QUANTIFYING BEDROCK STRENGTH WITH RESPECT TO FLUVIAL ERODIBILITY ALONG THE COLORADO RIVER: COMPARING IN SITU AND LABORATORY METHODS

Bedrock strength should be an important control on the erosion of the Colorado drainage, including fluvial incision. However, quantifying the fluvial resistance offered by a rock mass remains a difficult task, with various field and laboratory methods being used by different workers. It has recently been suggested that tensile strength is superior to other measures of rock-mass strength in capturing erodibility. To examine the relation between both laboratory-tensile and field-compressive strength of river-level bedrock and fluvial geomorphic metrics, we collected data for 18 rock units along the Colorado River in Glen and Grand canyons. Compressive strength was measured with a Schmidt hammer and tensile strength was obtained in the laboratory via Brazilian-splitting tests.

Initial results show only a very weak correlation between Schmidt-hammer compressive strength and Brazilian-splitting tensile strength. These results are surprising given previous studies indicating a positive relation between the two measures. Moreover, reach (10 km) and canyon-scale (100 km) averages of channel width, gradient, and unit stream power of the Colorado River do not correlate strongly to averages of tensile strength, whereas greater field-compressive strength does significantly correlate to a narrower, wider, and higher stream power reaches. Assuming there is a direct or indirect relation between bedrock strength and the geomorphology of the Colorado River, these results suggest that Schmidt-hammer field measurements capture the mechanical heterogeneities and anisotropy of rock masses better than the corings used in the splitting tests.