Lake turnover explained

It starts with one odd fact about water

Most liquids get denser as they cool. Fresh water is densest at about 4 °C (39 °F); colder than that, it gets lighter again, and ice floats on top.^[1][2] That one quirk explains why lakes sit in layers for part of the year and mix from top to bottom at other times.

Summer: a lake in layers

In spring and summer the sun warms the surface. Warm water is lighter, so it floats on the colder, denser water below, and the lake settles into three layers:^[2]

• the epilimnion, the warm top layer, the only one the wind keeps stirring;^[3]
• the metalimnion, often called the thermocline, the band between warm and cold water where the temperature drops quickly;^[2]
• the hypolimnion, the cold bottom layer.^[2]

The layers above cut the bottom layer off from the air all summer. As the summer goes on, its oxygen is used up, and in some lakes it runs out completely; many water animals can’t survive in it then, so there is little point fishing the bottom.^[2]

Fall turnover

In fall the surface cools and the layers weaken until the lake mixes again: that is fall turnover.^[2] With the lake about one density from top to bottom, the wind can turn it over completely and bring the bottom water up through the whole lake, and the water’s taste and smell can change for a while.^[3] The deep water is no longer cut off from the rest of the lake.

Winter and spring

Under the ice the order flips: the coldest water, near 0 °C (32 °F), lies just under the ice and is lighter than the slightly warmer water deeper down.^[2] For a short while after ice-out the lake is about one temperature from top to bottom, and the wind mixes it once more: spring turnover. Then the surface keeps warming and the summer layers form again.^[2]

Fishing through the seasons

• Summer. Many anglers fish above or around the thermocline, where the water is cooler than at the surface and the bottom layer’s lack of oxygen is still below them. Find its depth with a thermometer lowered on a line, or watch for a faint band on your sonar, then fish structure at that depth.
• During turnover. With the whole lake mixing, fish can be at almost any depth. Cover water and try several depths rather than sitting on one summer spot.
• After turnover. With the whole lake open to them again, fish can use the deep water too. Deep structure that was empty in August can be worth a look.

Use the depth map

Once you know the depth of the thermocline on your lake, the depth map in HuntGrid Fishing shows where that depth meets the bottom. Set the depth band to the range just above it, and the map lights up the edges, humps and points at that depth so you can go straight to them. Our guides on reading a lake depth map and water temperature go further.

Sources

The facts and figures above come from these official pages, read on October 17, 2026. The words and drawings are ours.

1. Water density, U.S. Geological Survey
2. Water-quality and lake-stage data for Wisconsin lakes, water year 2014 (Open-File Report 2016–1131): water temperature, stratification and dissolved oxygen, U.S. Geological Survey
3. Elimination of thermal stratification in reservoirs and the resulting benefits (Water-Supply Paper 1809-M), U.S. Geological Survey

Further reading

More on the subject from other agencies. Their terms don’t let us reuse their material, so nothing above is taken from them.

• Pond turnover, Missouri Department of Conservation