For those who don't know about it, there is a trail from Scappoose most of the way to Vernonia, parallel to Scappoose-Vernonia Rd., known as Crown Zellerbach Trail. It's not paved, but pretty smooth, very passable on a loaded tour bike, and peaceful. Just finished an overnight trip from NE Portland to St. Johns Bridge to Highway 30, the C Z trail, camped at Vernonia Lake, which has an out of the way "primitive" camping area, had a nice breakfast in a restaurant, then took the Banks-Vernonia trail to Banks, rode on to Hillsboro, and took the Max home from there.
What I want to investigate next is that there is a road just north of Linnton, Marina Way, which is marked "Dead End," but it looks from above like on a bike you can get through to the Sauvie Island bridge, with just a short walk across some grass.