means The channel or ground over which a river flows, the depression in the earth that holds and shapes its course.

from A plain compound of two old Germanic words: 'river,' which came into English from Old French 'riviere,' itself from Latin 'riparius' (of the bank), and 'bed,' from Old English 'bedd' — the same word for a sleeping place, a garden plot, or the bottom on which anything rests. The riverbed is literally where the water lies down.