Rivers Shaped by the Most Erosion Resistant Material
Alluvial rivers are formed by, and are an expression of, the water and sediment that they convey. They are the primary arteries of water and nutrients on land, making them the lifeblood of communities and commerce. While a myriad of environmental and geological factors have been proposed to control alluvial river size, near-universal scaling relations between channel geometry and discharge suggest a common organizing principle. Here we use a global dataset, and a novel field study, to support a simple hypothesis: river geometry adjusts to the threshold fluid entrainment stress of the most resistant material lining the channel. This threshold condition describes the averaged hydraulic state of natural rivers, and is compatible with dynamics; erosion and deposition on channel banks, associated with meandering, for example, represent higher-order variations in fluid stress around the mean state. This greatly extends the applicability of threshold channel theory, which was originally developed to explain straight gravel-bedded rivers with uniform grain size and stable banks. We show how increasing the relative threshold of bank to bed material leads to a proportionate reduction in channel width and increase in channel depth; in this manner, muddy banks encourage sand-bedded rivers to adopt a meandering (rather than braided) morphology. The parsimonious “threshold-limiting material” model provides guidance for river management and restoration practices, and may aid in the reconstruction of past climates on Earth and other planetary bodies using alluvial river deposits.
Preprint Available Here: https://eartharxiv.org/pvqag/