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How do the epilimnion, thermocline, and hypolimnion differ?

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lake Lake Ann in North Cascades National Park, Washington, U.S., viewed from the park's Maple Loop Trail. The North Cascades National Park is a large wilderness area that preserves majestic mountain scenery, snowfields, glaciers, and other unique natural features.

In deep, permanent freshwater lakes, thermal stratification often creates three distinct vertical layers during the summer months. These layers differ primarily in their temperature, density, and mixing characteristics:1

Epilimnion: This is the uppermost, relatively warm layer of a lake. It is well-mixed by wind action, resulting in a more uniform temperature and higher concentrations of dissolved oxygen compared to deeper layers.231
Thermocline (or Metalimnion): This is the middle transition zone that separates the warmer epilimnion from the colder, deeper water. It is characterized by a rapid decrease in water temperature with increasing depth.145
Hypolimnion: This is the bottom layer of the lake, located below the thermocline. It consists of cold, stagnant water that remains at a relatively uniform temperature. Because it does not mix with the surface layers during the summer, it often contains lower concentrations of dissolved oxygen.31

These layers are typically seasonal in temperate regions; as surface waters cool in the autumn, the density differences decrease, eventually leading to a complete mixing or "overturn" of the lake water.16