This paper discusses a hitherto neglected aspect of fluvial downstream fining: the transition from a gravel bed to a sand bed. From the few known examples and by additional study of new sites, some general features of such transitions are given. These transitions are generally rapid and involve a change in bed texture from unimodal gravels through bimodal sediments to exclusively sand material and are often associated with a break in slope. We propose three possible causes: local base level control an excess of sand supply, and abrasion/breakdown of fine gravel. We further argue that such transitions occur in a variety of fluvial environments and are geographically more widespread than previously supposed, and on this basis we suggest they may also aid in interpreting ancient sediments showing bimodality and rapid proximal-to-distal changes in grain size.

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