Spatially variable erodibility in bedrock channels produced by weathering

Erosion of bedrock-floored channels regulates the evolution of many landscapes, and accurate prediction of channel erosion rates is a primary goal of landscape evolution modeling. It is hypothesized that bedrock weathering, typically neglected in models of bedrock channel evolution, introduces spatial variation in the rock erodibility across the geometry of such channels. To test this hypothesis, in situ measurements of rock compressive strength were made using a Type N Schmidt hammer in eight channel cross-sections located in arid, humid, and subalpine environments. Average compressive strength decreased 20 - 50% in transects rising 0.5 - 2 m from thalweg to channel margin in arid Utah. Average compressive strength decreased 30 - 40% along transects rising 2 - 5 m from waterline to channel margin in subalpine Colorado. Analyses of the extent of chemical weathering was also conducted on two transects using ICP-MS data and four indices of weathering. No significant chemical weathering was observed in the subalpine channel, and in the arid channel, the most intense weathering was found just above the perennial low-flow on the sub-aerially exposed margin. While more intense weathering is usually associated with wetter climates, bedrock channels show similar decreases in rock strength with height above the channel in arid channels. Finally, the spatial variability of rock erodibility is seen to occur at scales much smaller than previously accounted for in erosion calculations, and is controlled by a number of factors, including lithology, climate, solar exposure, and the availability of water