The location of "Owl Pipe" is in the same park where they filmed scenes of Glastonberry Grove, Windom Earles Cabin, Audrey and Jack's picnic, Donna and James' picnic and the scene where Hawk finds Major Briggs wandering out of the woods after he escapes his captor, Windom Earle. In fact, the pipe is located just yards away from where Briggs stumbles into the road. You can see a lager section of the pipe system in the background as Hawk drives up to Major Briggs: see below.

That larger pipe is seen as the camera speeds eerily through the small ravine (seen below) then up to the owl pipe.