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Why does a lake’s surface water sink and mix with deeper water in autumn?

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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 autumn, a lake’s surface water sinks and mixes with deeper water due to changes in water density driven by cooling temperatures.

During the summer, lakes often experience vertical stratification, where a warm, less dense upper layer (the epilimnion) floats atop a colder, denser lower layer (the hypolimnion). As autumn begins, the surface water loses heat to the atmosphere, causing it to cool. Because water becomes denser as it cools toward 4 °C (39 °F), the surface water eventually reaches a density that causes it to sink.12

This sinking process, combined with the mechanical energy provided by wind, facilitates the circulation or "overturn" of the entire water column. This mixing continues until the lake reaches a uniform temperature or until surface ice forms, which protects the water from further wind-driven mixing.12