I also found this. 

http://nwda.orbiscascade.org/ark:/80444/xv52194

There are probably some plans, maps and photos of the tunnel project in this collection somewhere.

So there definitely is a tunnel out there.  I suppose the entrance was blocked off long ago.  Thanks for the link to the court case, Kirk.  The court decision provides a good summary of PGE's history on the Clackamas, at least through 1947.  I didn't know about the proposed dam at Carver.  That would sure have changed things out in that area.  Interesting reading.

There must be a pretty good pile of tailings out there somewhere.

D.2

One of my photos is in that video

From the video

 Mine from 2011