Abstract
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.
Subject
Earth-Surface Processes,Geophysics
Cited by
36 articles.
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