Field evidence for the influence of weathering on rock erodibility and channel form in bedrock rivers

Abstract

Erosion processes in bedrock-floored rivers shape channel cross-sectional geometry and the broader landscape. However, the influence of weathering on channel slope and geometry is not well understood. Weathering can produce variation in rock erodibility within channel cross-sections. Recent numerical modeling results suggest that weathering may preferentially weaken rock on channel banks relative to the thalweg, strongly influencing channel form. Here, we present the first quantitative field study of differential weathering across channel cross-sections. We hypothesize that average cross-section erosion rate controls the magnitude of this contrast in weathering between the banks and the thalweg. Erosion rate, in turn, is moderated by the extent to which weathering processes increase bedrock erodibility. We test these hypotheses on tributaries to the Potomac River, Virginia, with inferred erosion rates from~0.1 m/kyr to >0.8 m/kyr, with higher rates in knickpoints spawned by the migratory Great Falls knickzone. We selected nine channel cross-sections on three tributaries spanning the full range of erosion rates, and at multiple flow heights we measured (1) rock compressive strength using a Schmidt hammer, (2) rock surface roughness using a contour gage combined with automated photograph analysis, and (3) crack density (crack length/area) at three cross-sections on one channel. All cross-sections showed significant (p < 0.01 for strength, p < 0.05 for roughness) increases in weathering by at least one metric with height above the thalweg. These results, assuming that the weathered state of rock is a proxy for erodibility, indicate that rock erodibility varies inversely with bedrock inundation frequency. Differences in weathering between the thalweg and the channel margins tend to decrease as inferred erosion rates increase, leading to variations in channel form related to the interplay of weathering and erosion rate. This observation is consistent with numerical modeling that predicts a strong influence of weathering-related erodibility on channel morphology.

The SPACE 1.0 model: a Landlab component for 2-D calculation of sediment transport, bedrock erosion, and landscape evolution

“…ω cr is the threshold stream power for detachment of bedrock, which may vary significantly depending on the relative dominance of plucking or abrasion (e.g., Hancock et al, 1998;Whipple et al, 2000) as well as the weathered state of the bedrock (Hancock et al, 2011;Johnson and Finnegan, 2015;Small et al, 2015;Murphy et al, 2016;Shobe et al, 2017). Howard (1998), Hancock and Anderson (2002), Turowski et al (2007), and Lague (2010) employed a similar exponential decline in rock erosion rate with increasing sediment thickness.…”

Application of a Luminescence‐Based Sediment Transport Model

“…Difficult Run is a 164 km 2 suburban watershed with a protected second‐growth forested riparian zone in the piedmont physiographic province. The river is a gravel bedded pool‐riffle stream (Hupp et al, ) with a bedrock gorge and knickpoint near the river mouth (Shobe et al, ). Our study is located upstream of this gorge.…”

Mechanical weathering and rock erosion by climate‐dependent subcritical cracking

“…Physical rock breakdown (aka mechanical weathering; here comprising all in situ rock fracturing at and near Earth's surface) represents a critical rate-limiting factor for a broad range of geologic and geomorphic processes [e.g., Shobe et al, 2017;Collins and Stock, 2016;Lebedeva and Brantley, 2017;Heimsath et al, 1997;Kirchner et al, 2006;McAuliffe et al, 2006;McFadden, 2013]. There has been a limit, however, in our understanding of the dominant drivers of mechanical weathering on Earth and other terrestrial planets.…”