Watercourse

n. stream, small river; water channel, river bed; canal

A watercourse is any flowing body of water. These include rivers, streams, brooks, anabranches et cetera.
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Noun
1. natural or artificial channel through which water flows
(hypernym) channel
2. a natural body of running water flowing on or under the earth
(synonym) stream
(hypernym) body of water, water
(hyponym) branch
(part-meronym) ford, crossing
3. a conduit through which water flows
(synonym) waterway
(hypernym) way
(hyponym) canal

A way or course through which water may or does flow. (DOI4)

Synonyms and related words:
adolescent stream, aqueduct, arroyo, beck, bed, bourn, braided stream, branch, brook, brooklet, burn, canal, channel, conduit, course, creek, creek bed, crick, culvert, donga, dry bed, duct, flowing stream, flume, fluviation, fresh, freshet, gill, gulch, gully, gullyhole, headrace, irrigation ditch, kill, lazy stream, meandering stream, midchannel, midstream, millstream, moving road, navigable river, nullah, race, racing stream, river, river bed, riverway, rivulet, run, rundle, runlet, runnel, sike, sluice, spill stream, spillbox, spillway, stream, stream action, stream bed, streamlet, streamway, subterranean river, swash, swash channel, tailrace, wadi, water carrier, water channel, water furrow, water gap, water gate, waterway, waterworks