Plural of distributary; branches of a river or stream that flow away from the main channel, especially in deltas.
From 'distributary,' which is formed from 'distribute' + '-ary' (Latin '-arius,' one who does something). Created as a geographic term to parallel 'tributary,' which flows toward the main channel rather than away.
Geographers invented 'distributary' specifically to mirror 'tributary'—it's a beautiful example of English speakers using parallel word-formation to express opposite concepts, and it appeared relatively recently in the 1800s.
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