early 13c., from Anglo-French rivere, Old French riviere "river, riverside, river bank" (12c.), from Vulgar Latin *riparia "riverbank, seashore, river" (cf. Spanish ribera, Italian riviera), noun use of fem. of Latin riparius "of a riverbank" (see riparian). Generalized sense of "a copious flow" of anything is from late 14c. The Old English word was ea "river," cognate with Gothic ahwa, Latin aqua (see aqua-). Romanic cognate words tend to retain the sense "river bank" as the main one, or else the secondary Latin sense "coast of the sea" (cf. Riviera).
U.S. slang phrase up the river "in prison" (1891) is originally in reference to Sing Sing prison, which was literally "up the (Hudson) river" from New York City. Phrase down the river "done for, finished" perhaps echoes sense in sell down the river (1851), originally of troublesome slaves, to sell from the Upper South to the harsher cotton plantations of the Deep South.
A wide, natural stream of fresh water that flows into an ocean or other large body of water and is usually fed by smaller streams, called tributaries, that enter it along its course. A river and its tributaries form a drainage basin, or watershed, that collects the runoff throughout the region and channels it along with erosional sediments toward the river. The sediments are typically deposited most heavily along the river's lower course, forming floodplains along its banks and a delta at its mouth.