A Field Study on the Lithological Influence on the Interaction Between Weathering and Abrasion Processes in Bedrock Rivers

Abstract

Lithology is an important control on the efficiency of bedrock incision and thus the pace of landscape evolution. Rock strength is commonly considered the limiting lithologic factor that resists erosion. Yet, rock strength is a dynamic property that oscillates during advection of rock to the channel surface as damaging processes weaken rock and erosion exposes fresh rock. We approach the problem by investigating damage on bedrock surfaces that vary by the frequency they are eroded in channels of different lithologies. Our data set includes measurements of channel slope and width to characterize channel morphology, and Schmidt hammer rebound, P wave velocity, slake durability, and porosity to characterize the mechanical properties of channel surfaces. The average damage accumulation rate of lithologies ranges over 41% of the mean. We find a range of damage patterns among the different lithologies. Local surface damage increases with erosion frequency in channels comprised of coarse-grained bedrock but decreases with erosion frequency in channels comprised of fine-grained bedrock. We interpret these patterns to develop from lithological influences on weathering, abrasion, and the threshold of damage to erosion. The cross-channel damage patterns between channel floors and margins are well correlated with stream power demonstrating links between microstructural rock properties, reach-scale morphology, and landscape-scale processes. We conclude that the morphodynamics of bedrock channels are sensitive to the lithological influences on the direction and magnitude of feedback in the coevolution of bedform morphology and the mechanical properties of the surface.