvial surfaces which are actively accumulating sediment have caps that are considerably finer than the underlying substrate. This study demonstrates that it is possible to explicitly link changes in a sediment mass balance to changes in channel form when the mass-balance sediment budget is partitioned by grain size classes and the grain size characteristics of different alluvial surfaces can be defined.