Toward the source of a river; in the direction opposite to the flow of water downstream.
Compound word: 'up' (toward higher ground) + 'river' (body of flowing water); both have Old English/Germanic roots; the direction convention reflects natural geography.
Salmon swimming upriver are actually swimming against gravity while fighting current—they can jump up to 12 feet vertically because they've evolved powerful muscles specifically for this exhausting, sometimes fatal journey home to spawn.
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