9706011 Granger The rate and pattern of river incision through bedrock determines how landscapes evolve in response to their tectonic and climatic environment. Understanding long-term river incision processes is important for understanding landform evolution, patterns of tectonic uplift, the geomorphic record of climate change, and the environmental impacts of land use and dam construction. However, because long-term river incision rates have previously been very difficult to measure, river downcutting rates remain poorly constrained, and theoretical models of river incision remain largely untested. Recently, cosmogenic nuclide techniques have provided new tools for estimating both long-term river incision rates and catchment erosion rates. It is now possible to infer detailed histories of river incision and catchment erosion rate over timescales relevant to river profile evolution. This project uses comogenic 26Al and 10Be in river sediments preserved in caves alongside the Green River, Kentucky, to infer the history of river incision and catchment erosion over the past 5 million years. Because caves provide an environment protected from erosion, cave sediments may be left undisturbed for millions of years. The Green River flows alongside numerous caves, including the Mammoth Cave system, by far the world's largest cave system with over 500 km of surveyed passage. These caves are a vast repository of river sediment, recording the river's incision history over the past 5-10 million years. Intense sampling of the Mammoth Cave system sediments will allow a detailed reconstruction of river incision and catchment erosion over the past 5 million years, a timespan which includes several major climate changes. Additional sampling along the length of the Green River will allow a study of the river's longitudinal profile evolution over the same timespan. These data will provide a record of river incision and catchment erosion unprecedented in its scope and detail. This research is des igned to explore the influence of sediment supply on river incision, by simultaneously measuring both catchment erosion (the source of river sediment) and river downcutting rates. It should also help clarify the influence of climate change on river downcutting and landscape evolution. This study should provide much-needed data to test and constrain the most fundamental hypotheses of river incision through bedrock.
