Investigating rates and mechanisms of lateral erosion in a small bedrock river using erosion pins, structure-from-motion photogrammetry, and luminescence dating

Bedrock rivers play a critical role in landscape evolution by leaving signatures of past climate shifts, base level changes, and tectonic uplift in the landscape. While much is known about the mechanics of vertical bedrock incision, the fluvial processes driving lateral bedrock erosion and the timescales over which it occurs remain poorly understood. In order to advance our understanding of how bedrock rivers erode laterally over time, we investigate past and present erosion rates at Kings Creek, an incised stream located within Konza Prairie LTER, Kansas, U.S.A. The geology of the study area consists of horizontally bedded alternating layers of limestone and mudstone. Erosion pins are installed in the bedrock of the channel banks and beds to measure modern bedrock erosion rates in resistant limestone and erodible mudstone lithologies. We use structure-from-motion (SfM) photogrammetry to create high resolution digital elevation models of channel beds and banks to evaluate patterns and mechanisms of bedrock erosion. Three terrace levels at the study site indicate that the channel formerly occupied higher stream levels and was laterally mobile, likely owing to changes in regional climate during the late Pleistocene and Holocene. Many of these terraces are mapped as fill terraces, yet strath terraces exist in some locations, which serve as records of past lateral bedrock erosion and valley widening. Single-grain optically stimulated luminescence (OSL) dating of fluvial deposits overlying strath terraces will yield an age of terrace occupation, as well as characterize deposition rates and the duration of lateral channel migration during this time period. SfM results and erosion pin measurements indicate that modern bedrock erosion rates on channel banks are much higher than originally anticipated owing to several high magnitude flow events during an exceptionally wet year. These results, coupled with OSL ages of alluvium overlying the strath terraces, provide information about rates and patterns of past and current lateral erosion, improving our understanding of the timing and processes linked to how bedrock rivers erode laterally to form wide bedrock valleys.