bed load

Bed load is a term to describe the larger particles (relative to the suspended load) that are carried along the bottom of a stream.Generally, bed load downstream will be smaller and more rounded than bed load upstream. This is due to attrition and abrasion which cause the stones to bump against each other and against the river channel, thus removing its rough texture and making it smaller.
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Sediment particles resting on or near the channel bottom that are pushed or rolled along by the flow of water.

A body of coarse particles that move along the bottom of a stream.