Earth and Planetary Sciences ETDs

Publication Date

8-31-2011

Abstract

The Green and Colorado Rivers comprise the drainage system of the western slope of the Colorado Rockies and Colorado Plateau. Comparison of river profiles and rates of incision between these rivers provides a natural laboratory for resolving controls on river evolution. Disequilibrium profiles in both rivers are evident by numerous knickpoints and convexities. By compiling existing age constraints and applying cosmogenic burial dating techniques to previously undated bedrock strath terraces, we determine spatial and temporal patterns of incision and profile evolution over the last 10 Ma. In several cases, incision rates are faster below knickpoints than above, suggesting that knickpoints are dynamically evolving and likely migrating upstream. Reconstruction of paleo-profiles from the 640 ka Lava Creek B terrace suggests rates of knickpoint migration of >150 m/ka in soft rock. Hard bedrock often coincides with knickzones and appears to slow knickpoint migration (<50 m/ka). Semi-steady average incision rates of 150 m/Ma over the last ten million years on the upper Colorado has resulted in 1.6 km of incision. The Lees Ferry knickpoint (ca. 950 m elevation) is interpreted to be an upstream-migrating knickpoint initiated by integration of the system through Grand Canyon at about 6 Ma. A burial date of 1.5 +/-0.13 Ma, on a 190-m-high strath terrace 169 km above the knickpoint indicates a rate of 126 m/Ma and is three times older than a cosmogenic surface age of the same terrace. Thus high terraces dated by surface techniques are misleading. This plus a compilation of available incision rates across Lee's Ferry knickpoint show moderate rates of 150- 175 m/Ma below Lees Ferry, ca. 100- 130 m/Ma above the knickpoint in long term rates and 230-300 m/Ma above the knickpoint in very low and young terraces with lower rates farther upstream (e.g. 100 m/Ma on the San Juan and 150 m/Ma near Grand Mesa). Previous authors noted convex features in tributaries above the knickpoint are at elevations between 1200 and 1400 m suggesting they are all responding to a change in incision rate on the Colorado River. Thus longitudinal profiles and incision rates are consistent with diffuse knickpoint propagation extending perhaps 300 km above Lees Ferry on very short time scales. Very high short term rates of 300-500 m/Ma over ~500 ka at Lee's Ferry, and upstream of the knickpoint (e.g. Navajo Mountain, Fremont River and Trachyte Creek) partly result from minimum estimates of age but still may suggest incision rates had increased ca. 500 ka due to knickpoint propagation following slower average incision in the last 1-2 Ma. A new cosmogenic burial isochron date of 1.48 +/-0.12 Ma on an abandoned meander 60 m above the river in upper Desolation Canyon gives an incision rate of 40 m/Ma. Thus, the Green River below Canyon of Lodore displays much slower incision rates relative to a similar distance upstream on the Colorado River. The combination of higher gradient, higher discharge, and higher incision rates over the last several million years, for the upper Colorado River relative to the Green, is interpreted to be due to differential rock uplift of the Colorado Rockies relative to the Canyonlands and Uinta Basin regions. This may be driven by mantle bouyancy associated with the Aspen Anomaly of central Colorado. The overall conclusions of this paper are that: 1) differential incision across the Lees Ferry and Desolation knickpoints records upstream-propagating incision transients in a disequilibrium river system; 2) the upper Colorado River system is incising faster than the Green River over the last several million years due to rock uplift of the Colorado Rockies relative to the central Colorado Plateau.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Kirby, Eric

Second Committee Member

Crossey, Laura

Third Committee Member

Coblentz, David

Project Sponsors

National Science Foundation

Language

English

Keywords

Incision, uplift, colorado plateau, colorado river, incision rates, Lava Creek B ash, Cosmogenic burial dating, knickpoint

Document Type

Thesis

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