Gully erosion and valley aggradation in the upper Republican River basin, Nebraska

This dissertation examines mechanisms of sediment erosion and deposition in the upper Republican River basin, and uses results to characterize floodplain aggradation, valley incision and gully erosion in semiarid regions in general. Research demonstrates that tributaries to the upper Republican River have incised and aggraded in response to late Holocene climatic variability. Ages of incision and aggradation episodes appear to coincide with other Great Plains alluvial chronologies, suggesting widespread regional synchroneity. Floodplains within the study area underwent slow valley aggradation during the late Holocene until about 1200 to 800 yr B.P. Widespread valley incision during this period corresponds with regional climatic conditions of the Medieval Warm Period, in which multi-decadal droughts stressed grassland vegetation and increased runoff associated with convective storms.; Results also provide evidence for mechanisms of aggradation and pedogenesis on semiarid cut and fill floodplains. Sheetflow deposition on unchanneled valley floors represents an important mechanism of valley aggradation. Rates of sediment deposition on cut and fill floodplains strongly determine the degree to which pedogenic features develop within aggrading alluvium. Three pedofacies common to semiarid alluvial deposits correspond with increasing aggradation rates: (1) cumulic soils; (2) multiple buried soils; (3) no pedogenic features. Radiocarbon age determinations from multiple alluvial fills indicate that floodplain aggradation greater than approximately 0.5 cm yr−1 limits soil formation: this represents a threshold rate of pedogenic assimilation. Floodplain sediments and soils exhibit a strong positive relationship between aggradation rate and total CaCO ₃ percent, and a negative relationship between aggradation rate and organic carbon percent.; This research also characterizes the morphology and distribution of contemporary gullies within the study area. No relationship exists between gully morphology and intrinsic geomorphic variables such as drainage area or channel order. Gully headcut morphology does reflect the hydraulic mechanisms of overland flow and/or shallow subsurface flow. Discharge records of small Nebraska watersheds provide insight into the runoff magnitudes responsible for driving valley-bottom gully incision. A record of sediment from an enclosed stock pond helps estimate total gully derived sediment yield in the study area at 0.43 Mg km −2 yr−1.