EVIDENCE OF, AND A PROPOSED EXPLANATION FOR, BIMODAL TRANSPORT STATES IN ALLUVIAL RIVERS

Gravel-bedded rivers organize their bankfull channel geometry and grain size such that their bankfull fluid shear stress (τ) is close to the threshold of motion. Sand-bedded rivers on the other hand typically maintain bankfull 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) re-analysis of global channel geometry datasets, with explicit consideration of the slope-dependence of critical shear stress (τ_c); and (2) examination of a longitudinal river profile as it transits from gravel to sand-bedded. Data reveal that the transport state of alluvial river-bed sediments is bimodal, showing either near-threshold (τ/τ_c~1) or suspension (τ/τ_c>>1) conditions, and that these regimes correspond to the respective bimodal peaks of gravel and sand that comprise natural river-bed 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 erosion-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.