We just "restored" a beaver pond this summer. Which entailed digging out the pond deeper so we have the possibility of keeping the fish alive through the winter (it's at 9200ft altitude). The big challenge is a dam that can withstand spring runoff and won't develop leaks, and a spillway that can handle runoff without eroding.
Because I plan on putting native Colorado River cutts in the pond, I had to have a system to let water out without all the fish going out with it. Besides, the pond a mile downstream has rainbows and brookies in it--and I want to keep those non-native "downstream" fish out.
So we have a square pipe with a vertical riser with the end of the riser at the max level we want the pond to be at, and the square pipe has levels with a board and places on the vertical section that acts like a valve that can raise or lower the water level.
The system your describing for lining the pond sounds like a good one. What altitude are you at in Montana? Are you down in the valleys or up in the hills?
I can't help but put in a pitch for you to transplant
native fish into your pond when it's finished
Casey
Oh, and remember to put lots of structure in the pond.