If it helps, assume I'm a member of the Chinook tribe. Being so far removed from the Eastern Seaboard, how long would it take for the news to reach me? What systems allowed the spread of news from tribe to tribe among Native Americans in the late 15th and early 16th centuries?
By the time Lewis and Clark had arrived in the Pacific Northwest, the Chinookan speaking people's of the Columbia were very familiar with trading with Europeans (which was a major source of strife since the American explorers had to deal with the increased prices).
The first verifiable (the British contested that they had discovered the Columbia years earlier) contact between Europeans and Chinookan speaking Indians was the 1792 expedition of Robert Gray.