The width of bedrock-bound rivers

The hydraulic geometry of a river is defined by three quantities, width, depth and velocity, the product of which gives the discharge. While depth and velocity can be predicted from fluid flow physics, prediction of channel width remains a stubbornly difficult problem. In alluvial channels, the geometry of a river is set by persistent erosion and deposition of sediment on channel margins. It has been argued that some bedrock rivers have the same hydraulic geometry as alluvial rivers, but there are clear contrasting examples of rivers with high bedrock exposure that are narrower and deeper than alluvial channels. Here we explore how rock exposure on river banks impact channel width using observations from small drainage basins to the largest bedrock rivers on Earth. We find that bedrock-bound channels, where both banks are rock, are distinctly narrower and deeper than alluvial channels. Our results also suggest that even where there is substantial bedrock exposed in a channel, alluvial erosion and deposition processes may still dominate channel morphology. But when channels are bound by rock, they tend to adopt geometries that do not occur in alluvial channels and should be considered a distinct class of channels.