This is the tunnel and pool which is the subject of the FP. The bigger fish was sitting in the eddy on the left, facing parallel to the big rock. I was casting to it from the upstream side of the gravel beach you can see, from a little behind the piled up rocks. The mouth of the tunnel is about 6' high.
This bottom picture is one of the few pictures of me with a fish in existence, documentary evidence I do actually catch some! The fish is a 3.5 lb rainbow jack. Not the one I caught in front of the tunnel unfortunately, but from a bit further upstream.
Thanks to Henning Koehn, a very congenial fishing partner, for the pictures.