This occurs during what is called the "fall lake turnover." During summer, lake surfaces heat up, while the lower water remains cooler. This is called stratification. The lower layer tends to become anoxic (without oxygen) because the layers do not mix. As the temperature decreases in the fall, the top layer becomes denser and tends to sink to the bottom and, with the assistance of some wind action, mixing occurs. The clarity of the lake usually decreases because mixing brings up nutrient-rich water from the bottom of the lake and causes the lake to look cloudy. Sometimes there is a distinct odor as decomposing plants, algae and other matter comes to the surface. The timing and duration of fall turnover depends on the size and depth of the lake. Sometimes it can occur in a few days, sometimes it takes a week or longer until the water becomes thoroughly mixed, oxygen mixes throughout the water, and the temperature becomes uniform from top to bottom.