THE INFLUENCE OF WEATHERING ON THE SPATIAL DISTRIBUTION OF ERODIBILITY IN BEDROCK RIVER CHANNELS

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.