I've always wondered why the crappie and shad apparently go to creeks in the winter. The creek at my lake is only about 10 feet deep and is literally 10 degrees cooler than the lower end near dam so I would think more fish would stay in main lake near dam so its close to 10 degrees warmer? I've heard the fish break into two groups, deep main lake crappie and a shallower bunch in the backs of coves and creeks in more stable current-less places that hopefully will warm faster on the warmer days.
shallow water warms up faster than deep water
I've thought about this too, how the shad really are usually much colder in the creeks now than in the main lake. To me the main reason they are in the creek is much like why crickets group up under street lights at night. The shad simply get "fooled" by the man made conditions that they are in.
From exploring several creeks during winter time I've found that the more the mouth of a creek was tapered evenly like a man made channel, getting shallower and narrower further upstream, the less threadfin stick around in that sort of creek during these prolonged severe cold snaps in the winter. The exception is where there is a treatment plant releasing water, or some other warm water source, where a more evenly tapered creek (i.e. creek "x") will draw them in.
For threadfin, lakes are not their normal habitat. Threadfin shad come from river systems. In the river if the water was generally deeper and warmer a good instinct would be to move faster in the direction of better water conditions. This is how they get trapped. In a lake there are big shallow muddy flats that often get shallower and shallower towards the main lake. These flats warm up when it is sunny and draw in the shad. When temps drop the shad retreat to deeper, warmer water, which is often initially in an upstream direction. So they get "stuck" in less than optimal conditions. If they were "smarter" they would have headed to the main lake.
I imagine there are times, especially on a smaller shallow lakes or big ponds, where more shad might survive in the creek better versus the main lake during these cold snaps? These would be like the oxbox lakes shad might get stuck in in the time before dams were built. It does seem like shad will sort of "bury" themselves in the mud in the creeks when it is cold. This creek "mud sucking" survival strategy makes more sense in a river than in a big lake. They might as well scrape the bottom 60' deep in the main lake, where conditions will generally be more constant and forgiving, and where the ground temps will be more constant.