If one had to guess, I would imagine a scenario that looked like this. Lewis and Clark (or some other Anglo explorers) asked a band of Indians up river from Portland (the mouth of the Columbia) who those Indians were downstream. The Indians they asked spoke some version of Chinook. In Chinook, the Indians answered "Those are the Indians downstream" which the white explorers took to be a proper noun rather than a locator. Isn't it fun to make up history?