As rivers cut a path through bedrock, what influences the relationship between channel width, slope, and erosion rate? Data from naturally incising bedrock rivers show differing relationships among these parameters. For instance, while some studies show a strong inverse relationship between erosion rate and channel width, others show no such correlation. To explain the field data, Yanites and Tucker created a model of optimized channel geometry that accounts for water discharge, sediment supply, and base‐level fall. They found that including the erosion‐inhibiting effects of immobile sediment is necessary to explain the observed disparate relationships. Essentially, as more sediment is added to a river, the channel geometry must widen and steepen to transport the supplied sediment and continue eroding bedrock at the rate of base‐level fall. For channels carrying significant bed load, they found that channel width can be the primary morphological control on the erosive potential of the river, a prediction matched by field data. Further, trade‐offs between increasing sediment supply (which widens the channel) and erosion rate (which narrows the channel) can generate rivers with no relationship between channel width and erosion rate. The model helps explain the conflicting field observations of controls of channel geometry and could contribute to improved understanding of landscape evolution. (Journal of Geophysical Research‐Earth Surface, doi:10.1029/2009JF001601, 2010)
EOS: Research Spotlight: What controls bedrock channel geometry?
By: Ernie Tretkoff
Tuesday, February 01, 2011