Actually, the Oak Grove Fork confluence is a long way downstream from there. Near Ripplebrook RS and Three Lynx.
That little tiny creek that dumps into the Collawash is actually the mainstem Clackamas. I always figured someone really screwed up when the were originally mapping it.
Back in the Old Days when they let the summer runs through the dams (or trucked them up), that was some good fishing. The confluence, and the lower couple of miles of the Collawash. Long drive up, but fishing for summers in the snow was a hoot. Used to always be a bunch of bears up around there.