RIVER PROFILE EVOLUTION ACROSS A LITHOLOGIC BOUNDARY: INSIGHTS FROM MODELING BEDLOAD TRANSPORT

Spatial variation in bedrock erodibility can lead to the development of topographic relief even in tectonically dead settings. Variation in bedrock properties also gives rise to differences in fluvial process across lithologic boundaries. This study demonstrates the importance of explicitly accounting for fluvial process in landscape evolution across a major lithologic boundary.

This work takes its motivation from the North American High Plains. In this setting, modern rivers exiting the Rocky Mountains have incised into the sedimentary units of the plains. As mountain streams flow over hard, crystalline rocks, they carry a gravel bedload comprising highly resistant, quartz-rich clasts. Results from numerical modeling demonstrate that because these grains are slow to abrade downstream, they contribute to rapid downstream thickening of alluvial cover on the sedimentary streambeds of the plains. As alluvial cover thickens, erosion of the underlying bedrock is hampered.

This study examines the “tools versus cover” competition in a lithologically heterogeneous environment and finds that the juxtaposition of highly contrasting rock types can concentrate a river’s erosive power near the lithologic boundary. This gives new insight into the erosional patterns expected to arise in a geologic setting such as the High Plains, and therefore forms a baseline for better understanding the magnitude of external forces (climate, tectonics) that have also shaped the landscape.