Event Title

Large-Scale Geologic Control of the Colorado River's Profile through Glen and Grand Canyons, UT and AZ: Testing J.W. Powell's Hypothesis

Presenter Information

Rob D. Mackley
Joel L. Pederson

Location

Space Dynamics Laboratory

Event Website

http://water.usu.edu/

Start Date

3-25-2004 11:50 AM

End Date

3-25-2004 11:55 AM

Description

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.

Data collected at over 90 study sites within Glen and Grand canyons supports Powell’s 1869 hypothesis of river-level bedrock control. These data include 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.

These results are the first to present meaningful relations between bedrock driving and hydraulic resisting forces for a large-scale, natural river encountering heterogeneous rock-types. They 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. The relatively steep gradient transition between Glen and Grand canyons is likely the result of a dynamic equilibrium between bedrock and the river rather than a transient tectonic knickzone brought on by recent tectonic uplift. Rock-strength variables may also have an influence at smaller spatial scales within the canyons. Previous workers have shown 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 should impart an influence on the caliber and yield of sediment transported to the Colorado River. Thus, there is an indirect control by bedrock through its influence on hillslope-to-river sediment production and a direct control on the profile by river-level bedrock resistance to incision.

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Mar 25th, 11:50 AM Mar 25th, 11:55 AM

Large-Scale Geologic Control of the Colorado River's Profile through Glen and Grand Canyons, UT and AZ: Testing J.W. Powell's Hypothesis

Space Dynamics Laboratory

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.

Data collected at over 90 study sites within Glen and Grand canyons supports Powell’s 1869 hypothesis of river-level bedrock control. These data include 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.

These results are the first to present meaningful relations between bedrock driving and hydraulic resisting forces for a large-scale, natural river encountering heterogeneous rock-types. They 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. The relatively steep gradient transition between Glen and Grand canyons is likely the result of a dynamic equilibrium between bedrock and the river rather than a transient tectonic knickzone brought on by recent tectonic uplift. Rock-strength variables may also have an influence at smaller spatial scales within the canyons. Previous workers have shown 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 should impart an influence on the caliber and yield of sediment transported to the Colorado River. Thus, there is an indirect control by bedrock through its influence on hillslope-to-river sediment production and a direct control on the profile by river-level bedrock resistance to incision.

https://digitalcommons.usu.edu/runoff/2004/AllPosters/4