Geologic controls on bedrock channel width in large, slowly-eroding catchments: Case study of the New River in eastern North America

• Bedrock channel width can be extremely variable along large passive margin rivers.
• High aspect ratio (width/depth) occurs in bedrock with closely spaced joints.
• Narrower channels occur where erosion is dominated by abrasion.
• Morphology reflects local properties, not necessarily variation in uplift rate.

We have investigated the geologic controls on hydraulic geometry of bedrock rivers using a single large catchment, the New River, from a stable tectonic setting with variable, resistant lithology but spatially stable climate. Our survey of channel width at 0.5 km spacing along 572 km of the river shows major variation that only roughly fits the expected scaling relationships between width, drainage area, and slope. Considerable variations in width, including steps in trends and large spikes, relate to physiogeologic boundaries that the river passes through. A large fraction (15%) of the river's length classifies as bedrock reach, showing that it behaves more like a bedrock river than an alluvial river. Unlike established trends, the channel is wider in bedrock than in alluvium. Field observations show that aspect ratio (width to depth) is not constant, but fluctuates systematically with width from wide, shallow reaches to narrower, deeper reaches. Our observations of bedrock properties suggest that susceptibility to fluvial plucking versus abrasion may control this anomalous channel morphology. One end member form with aspect ratio as high as 500, which we term the incision plain, is associated with very closely spaced discontinuities (~ 10 cm) in otherwise hard rock. We propose that the closely spaced discontinuities enable efficient plucking that leads to widening by lateral erosion. This morphology locally occurs in other passive margin rivers and may be a fundamental fluvial form that is similar to, but the inverse of, slot canyons. The other end member, which we term channel neck, is narrower and deeper with complex flow paths through blocky bedrock. This form occurs where discontinuity spacing is longer (> 0.5 m) and erosion is abrasion dominated. These results imply that changes in channel width do not necessarily reflect variations in uplift rate, but instead may result from complex response to bedrock properties.