Every autumn, something remarkable happens beneath the surface of America’s deepest lakes. As temperatures drop and winds pick up, these massive bodies of water literally flip upside down in a process called lake turnover. This fascinating natural phenomenon affects everything from fish behavior to water clarity, yet most people have never heard of it.
Lake turnover is the process that describes when a lake de-stratifies and is able to mix from top to bottom. What makes this so dramatic is that for many lakes deeper than about 20 feet, distinct and thermally separated layers of water form during the majority of the year, and these layers prevent the lake from mixing and aerating. Come fall, this stable system completely breaks down, creating a massive mixing event that can be seen, smelled, and felt by anyone near the water.
Lake Mendota, Wisconsin – The Academic Superstar
Lake Mendota, within the Yahara Watershed, is “dimictic,” meaning it undergoes turnover twice during the year. Located on the University of Wisconsin-Madison campus, this lake has been studied more extensively than perhaps any other in America. With its large size and deep water, Lake Mendota is the last lake in the Madison Chain to turnover.
The turnover here is so predictable that researchers can almost set their calendars by it. When the lake’s water reaches approximately 39 degrees Fahrenheit, the surface water becomes denser than the other water in the lake’s water column. Students often notice the telltale signs first – the water turns murky, and there’s an unmistakable earthy smell that signals the process has begun.
Lake Monona, Wisconsin – The Urban Lake’s Drama
Also part of Madison’s chain of lakes, Lake Monona turns over earlier than Lake Mendota due to its characteristics. This lake sits right in the heart of Madison, making its autumn transformation visible to thousands of daily commuters. The turnover here creates quite a spectacle as the normally clear water becomes cloudy and takes on that distinctive “turned over” appearance…