From Longman Dictionary of Contemporary Englishriver bedˈriver bed noun [countable] SGthe ground at the bottom of a river → sea bed
Examples from the Corpus
river bed• The entrepreneur made a million dollars out of Pet Rocks-rocks that you could find in any river bed.• By proceeding a little further, a scrambling descent to the river bed may be made.• It was thus possible to assemble a bridge pier and lower it complete on to the river bed.• To my astonishment I find that we use the river bed as a road. 