'downstream-moving' originates from English, specifically the compounds 'downstream' + 'moving', where 'down' originally meant 'toward a lower place', 'stream' meant 'a flow of water', and 'move' meant 'to change position or go from one place to another.'

'downstream' formed in English from Old English elements (compare Old English 'dūn' for 'hill' / 'down' and 'strēam' for 'stream'), and 'move' comes from Latin 'movere' via Old French and Middle English; the compound 'downstream-moving' is a modern English compositional formation combining those established words.

Initially the parts referred separately to direction ('down') and a flowing body ('stream'); combined with 'moving' the compound came to mean specifically 'moving toward the lower part of a flow' and retains that literal directional sense.