All summer the water in lakes has been stratified with warm surface water and cooler water in the depths. Fish often hold in the cooler water down deep if there is enough oxygen in it to support them. You can pattern them and consistently catch them since they don't move around much.
As the air cools in the early fall, the top layers of water cool. Since warm water is lighter than cool water, this cooler water sinks. Suddenly, the layers of water mix and the lake becomes a more consistent temperature in the areas the fish live in. Oxygen is mixed through-out these layers and fish roam freely. This mixing is the fall turnover.
You can tell when this happens in a couple of ways. The normally clear water becomes murky without rain causing it. You may notice a stale, rotten odor as the unoxygenated deeper water comes to the top. And the fish you had counted on to be schooled up deep are no longer in the same places.
The fish also go on a feeding spree. You will find them in shallow water where they haven't been for months. They know the changes mean cold weather is on the way and they need to stock up to get ready for the lean months. Females start to develop their eggs so they will be ready to spawn when the temperatures warm again.
Fishing can be tough after the turnover. The fish are scattered and hard to pattern. They can be anywhere and will feed at any time. The way to combat this situation is fish fast and hit a lot of different types of structures. Don't expect to catch a bunch of fish in one place since they are less likely to be schooled up.
Use lures that cover lots of water. Top water, crankbaits and spinnerbaits are better than a worm dragged across the bottom for bass. Look for the fish around the mouths of creeks and on top of open water structure.
Fall can be a great time of the year to catch fish. Anytime of year is a great time to go but after the turnover you are likely to find some catchable fish in shallow water and the weather is more comfortable for you, too. And the lake is usually less crowded. What more could you ask for?
Let me know how you fish the fall turnover at email@example.com