BEDROCK STRENGTH AND RIVER MORPHOLOGY DATASETS FOR THE COLORADO RIVER SYSTEM

There has been renewed debate over the mechanisms and timing of both uplift and erosion in the Interior West. Yet, in order to understand the region’s long-term landscape evolution and patterns of topography a third factor of bedrock properties must be considered. We are completing a large dataset of bedrock strength and exploring it in the context of reach-scale topographic metrics for the upper Colorado River system. Included are rock-strength measures such as laboratory tensile strength, Schmidt-hammer compressive strength, approximate shale proportion, and Selby rock mass strength classification. To estimate the strength of units too incompetent to test directly, such as the shales prevalent in the region, we utilize functional relations between reach-scale rock strength and valley-bottom width and unit stream power.

The highly variable Colorado River drainage can be broken up into both alluvial and bedrock reaches underlain by a variety of rock types including quartzite, sandstone, shale, limestone, and crystalline basement. Initial results indicate significant correlations exist between rock strength and channel and valley-bottom width, as well as gradient and unit stream power. High rock strength generally coincides with narrow and steep canyon reaches in this landscape, but exceptions exist where perhaps no single rock-strength measure can capture erodability completely. This implies that bedrock is frequently a first-order control on topography and river form in this complex landscape, which is intuitive yet often overlooked.