Hey, Keith here. The nights are finally starting to cool off, and that means something important is happening underwater that most bank anglers never stop to think about.

Every lake and pond you fish has an invisible wall hiding somewhere in the water column, and that wall has been holding bass at a very specific depth all summer long.

Right now, that wall is breaking down, and the fish on the other side of it are about to look for new water.

Today, I want to slow down and explain exactly what a thermocline is, how to tell when your water is flipping, and how you can use all of it to catch more bass from the bank this fall.

I get asked about this word more than almost any other term in fishing, and I think that's because it sounds a lot more complicated than it actually is.

A thermocline is simply the layer in a body of water where the temperature changes fast as you go deeper.

Picture your pond or lake stacked like a cake.

The top layer is warm because the sun has been beating on it all summer, and that warm water stays near the surface because warm water is lighter than cold water.

The bottom layer stays cold and heavy because sunlight doesn't reach as far down to warm it. 

Sandwiched between those two layers is the thermocline, a thin band where the temperature drops fast over just a few feet.

Above it, the water is warm but often thin on oxygen by late summer.

Below it, the water is cool but can run so low on oxygen that almost nothing wants to live down there.

Right in that thermocline band, bass find the best of both worlds.

The water is cool enough to be comfortable and still holds enough oxygen to support a healthy fish, so bass—and the baitfish they eat—both stack up right on that line.

This layer does not show up overnight, and it does not sit still all year either.

It typically starts forming in early summer once surface temperatures climb into the mid-seventies. As the season goes on and the sun keeps cooking the surface, that layer sinks deeper into the water column.

By the heart of summer, it can reach anywhere from 25 to 40 feet on a lake with real depth to work with, which is exactly why boat anglers lean so hard on their electronics once August rolls around.

Here's the part that matters most for us as bank anglers though.

That layer does not last.

As fall arrives and the nights cool off, the surface water starts losing heat faster than the water below it, and the thermocline begins rising back toward the surface and losing its grip. 

Eventually the whole lake reaches a point where the top and bottom are close enough in temperature that wind alone can mix the entire water column back together.

When that happens, the invisible wall that split summer bass into a very specific comfort zone disappears completely, at least until next year.

That collapse is what anglers call turnover, and it is the whole reason this time of year gets so interesting.

You don’t need a graph or a temperature gauge to know when your water is turning over. Your eyes and your nose will usually tell you first.

Watch for water that suddenly looks darker, dingier, or more stained than it has all summer, sometimes with a thin film or a bit of foam sitting on the surface.

A lot of anglers describe a faint smell during turnover too, something close to rotten eggs or decaying leaves, which makes sense once you remember that all that bottom water sitting low on oxygen is finally getting stirred up and pushed to the surface.

The most reliable sign of all, though, is what happens to the bite itself.

If you're working water that you know holds fish and getting absolutely nothing, no bites, no bait activity, no signs of life at all, there's a good chance you rolled up on a pond or a section of lake that is turning over right now.

When water turns over, it’s time to focus on extremely shallow water. This water isn’t turning over and remains unchanged.

I like to throw a weedless white Fluke Jr. without a weight up and down the bank when I start to smell the turnover.