ASSESSING SEDIMENT AND ORGANIC CARBON ACCUMULATION IN SEMINOE RESERVOIR, WYOMING

Surface water impoundments preserve records of sedimentary, hydrologic and organic processes. In Seminoe Reservoir, Wyoming, stratigraphic variations in grain size document the history of sediment transport, and organic carbon accumulation within the sediments records biologic processes in the basin and the lake. Deposition of both sediment and organic matter occurs as a function of hydrologic conditions and relates to climatic variations as well. To investigate the processes preserved in reservoir strata, sediment was sampled by trenching, augering, coring and collecting water-column suspended sediment. Analyses of the samples are used in conjunction with available environmental data collected since the North Platte River was impounded to illustrate patterns by which sediment and organic carbon have accumulated in the reservoir. Using these data and results, we explore the following hypotheses: 1) sediments in the delta preserve longitudinal gradients in transport capacity averaged over time, 2) hydrologic signals occurring over short timescales are less likely to be preserved than those which take place over long timescales, 3) organic carbon accumulation is positively correlated with sedimentation rate, and 4) sediment-associated organic matter content is negatively correlated with median grain size.

Grain size distributions and organic content were determined for samples at various locations and depths along the direction of transport in the reservoir delta. In order to determine a grain size distribution, sediments ranging from cobble to silt were analyzed using automated optical imaging techniques. Grain sizes throughout the length and depth of the delta are correlated with discharge and settling velocity in order to better understand the relationship between hydrologic signals and the sedimentary record. To determine organic matter content and isotope values, samples were processed at the University of Wyoming Stable Isotope Facility using a mass spectrometer. A map showing overlain grain size and organic content illustrates their relationship as well as the relationship between grain size and longitudinal location of deposition. Our results are useful in understanding sedimentary, hydrologic and organic processes in the lake as well as management of the surface water resource.