The influence of weathering on the spatial distribution of erodibility in bedrock river channels

This is a presentation that was given on October 20th, 2014 at the Geological Society of America annual meeting in Vancouver, BC, Canada. The talk presents rock strength, surface roughness, and crack density data from tributaries to the Potomac River in Virginia. Our data support the idea that bedrock channel erodibility is greater on the channel margins than at the thalweg, which we hypothesize to be the result of weathering damage preferentially accumulated on the channel banks. This work was published in Shobe et al (2017; Earth Surface Processes and Landforms). Note: This presentation contains figures reproduced from Hancock et al (2011; Journal of Geophysical Research: Earth Surface) Abstract: Weathering may yield spatial variations in bedrock strength across rock-floored channel cross-sections, because weathering is dependent on geomorphic stability that varies in an active channel. Modeling results suggest that the channel erosion rate dictates the weathering variance possible in any given channel, but this has not been tested in the field. We test if average cross-section erosion rate can exert a first-order control on weathering intensity. We present data from three Potomac River tributaries undergoing transient response to ~30 m of baselevel drop following the passage of Great Falls. Erosion rates along tributary profiles vary from <0.5 to ~0.8 m/ky. We selected nine channel cross-sections: one below, within, and above the major knickzone on each tributary. Cross-sections within each tributary exhibited consistent lithology. At each section, we quantified weathering through measurements of compressive strength, surface roughness, and crack density at multiple heights above the thalweg as a proxy for degree of geomorphic stability. At each height, we 1) obtained 60 compressive strength measurements with a SilverSchmidt hammer, 2) used a Fowler contour gage and photograph analysis to quantify rock surface roughness along six roughness profiles, and 3) measured the length, width and other properties of all cracks >2 cm length in a ~3 m2 area for selected sites. All nine cross-sections showed significant declines in compressive strength (~10% to ~50%) between the thalweg and the channel margins. Six of nine cross-sections showed significant increases in surface roughness with height. We observed three to seven times more crack length per rock area in channel margins than in the thalweg, with an average difference of 0.2 ± 0.05 mm/cm2 of visible cracks. In nearly all cases, our proxies suggest an increase in weathering with height. Differences in weathering measurements between the thalweg and the channel banks in each cross-section are inversely related to cross-sectional unit stream power (R2 = .69 for compressive strength, R2 = 0.68 for crack length per area), providing evidence that erosion rate dictates the accrual of weathering effects along the channel margin. We conclude that the interaction between the rates of erosion and weathering influences the distribution of bedrock erodibility here.

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