I've always assumed it is a little of what both you and Dungee said. I don't believe that fish migrate from the dam to 2 Mile canyon, but I do think that the spring mudline pushes them down canyon a ways and gives cover for the shad to grow and get a headstart before they get destroyed by the stripers. Then in the fall as the mud has settled out of the water the stripers head back while chasing the shad all the way to the mud line. As the water temperature cools they can tolerate the shallower water where in the summer they have to have access to deeper water to reset their metabolism. When the lake was full there was deep water at Striper City (Trachyte Canyon) so they could retreat to the deep pockets, but now they are relegated to wait until the temperature drops in late fall/winter. I've seen the same thing in San Juan and Escalante just not to same scale as up north because the Colorado river is massive in comparison. Just my observations/hypothesis.