Does Okoboji turn over?

Yes, though scientists prefer to call this “mixing”. The surface and bottom waters of all lakes mix, but West Okoboji – compared to other Iowa lakes - is unique because it only mixes twice a year, in the spring and fall, because it is so deep, while shallow lakes can mix on a daily basis. During the summer, the lake surface heats faster than the water underneath. Because cold water is denser than warm water, thermal layers develop with warm surface water overlying a layer of cold, deep water. When the wind blows, these layers mix, but not in deep lakes like Okoboji. The upper warm layer is about 30 feet deep and stays stable all summer long no matter how hard the wind blows until fall. Then, shorter days and dropping temperatures cause the surface water to cool and become denser. The denser water sinks, replacing and mixing the underlying water, which mixes its way up to the surface in what is popularly called ‘turnover’. From fall through winter water temperatures stay fairly uniform under the ice- just under freezing. When spring arrives, however, the melting ice cools the underlying water, making it denser, and once again causing it to sink and mix with the bottom water the mixes up to the surface. Each time this happens, the bottom waters bring organic material with them to the surface, in a sense fertilizing the lake and starting a new cycle of aquatic life over. In the

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