As you can tell by the graph, the normal winter level flucuates from about 676 to 677.5 most of the time. The lowest that I have ever seen it was 675.3 one time a couple of years ago. Rainfall influences it probably more than anything else, along with the air temp. The colder it is, the more heat people use and thus more hydropower if available.
TVA runs at least 1 turbine all the time during the sauger spawning run and usually holds water on the weekend if they can. They usually run more water during the week as people are going to work, cooking, etc.