The lake thermocline, explained for anglers and swimmers

Through late spring and summer, many deep lakes split into layers. The sun warms the top, and warm water is lighter than cold water, so it floats on top. The result is three zones:

• Epilimnion: the warm, well-mixed surface layer, stirred by wind.
• Thermocline: a band where the temperature drops quickly with depth, often several degrees per few feet.
• Hypolimnion: the cold, dark bottom layer, cut off from the surface until fall.

Why anglers care

Many fish hold near the thermocline in summer. Above it the water can be too warm; below it oxygen can run low, because the deep layer does not mix with the air for months. Fish finders often show the thermocline as a faint band of clutter, and baitfish and game fish stack up just above or within it.

Why swimmers notice it

Dive down a few feet on a hot day and you may hit noticeably colder water. On deep, clear mountain lakes the cold layer can be close to the surface even in August, which is part of why those lakes feel so cold.

When it disappears

In fall the surface cools until the whole lake is close to the same temperature. Wind then mixes it top to bottom. That event is called lake turnover. Shallow lakes often never form a lasting thermocline because wind keeps them mixed all summer.