The role of bedrock on the longitudinal profile of the Colorado River has intrigued workers for over a century. The river\u27s profile exhibits large-scale (10 to 100 km) variations in geomorphology that are qualitatively associated with changes in rock type. This study provides the first bedrock-strength data to quantitatively test the relation of bedrock-resisting to hydraulic-driving forces in Glen and Grand canyons. The intent of this study is to explore the role, if any, that bedrock has on large-scale geomorphic variations along the profile of the Colorado River. Rock-strength data collected at 84 sites along the river corridor in Glen and Grand canyons include intact-rock strength, fracture spacing , and other characteristics associated with Selby rock-mass strength (RMS). These strength data were statistically related to measurements of channel width, gradient, and calculations of unit stream power.
At the canyon scale (100 km), rocks in Grand Canyon have significantly higher intact-rock strength, lower fracture spacing, and higher RMS than those in Glen Canyon. These observations correspond to the fact that Grand Canyon is steeper and narrower, and has greater mean unit stream power. Furthermore, smaller scale, reach-average values of rock strength correlate significantly to width, gradient, and unit stream power, although there are outliers related to local-scale effects such as rapids. The Colorado River runs in a narrower and steeper channel in reaches confined by resistant bedrock ( e.g., Upper Granite Gorge, RM 77-114). In contrast , reaches floored in weaker bedrock (e.g., lower Marble Canyon, river miles 37 to 58) are associated with wider channels and lower gradient.
This study confirms previous research linking rock type to the geomorphology of the Colorado River. Results imply that knickzones in the profile are persistent features that reflect a dynamic equilibrium between hydraulic-driving and bedrock-resisting forces, rather than transient waves of incision due to tectonics or drainage integration. They support the hypothesis that bedrock sets the long-term, large-scale template for the Colorado River. Bedrock hypothetically acts as a direct control on the river\u27s width and gradient, particularly when the river is in contact with bedrock. Rock-strength and weathering properties of bedrock within tributary catchments, where debris flows initiate, act as an indirect control through their influence on hillslope-to-river sediment production during episodes, such as today, when the river is not on bedrock
