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Upstream: adverb - contrary to the direction of flow of a river or stream, further up or along a moving body of water.
If you have trouble remembering these two, just use the analogy of stairs. Water always flows down, never up. So: as you climb up the stairs, so you go 'upstream', toward the source of the river in the hills.
'It's much easier to swim or paddle a boat downstream, as you follow the flow toward the sea.'
'If you intend to drink from a stream, it's probably best to make sure there isn't a dead sheep lying in the current upstream.'