HR: 1330h
AN: H52A-1140    [PDF]
TI: Is the Profile of the Colorado River in the Colorado Plateau in Equilibrium With Bedrock?
AU: * Mackley, R D
EM: mackley@cc.usu.edu
AF: Geology Department, Utah State University, 4505 Old Main Hill, Logan, UT 84322-4505
AU: Pederson, J L
EM: bolo@cc.usu.edu
AF: Geology Department, Utah State University, 4505 Old Main Hill, Logan, UT 84322-4505
AB: The Colorado River sets the pace for the erosion of the Colorado Plateau and part of the Rocky Mountains, and the evolution of its long-profile may hold the key to resolving debate over the region's late Cenozoic erosional and tectonic history. Significant reach (10 km) and canyon-scale (100 km) gradient variations occur along the modern profile of this mixed bedrock-alluvial river, and there are multiple hypotheses for the controls on its gradient: 1) knickzones related to active tectonics in western Grand Canyon, 2) coarse bed material from tributary side-canyons, and 3) bedrock resistance as a direct control with respect to channel substrate or an indirect control in terms of hillslope processes in tributary catchments and the production and comminution of bed material. The goal of this study is to test for a spatial relation between hydraulic driving forces and bedrock resisting forces at reach and canyon-scales within and between Glen and Grand canyons. The river encounters diverse rock types that vary in hardness and fracture density. Field and GIS results aimed at testing the bedrock resistance hypothesis for Glen and Grand canyons indicate a spatial relation between rock-mass strength (RMS) and channel gradient. Reach-scale gradients vary as much as a factor of 20 through Grand Canyon, with steep reaches having significantly higher compressive strengths and wide fracture spacings. Glen Canyon, on the other hand, has dramatically lower channel gradient, weaker RMS properties, and reaches with steep gradients that do not correlate to either debris fans or changes in bedrock type but do coincide with geologic structures. The presence of a bedrock or debris-fan frequency correlation to the majority of gradient variations argues against tectonic origins of knickzones and suggests a relation between bedrock resistance and the profile of the Colorado River exists. However, when these results are combined with the distribution of tributary debris fans and their control on the channel organization as detailed by other workers in Grand Canyon, there is a complex spatial correlation between bed material, rock-type, and channel gradient, preventing the direct inference that the river is in equilibrium with bedrock.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1884 Water supply
DE: 8010 Fractures and faults
DE: 8020 Mechanics
SC: Hydrology [H]
MN: 2003 Fall Meeting