From Peaks to Prairie: Two Natural Experiments in Decadal Landscape Evolution

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Erosion and sedimentation constantly re-shape the Earth's surface. Over geologic time, these processes wear down mountains and move vast quantities of broken rock to the sea. On human time scales, the rates are often so slow as to be barely perceptible. But when disturbance strikes a landscape such as in the form of wildfire, flooding, earthquakes, livestock overgrazing,or climate change, the pace of erosion and sediment transport can be altered dramatically. Geologists and engineers have long sought to develop mathematical models of landscape evolution that can predict how a given landscape will response to disturbance. Such models are crucial to understanding how the geological cycle operates and, on shorter time scales, for predicting how landscapes are likely to react to environmental changes. However, these models need to be tested rigorously against actual field data. This project will measure historical and contemporary erosion and sedimentation at two sites in Colorado to provide case studies with which to test models. The first site, located on the High Plains east of Denver, has shown unusually rapid gully growth during the 20th century. The history of the channel network will be reconstructed using a combination of historic aerial photographs, measurements of the volume of sediment trapped behind earthen dams built in the early 1960s, dating of sediments within the valleys, and measuring the concentration in soils of exotic radionuclides that were released by Cold-War era nuclear weapons testing. In addition, the new technology of high-precision ground-based laser scanning will provide images at unprecedented resolution of the year-by-year growth of channel heads. The second site lies in a drainage basin in the Colorado Rockies where intense wildfire followed by heavy rainfall in 1996 triggered dramatic erosion and sedimentation. Repeated field measurements following the fire provide a detailed record of dramatic changes in the watershed area. These original measurements will be supplemented with airborne and ground-based laser-derived maps of the terrain. Data from both sites will be used to test one of the leading computer models of landscape response. The databases will provide a resource for the scientific community now and into the future as models continue to become more sophisticated.Erosion and sedimentation are natural processes that have helped shape our planet throughout its existence, and they have direct and wide-ranging impacts on society. The constant flow of sediment from mountains to the sea fills in hydropower reservoirs, decreasing their generating capacity and operational lifespan. Sedimentation within river channels can increase flood risk to surrounding communities, while incision along streams and gully networks can damage roadways and other infrastructure and reduce the quality of agricultural land. Changes in the amount or size of sediment carried by streams will alter the health of river ecosystems. Erosion can even exhume buried hazardous wastes, releasing them to stream networks where they can contaminate water supplies. All of these processes are often affected by human development. For this reason, computational models of landscape response to both natural and human disturbance are needed to support careful planning that mitigates the potential effects of future changes in land use and climate. This project supports the continued development and refinement of these models and will result in field-based tests that will contribute to sustainable living in an ever-changing landscape.