Doctoral Dissertation Research: Responses of Gully Erosion and Sedimentation to Environmental Change in the Upper Republican River Basin, Nebraska

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Many fluvial systems of the central Great Plains have flashy discharge regimes and high sediment yields. As a result, their sedimentary processes are extremely sensitive to changing hydrologic conditions. Evidence in the form of cut-and-fill terraces and alluvial stratigraphy with high resolution depositional units suggests that both hydrologic and sedimentologic adjustments have taken place throughout the Holocene. Morphologic properties of currently active gullies and valley fills also indicate that contemporary gully erosion rates are sensitive to changes in modern environmental conditions. This general objective of this doctoral dissertation research project is to quantify gully erosion and valley aggradation rates in the upper Republican River basin of western Nebraska and to determine how these rates have varied in response to environmental change. This research will examine how gully systems have (1) activated and stabilized on decadal time scales in the late 20th century; (2) incised and aggraded in response to climate changes of the Holocene; and (3) extended headward to shape watershed morphology during late Quaternary. This research will integrate field, laboratory, and GIS analyses. Historical gully erosion will be documented by examining aerial photographs to determine the magnitude of headward gully migration on a decadal time scale since the 1950s. The spatial distribution of gully-derived sediment downstream from active headcuts will be established using a 137-Cs sediment tracing technique. Holocene valley aggradation rates will be estimated on the basis of radiocarbon age determinations of organic matter within alluvial fills and terraces. Sedimentologic and pedologic analyses of these same fills will permit interpretation of the depositional and soil-forming processes active during aggradation. Extension of drainage networks will be investigated by using soil-stratigraphic and morpho-stratigraphic evidence to infer the headward limits of networks at various times throughout the late Quaternary. These limits can define network planforms that will be analyzed in a GIS environment to determine changes in hydrologically significant drainage network parameters over millennial time- scales.Results of this research will help document the response of semi-arid fluvial systems to environmental change. As scientific concern mounts over the potential impact of future climate change in sensitive semi-arid regions like the Great Plains, realistic appraisals of past hydrologic variability become increasingly necessary. This research will improve understanding of that variability by examining its impact on gully erosion and valley aggradation. By investigating these processes over temporal scales ranging from decades to millennia, the project will establish a better understanding of the complete range of potential process rates, and will document the many factors that may govern changes in these rates. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.