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bed load

American  

noun

Geology.

  1. the sand, gravel, boulders, or other debris transported by rolling or sliding along the bottom of a stream.

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Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.