Funny, I just backpacked the Chiliwack River/Copper Ridge loop last week. Chiliwack lake feeds the river just north of the border.
That is the Chilliwack River. I am not sure what the name of the river is that feeds the lake from the US side just a couple miles south of the lake. It is fed from the Baker Chain of mountains anyways. The Chilliwack river changes into the Vedder River in my home town of, guess what....Chilliwack. Seems to change from Chilliwack River to Vedder River soon as it passes beneath a bridge. I understand that the river was interfered with some 100 or so years ago when someone, over a land or water dispute, dropped some trees and diverted most of the flow from the Chilliwack to the Vedder. Now no water from the Chilliwack River at that point, makes it to the Chilliwack River further down stream in the city. Word of advice. When hiking up that way, don't cross that clearing. That's the border and the US Border PAtrol has siesmic sensors all along that strip.
LMAO ... I got a good bit of entertainment reading this thread. Hope to ride out west and see some of you one day. It's still a dream to ride the PCH as well. We're planning a trip to Badlands/Black Hills/Devils Tower for next year.