I am a distant cousin of Clem Bergevin's. My great-great-grandmother was Emma Allard (who married James Ladouceur', a neighbor of Oliver Allard's family in Lowden/"Frenchtown" near Walla Walla), Mary Allard's sister.
In the mid-1970s or so, as a teenager, I attended a large Bergevin Family reunion in Walla Walla, where there was a HUGE family tree spread out over a fence, about 30 feet's worth. I just returned from Walla Walla, where one of the things I tried to do was get in contact with one of the Bergevin clan who may have custody of this. I suspect it is one of Clem's family, as he has been active with family history. In fact, you can read some of his history of the Frenchtown area at:http://museum.bmi.net/Pioneer%20Trails/frenchtown_story.htm
Another aside: the Bergevins have a winery in Walla Walla, the Bergevin Lane Winery. I believe the Bergevin owners are Gary Bergevin and one of his daughters. You might check their website.
I hope this is useful to you. Let me know if you find anything out.
(son of Gordon Livengood; son of George H. Livengood and Louise Ladouceur, who was the daughter of James Ladouceur and Emma Allard, who was the daughter of Oliver Allard and the sister of your matriarch, Mary Allard)