Simply, what was happening in Coast Salish society for these 300 years?
I'm from the Puget Sound area, and our common knowledge of the local people begins with a young Chief Seattle seeing Vancouver's ship, and ends with the founders of the city of Seattle naming the new town after him. (Yes, i am aware that's not his name, but I'm not sure how to spell it correctly)
I understand that Europeans and European-Americans took somewhat longer to get out to the west coast, but surely there had to have been some influence and effects from colonization during those centuries. Or maybe not? Were the Lushootseed-speaking people of Puget Sound largely untouched by outside societies during that time? I understand that's not really true - since Spanish, Russian, English, Haida, and other people all interacted with them - but mention it to contrast with the people east of the Mississippi or south of the Rio Grande.
There really is little compelling reason to believe that European or Asian influence reached the interior Puget Sound region prior to the late 1770s when sustained interaction with explorers/fur traders took place. There are some exceptions, however. Many early european explorers noted the presence of metal tools in use by natives. In a component dating to between the 15th and 16th centuries, excavators at the Ozette Site, near the Makah Reservation, in the far northwest coast of Washington State, discovered both ferrous metal and bamboo. The bamboo was preserved in an anaerobic environment beneath a mudslide. Ferrous metal and copper artifacts are found in a few sites on the coast in components thought to be pre-contact. The most persuasive arguments accounting for these seemingly out of place artifacts is that they likely were scavenged from shipwrecks or debris along the coast. In simpler terms the exotic material got here, but the people did not.
This theory is discussed at length here.
There is an additional hypothesis that Asian metal working diffused across the Bering Strait and down the Northwest Coast during the prehistoric period. I do not have a reference for that one. I'm sorry but I'm stuck in a cabin in a snow storm in the back woods of Northern California. If you PM me next week, I can maybe look it up for you.
As far as material culture or any other evidence of pre-late 1770s contact coming across land from the east or south, none that I know of exists.
George I. Quimby. 1986 Trade Beads and Sunken Ships. In Archaeological Essays in Honor of Irving B. Rouse, Robert C. Dunnell and Edwin S. Hall, Jr., editors, pp. 231–246. Mouton, The Hague.
K. Ames and H. Maschner 1999 Peoples of the Northwest Coast, their Archaeology and Prehistory. Thames and Hudson. London