Date of Award:

5-2003

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Department name when degree awarded

Geology

Committee Chair(s)

Joel L. Pederson

Committee

Joel L. Pederson

Committee

Janis Boettinger

Committee

John Schmidt

Abstract

Tectonics and drainage evolution are controlling overall landscape incision in eastern Grand Canyon. Superimposed on downcutting are dynamic responses of hillslopes, tributary streams, and the Colorado River to glacial-interglacial climate cycles. Five tributary stream fill terraces have been identified, and luminescence dating indicates aggradation was occurring 50- 34 ka {S3), 12-7 ka (S2), and 5-3.5 ka (S1). Seven Colorado River fill terraces have been identified, and luminescence and U-series dating indicate deposition was occurring 343-322 ka (M5), 151-118 ka (M4), and 71-64 ka (M3).

Aggradation by the Colorado River in eastern Grand Canyon begins during glacial advances and continues into the subsequent climate reversals. It appears to be driven by increases in sediment yield associated with glacial advances in headwater areas and glacial conditions elsewhere in the Colorado River drainage basin. Local catchments have buffered responses to climate change, with stream aggradation being driven by changes in sediment yield and hydrology of hillslopes. Tributary stream aggradation during full-glacial conditions (S3) is caused by a decrease in precipitation intensity and an increase in bedrock weathering. Aggradation during glacial-interglacial transitions (S2) and interglacial conditions (S1) is the result of increased erosion of older surficial deposits and decreases in vegetation cover.

In the context of the current working model for the response of drylands to climate change, eastern Grand Canyon is distinct in that significant tributary stream aggradation occurs during full-glacial climate conditions and throughout the tributary drainage networks. This suggests that landscape response to climate change in semiarid environments may be slightly different than in arid settings.

Bedrock incision by the Colorado River has been occurring at a rate of 0.13 mm/yr over middle-late Quaternary time. The technique used to make this calculation involves comparing snapshots of the river at analogous positions within its aggradation-incision cycles. The most accurate calculation results from tracing the elevation of the valley bottom through time. This method requires robust age control, but is advantageous because true bedrock incision is calculated and possible error associated with the use of short time intervals and thick fill deposits is eliminated.

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