Good post, Theo.
I can't speak for northern ponds, but we've had some experience with both spillway runoff and creek inflow during times of heavy rain. As Theo said, fingerlings tend to wash over the spillway, which is what happened to our rosies. After a stocking and a big rain, thousands were swimming in a creek downstream from the pond. Todd Overton had also brought us some great-looking 6-8" HSB and we worried that they washed away as well, but it seems that they stayed, judging by the amount we see feeding on pellets.
When the creek continues to run into the pond after a big rain, the gambusias actually swim into the running water and some end up in the creek. They look like tiny salmon swimming and jumping upstream.
From my experience, your larger fish should stay put, as Theo indicated. I'm sure some species of fish will hold tighter than others, but it seems that size is the main factor.
