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Lake Turnover

November 2015
Carver County Water Management Org. Water Column

While swimming in one of our many Minnesota lakes in the summer, if you let your feet hang down, you may notice that as you get deeper, the water gets colder. This happens because our lakes have different layers of water, kind of like a layered cake. These layers are called stratification. Lakes stratify because different temperatures of water have different densities. Generally the warmer the water, the lighter it is and the closer it is to the surface of the lake. Water is the heaviest at 39.2⁰F. Anything warmer or colder than 39.2⁰F will be lighter and sit on top. That’s why ice floats at 32⁰F; it is actually less dense than water.

Lakes typically stratify into three layers: epilimnion (top layer), metalimnion (middle layer), and hypolimnion (bottom layer). In the summer, the top layer is warm due to the sun, the bottom layer is cold and the middle layer is where the temperature change takes place. The blustery winds and temperature changes of fall and spring help mix the lake. In the fall, the top layer cools off and the cooler water sinks. This cooling and sinking continues until the temperature of the epilimnion and hypolimnion match, at which point the whole lake mixes from top to bottom. This is called lake turnover. Lake turnover happens again in the spring as ice melts off the surface and the surface of the water warms up to 39.2⁰F.

Lake turnover renews the lake and provides vital oxygen for the aquatic life at the bottom. Oxygen at the bottom of the lake is often very low and the full lake mixing in fall and spring brings oxygen filled water from the top down to the bottom.

While not all lakes stratify, most lakes in Carver County do. You can about what lakes in Carver County by visiting and looking at lake temperature profiles on our water quality page. Whether a lake stratifies or not depends on the lake’s depth, size of the lake’s surface area (called fetch), the adjacent landscape, and chemistry including salt. Salt makes water denser and prevents mixing.

Twin cities metro area lakes and rivers have seen an increase in salt content of their waters largely due to road salt use during winter months. There are now 26 lakes, 23 stream sections and 1 wetland in the metro area that are impaired by salt. Along with affecting lake turnover, too much salt can harm fish and other wildlife and affect the diversity of plant and animal life in waters.

Public agencies in the metro area are working together to reduce salt use while maintaining safety of winter roads. Many have certified their snow plow drivers through trainings in practices to reduce salt use while also maintaining safety. Homeowners and business owners can also help reduce salt use by following these steps.

1) Shovel. The more snow you shovel, the less ice is created and less salt is needed.
2) Don’t over apply. More salt does not equal more melting. Use only 4 lbs. per 1000 ft2. One lb. of salt is about 1 twelve oz. coffee cup.
3) Temperature matters. Typical salt (NaCl) used for safety is ineffective at melting snow or ice below 15⁰F. Instead make sure you’ve shoveled and use a small amount of sand for traction.
4) Sweep up extra. Salt and sand on dry pavement isn’t doing any work and will wash away. Sweep up the extra and reuse it.





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