MODELING THE FORMATION OF KNICKPOINTS AND SLOT CANYONS IN THE BOONE LIMESTONE

Bedrock channels controls rates of landscape response to changes in tectonic uplift. These channels are typically assumed to erode via mechanical processes, such as abrasion. However, bedrock channels incising carbonates can also erode via dissolution of the rock. Relatively few studies have examined the impact of chemical erosion processes in bedrock channels. Here, we explore a natural experiment, where channels in the Buffalo River catchment of Arkansas incise through a layered stratigraphy that includes sandstone and limestone strata with similar mechanical erodibility. Within the limestone reaches the channels steepen and develop waterfalls and slot canyons. This is counter to initial expectations, as the limestone, which can erode by both chemical and mechanical processes might be considered to be a weaker rock. Bedrock channels normally become wider and less steep within weaker rock layers. We use CHAMP, a bedrock channel evolution model, to explore the development of these steep and narrow channel reaches. Chemical and mechanical erosion rates scale differently with discharge. Consequently, when the full distribution of discharges experienced by a channel is considered, the two processes have different discharges at which they are most effective. The maximum mechanical erosion occurs during large flood events, whereas the maximum chemical erosion occurs at a much lower discharge, typically near the most common flows. Our model results demonstrate that this reduction in the most geomorphically effective discharge within the limestone can produce channel narrowing and steepening similar to that observed in the field.