Styles and rates of knickpoint migration from cosmogenic Be-10 in an extensional landscape

Knickpoints are important elements in transient landscapes as they represent disequilibrium in the hydrologic system. The upstream retreat of knickpoints is a primary mechanism for communication of base level fall through the landscape, and ultimately controls landscape response to changes in the system. Despite the importance of these features, little quantitative information is known on the style or rates of knickpoint migration. We present data from tectonically-controlled knickpoints in the Canyonlands Graben, Utah, to constrain rates and styles of bedrock erosion in this semi-arid landscape. Our results show that knickpoints migrate upstream by processes of groundwater sapping that cause undercutting and eventual collapse of beds with thickness of greater than 1m; this process resets the cosmogenic isotope system allowing us to estimate knickpoint migration rates of approximately 10m/ka. Bedrock erosion rates show that knickpoint zones are fundamentally transient features through which bedrock incision is accomplished.