'upstream-migrating' is a compound formed from 'upstream' + 'migrate' + the progressive/gerund suffix '-ing'. 'Upstream' itself comes from Old English elements 'up' + 'stream', where 'stream' referred to a current of water; 'migrate' originates from Latin 'migrāre' meaning 'to move (from one place to another)'.

'migrate' entered English from Latin 'migrāre' (via Old French 'migrer' in some routes) and became established in Modern English as 'migrate'; 'upstream' developed in Old English as a directional compound of 'up' and 'stream'. These elements were later combined with '-ing' to form descriptive progressive compounds like 'upstream-migrating'.

Initially, 'migrate' meant simply 'to move from one place to another' and 'upstream' meant 'toward the source of a stream'; over time the compound came to be used specifically for directional biological movements (e.g., fish moving to spawn) and for descriptive phrases indicating movement toward a source or origin.