Go here -
http://www.tva.com/river/lakeinfo/index.htm - select Old Hickory from the drop down menu. What you are looking for is any change in the volume released. For example any step up say from "0" to "5600" or from "5600" to "12,000" or from 0 generators to 1 or more or 1 to 2 or more is the triggering event for the best fishing. From 5600 cfs to 13000 cfs is manageable current and also good flow for fishing down there.