Wide bedrock valley development and sensitivity to environmental perturbations: Insights from flume experiments in erodible bedrock

Wide bedrock valleys and their genetic descendants, strath terraces, can serve as morphological records of past climate that reflect river discharge and sediment load during periods of valley widening. Understanding how changes in sediment load and water discharge create such distinct morphological features is limited by a lack of robust understanding of the specific processes of bedrock valley widening. We present results from the first set of flume experiments specifically devoted to exploring the conditions necessary to create wide bedrock valleys and how bedrock valleys develop through time. We ran six experiments in a weak bedrock substrate representing valley widening in an easily erodible bedrock, with differing amounts of water discharge, sediment load and base level fall. We evaluated valley width, valley wall height, channel mobility, lateral and vertical bed incision and sediment cover on the bed to explore the conditions necessary for the development of wide bedrock valleys and better understand the processes that affect valley widening rates. The results of the experiments show that wide bedrock valleys developed slowly and only under long periods of high sediment conditions, while vertical incision occurred much faster and was easily induced under different forcing mechanisms. We found that high sediment flux, enough to cover the channel bed, was a necessary condition for substantial valley widening. However, sediment cover on the bed was not by itself a sufficient condition to create wide bedrock valleys in our experiments; other factors were also required, particularly mobile channels within the valleys and some channel curvature to induce lateral undercutting. The results from this set of experiments suggest that the creation of wide bedrock valleys has several necessary conditions that must be met, and the development of a wide bedrock valley can be disrupted by slight changes in one of these necessary conditions.