Most tailwaters have enough regular flow to prevent much of a thermocline or at least mix the water. A fish has no clue that better water is a 100 yards away, imagine if a cloud of invisible poisonous air decended around you, you would have no clue which direction you could go to get out of it, and a fish has little no reasoning power.
The majority of fish kills in lakes there is little or nothing that can be done, it simply has to run its course. BUT the next one reported might be because a golf course dumped 40 gallons of old herbicide not realizing the drain empties directly into a small creek and then into the lake and killed a few thousand fish, that would be something to be investigated even if only to make sure it doesn't happen again.