LARGE-SCALE GEOLOGIC CONTROL OF THE COLORADO RIVER'S PROFILE THROUGH GLEN AND GRAND CANYONS, UT AND AZ: TESTING J.W. POWELL'S HYPOTHESIS

The longitudinal profile of the Colorado River through the Colorado Plateau may record the dynamic interplay between hydraulic-driving and bedrock-resisting forces and provide important insights into the ongoing debate about the erosional and tectonic history of the Colorado Plateau. From upper Glen Canyon to lower Grand Canyon (~650 km), the river’s profile is broadly convex, marked by large-scale variations in gradient. Powell (1869) originally recognized this, noting the river’s “mood” closely corresponded to the type of bedrock encountered at river-level.

Recent evidence collected at over 90 study sites within Glen and Grand Canyons supports Powell’s 1869 hypothesis with newly collected data that includes Selby (1980) rock mass strength (RMS) values, raw compressive strengths and fracturing, and calculated unit stream power. Canyon-scale data indicate that Grand Canyon’s lower channel width, steeper gradient, and higher unit stream power relates to statistically higher RMS and compressive strengths. Furthermore, 18 reaches defined by rock-type within the two canyons show a strong positive correlation with gradient and unit stream power, and have an inverse correlation to channel width.

Results support the interpretation that harder and/or less fractured rocks offer greater resistance to incision and that the increased energy required for incision along the river’s profile is provided by a higher gradient and deeper flow, and that the gradient transition between Glen and Grand Canyons is not a transient tectonic knickzone. At a smaller spatial scale, bedrock resistance may also be an indirect control. Previous workers have established that local delivery of coarse bed material is the dominant control on channel organization and gradient at the rapid-pool scale (100 m to 1 km. Rock strength and weathering properties in these catchments, rather than at river-level, should influence the yield and caliber of sediments transported to the Colorado River.