The deeper we go the water temperatures will warm to as much as 39 degrees at the reservoir’s bottom. Working down through the ice, we will ultimately find liquid water, which at the ice’s edge will be approximately 32 degrees. At this point the lake will have a coat of ice that will be colder than 32 degrees Fahrenheit at the lake surface. Let’s start with a Colorado lake that is moderately deep, say 50 feet, just prior to spring thaw. Turnover is best understood by walking through the four seasons and looking at the water processes that are occurring. Consequently, in the spring and the fall around ice-off and ice-up there is a short period when the water temperature of a lake is 39 degrees throughout, allowing for water to mix or turnover if you will. Without this property, large bodies of water would be frozen solid anywhere it gets cold enough to freeze, as frozen water would sink to the bottom until only a skim of liquid water existed on the surface. Which means ice is less dense allowing it to float on its liquid counterpart. In order to understand turnover, one needs to understand that water is densest at approximately 39 degrees Fahrenheit. There’s little doubt in my mind that many use the term “turnover” to describe any change in water conditions, especially if those conditions are visual, smelly and/or associated with a fish kill, all symptoms of events other than turnover. I’m fairly sure most of these “turnover” reports are actually algae blooms. Often these are made during the summer or early fall. Every year I see several conditions report stating the lake is turning over.
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