HOLOCENE GULLY INCISION AND STABILIZATION IN THE UPPER REPUBLICAN RIVER WATERSHED, CENTRAL GREAT PLAINS (USA)

From its origin at the confluence of the North Fork Republican River and the Arikaree River, the Republican River runs east from the northwestern corner of Kansas along the Nebraska state line. The river marks the boundary between the distal reaches of the Nebraska Sand Hills to the north and the thick Quaternary loess deposits of the central Great Plains. The deeply dissected fringe of the loess plateau is known regionally as the Arikaree Breaks. Modern canyon morphology exhibits several terrace levels within the Breaks as evidence of multiple gully cut and fill cycles that have cut through the loess and into the Pierre Shale below. Radiocarbon and OSL dating of terrace soils and sediments have yielded time control for the later gully cycles, but widening of previous gullies has left only small remnants of older valley fills. A recent episode of gully expansion appears to have followed a lengthy period of sediment storage and soil formation ending after 1600 yr BP. Linear expansion of the gully system progressed into the upper reaches of the canyons simultaneous with gully widening in the lower canyon. Stability in the new valley floor was again disturbed by gully development in the lower reaches of the canyons. Erosion associated with this episode corresponded with the onset of European settlement in the region. Though landuse likely exacerbated the gully expansion, the gully cycle may have been initiated with the mid-19th century drought. Comparison with regional climatic records derived from aeolian and fluvial sources will provide insight into the length and severity of climatic events required to impact gully cycling in the late Holocene.