Document Type

Thesis

Abstract

Field and modeling studies suggest that bedrock channels equilibrate to base-level change through geometry and slope adjustment to imposed discharge, sediment supply, and substrate erodibility conditions. It is hypothesized that differential weathering may lead to differences in erodibility across bedrock-floored channels. This cross-channel variation in erodibility is likely an important but largely ignored control on channel geometry. To test this hypothesis, in situ measurements of rock compressive strength were obtained using a Schmidt hammer in 32 channel cross-sections. The Schmidt hammer measures compressive strength, which has been shown to decrease with increased weathering and will therefore provide a semi-quantitative measure of the extent of rock weathering. Average compressive strength decreased 15- 55% along 18 of the 32 transects. The decrease in Schmidt hammer values with height can be attributed to mcreases in porosity, alteration of minerals, and weakening of cementation by weathering. Deviations from this trend may result from differences in lithology, erosion rates or rock aspect. Analysis of chemical weathering was conducted along three transects using ICP-MS data and four indices of weathering. Chemical weathering was shown to be most effective near the thalweg in some channels. This is likely due to the near-perennial availability of water near the thalweg. Compressive strength and geochemical weathering indices vary across nearly all channel cross-sections, indicating that all channels in this study show differential weathering. The prevalence and dominance of physical or chemical weathering varies across a channel, and this variation implies that there will be differences in erodibility across bedrock-floored channels.

Date Awarded

2011

Department

Geology

Advisor 1

Gregory S. Hancock

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