That's my understanding as well. I'd have to dig up the references again, but in researching a comment letter years ago, the conclusion was that similar to pronghorn, the northwestern edge of bison's (pre?)historic range was likely around the Columbia River in central Washington, certainly reaching the territory of the Yakama, though numbers were lower here at the edge of their population far from the large herds to the south and east.
Their range in the Inland Northwest is thought to have begun to contract (possibly due to increased human pressure via the horse and other efficient hunting methods, or climactic changes to range conditions, or both) a century or more before Lewis and Clark, with some small, roving herds still present in far eastern Washington and Oregon but little in terms of a resident population by the early 1800's.
Would be super cool to see them back in the canyon country around the Blues someday. Range conditions (ex. overgrazing by wild horses on the Yakama Reservation), habitat fragmentation and development for ag in central Washington and the Palouse likely preclude recovery elsewhere in our state.
Some more on the topic:
http://www.confluenceproject.org/blog/yes-buffalo-did-once-roam-here/"The number of animals that succeeded in reaching eastern Washington or Oregon either by getting through the mountains or by migrating around through the plains of Oregon was undoubtedly very small.[xvii] Had there been no human enemies Native American or foreigners the buffalo would likely have made their way eventually to the Pacific Northwest. Dr. Charles Pickering who accompanied Lieutenant Johnson of the Wilkes Expedition in 1841 crossed the Cascade Mountains eastward bound and visited Colville, Lapwai and other places of the interior. Pickering noted the absence of game in the region, stating that it was outside the range of buffalo but seemingly well-adapted to them.
The Spokane Indians told stories of their forefathers surrounding and killing the last buffalo in the Spokane Valley probably between 1810 and 1820. They also recalled that earlier in the previous century a large number of buffalo were killed north of Moses Lake or the Grand Coulee."
