Lake turnover - how and why does it happen?Lake waters turnover in the fall and spring. The reasons for the turnover are water density and air temperature. Water is most dense (heaviest) at 39º F (4º C). As the water temperature increases or decreases from 39º F, it becomes increasingly less dense (lighter).
Editor’s note: This is part of a series provided through the Minnesota Department of Natural Resources called “Minnesota’s Waterways: Protecting and Enhancing Watersheds in Minnesota.”
Lake waters turnover in the fall and spring. The reasons for the turnover are water density and air temperature. Water is most dense (heaviest) at 39º F (4º C). As the water temperature increases or decreases from 39º F, it becomes increasingly less dense (lighter).
In summer and winter, the water in most lakes is stratified, with less dense water at the surface and more dense water is near the bottom. Because of the difference in density, the water does not mix well between these layers.
But in late summer and autumn, air temperatures cool the surface water, causing its density to increase. The heavier water sinks, forcing the lighter, less dense water to the surface, until the water temperature at all depths reaches approximately 39º F. Because there is very little difference in density at this stage, the waters are easily mixed by the wind.
In the spring, the process reverses itself.
So why is this important? Over the summer, surface waters are warmed by the sun. Winds and storms cause some mixing of water, but because of the density-temperature relationship, in many lakes, the middle layer acts as a barrier to any mixing of the deeper waters. By the end of summer, the deep water becomes quite depleted of oxygen because no mixing has taken place.
As the days get shorter and cooler, water cools and sinks. When the majority of the water in the lake reaches an approximately uniform temperature, storms and winds begin to overturn and mix all of the water in the lake.
The deep water contains decaying matter and sulfurous gases; when it reaches the surface, it produces an odor that indicates the mixing has begun. Eventually the turnover mixes the entire lake, replenishing the deep waters with life-giving oxygen and cleansing the sulfurous fumes from the water. This allows fish to return to the depths where they will spend the winter months.
Learn more at the Minnesota Department of Natural Resources website, www.mndnr.gov.