WHAT CONTROLS GULLY INCISION TO BEDROCK? AN EXPLORATION OF CLIMATE, BASE-LEVEL, AND RANDOM CHANNEL INCISION ON THE HIGH PLAINS OF EASTERN COLORADO (Invited Presentation)

In low relief, soil mantled landscapes, such as the Great Plains of the United States, there are relatively few geomorphic processes that generate sufficient soil erosion to expose bedrock to sub-aerial physical weathering. At the West Bijou Creek field site in eastern Colorado it has been observed that gully headcut erosion is one such geomorphic process where soil is sufficiently eroded to expose underlying bedrock. The frequency with which gully headcuts are created and expose underlying bedrock, however is unknown. One typical explanation for gully headcut initiation is local base-level drop; a process that has been attributed to climate, landuse change, and tectonics. A contrasting hypothesis is that headcuts result from local geomorphic instabilities within gully channels without the influence of a local base-level drop. We used optically stimulated luminescence (OSL) to date sediment exposed at gully headcuts in order to quantify the time since the last bedrock exposure at seven headcut locations at our field site. We paired a numerical model of erosion/sedimentation with the OSL dates to understand how to interpret OSL ages of sedimentation at headcuts. The OSL dating revealed that gully headcuts have been active in our study area for hundreds to thousands of years, particularly during drought periods. However, the dates do not support base-level drop as the primary mechanism for gully headcut initiation. Field dating and observational evidence are more supportive of headcut initiation via fluvial scour, which is sufficient to locally erode resistant vegetation, creating a scour hole that can migrate upstream as a headcut. Finally, bedrock exposure frequency was estimated based on a headcut with the best documented retreat rate, suggesting that within this gully bedrock might be exposed 1.4% of the time.