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Volume 6, issue 3 | Copyright
Earth Surf. Dynam., 6, 583-594, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 25 Jul 2018

Research article | 25 Jul 2018

Evidence of, and a proposed explanation for, bimodal transport states in alluvial rivers

Kieran B. J. Dunne and Douglas J. Jerolmack Kieran B. J. Dunne and Douglas J. Jerolmack
  • Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, Pennsylvania, USA

Abstract. Gravel-bedded rivers organize their bank-full channel geometry and grain size such that shear stress is close to the threshold of motion. Sand-bedded rivers, on the other hand, typically maintain bank-full fluid stresses far in excess of threshold, a condition for which there is no satisfactory understanding. A fundamental question arises: are bed-load (gravel-bedded) and suspension (sand-bedded) rivers two distinct equilibrium states, or do alluvial rivers exhibit a continuum of transport regimes as some have recently suggested? We address this question in two ways: (1) reanalysis of global channel geometry datasets, with consideration of the dependence of critical shear stress upon site-specific characteristics (e.g., slope and grain size); and (2) examination of a longitudinal river profile as it transits from gravel to sand bedded. Data reveal that the transport state of alluvial riverbed sediments is bimodal, showing either near-threshold or suspension conditions, and that these regimes correspond to the respective bimodal peaks of gravel and sand that comprise natural riverbed sediments. Sand readily forms near-threshold channels in the laboratory and some field settings, however, indicating that another factor, such as bank cohesion, must be responsible for maintaining suspension channels. We hypothesize that alluvial rivers adjust their geometry to the threshold-limiting bed and bank material, which for gravel-bedded rivers is gravel but for sand-bedded rivers is mud (if present), and present tentative evidence for this idea.

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Short summary
What controls the size and shape of a river – its width, depth, and slope? The cross-sectional channel geometry of gravel-bedded rivers is understood to be a function of the river's fluid shear stress and the weight of the average particle on its bed; however, there is no satisfactory explanation for sand-bedded rivers. We analyze global datasets and individual river profiles and propose that accounting for riverbank cohesion could allow for an explanation for sand-bedded river channel geometry.
What controls the size and shape of a river – its width, depth, and slope? The cross-sectional...