in the direction that a river flows; toward the mouth of a river or in the direction of lower elevation.
Compound of 'down' (from Old English 'dūn,' meaning 'from a hill') and 'river' (from Old French 'riviere'). The direction is relative to water flow rather than absolute geography.
Navigation before maps was terrifying—going downriver meant following water, so 'downriver' was actually safe navigation. But 'downriver' doesn't always mean south; a river can flow east, west, or north downriver!
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