Lake turnover in fall and spring

Turnover is when a lake mixes from top to bottom. It happens because water is densest at about 39°F (4°C).

Fall turnover

All summer, a warm surface layer floats on top of cold, deep water, separated by the thermocline. In autumn, cool nights chill the surface until it is as dense as the water below. With nothing holding the layers apart, a few windy days mix the whole lake. Depending on location and depth this usually happens between September and November.

Spring turnover

In northern lakes that freeze, the process repeats in spring. Once the ice goes out and the surface warms to about 39°F, the lake mixes again before summer layering sets in.

What you notice

• Murky or discolored water as sediment and organic matter from the bottom are stirred up.
• A sulfur or musty smell for a few days, from low-oxygen bottom water reaching the surface.
• Odd fishing. Fish scatter throughout the water column because temperature and oxygen are suddenly similar at every depth. Many anglers find a slow week or two, then a strong fall bite once the lake settles.

Turnover is a normal, healthy process. It refreshes oxygen in the deep water and resets the lake for the next season.