![]() ![]() ![]() Large and very deep lakes will obviously take longer for the phenomenon to be completed.Īs winter approaches, the water that has now reached 39 degrees sinks to the bottom, allowing colder and less dense, buoyant water to remain at the surface to freeze. ![]() Very shallow lakes - with little or no thermocline - may experience little if any noticeable turnover. Not all lakes experience turnover to the same extent or in the exact same way, due to things like depth, bottom structure and size. Just an extra note, since this page seems to get a lot of views, especially in the fall of the year. Eventually the turnover mixes fresh oxygen into the entire lake mass, replenishing the deep waters with the life-giving stuff and cleansing the sulfurous fumes from the water, allowing fish to return to the depths where they will spend the winter months. The deep water contains an abundance of decaying matter and sulfurous gases when it reaches the surface, it produces a telltale odor that indicates the process has begun. Now, storms and sustained high winds can begin to perform the task of overturning and mixing all of the water in the lake - referred to as fall turnover. At about 50 degrees, the cooler water (with a higher oxygen content) at the surface begins to sink into and through the thermocline, forcing warmer and less dense water to the surface, eventually erasing the temperature stratification built up over the summer.Īt some point, the majority of the water in the lake reaches an approximately uniform temperature. Toward the end of summer, the deep water becomes quite depleted of oxygen because no mixing has taken place.Īs the days get shorter and cooler, and energy is transported away from and out of the lake, mixing becomes easier. The middle layer, known as the thermocline, acts as an effective barrier to any mixing of the deeper waters. Because of the density-temperature relationship, many lakes in temperate climates tend to stratify, that is, they separate into distinct layers. Winds and storms can cause some mixing and do add some oxygen atmospheric oxygen is added by the air-water interaction to the oxygen produced within the water by aquatic plants.īut there is too much difference in temperature between the surface water and that at depth to allow for complete mixing of all the water in the lake. Starting in the spring and over the course of the summer, surface waters absorb a lot of the sun's energy and can heat extensively, causing them to become quite buoyant. So how does all of this relate to lake turnover (which by the way can actually take place once or several times per season, depending on many additional factors)? Let's first consider the fall turnover. Everything beneath the surface of the ice never gets any colder than 39 degrees Fahrenheit. It also stops further energy loss from the lake. Why then does this not happen? The chemistry of the water molecule dictates that at 39 degrees (39.20 to be precise) Fahrenheit, water actually expands and becomes less dense, allowing it to float above the warmer water! The water that cools below that temperature, to 32 degrees, freezes and stays on the top, effectively capping the lake. meaning that the lake would freeze from the bottom up - eventually destroying all life beneath the waves. With continued cooling at the surface, the decreasing temperature should (or so it would seem) eventually cause all the water to eventually freeze solid and sink to the bottom. In any case, as water cools to that temperature, it does, as predicted, contract and become more dense, ultimately sinking to the bottom of the lake and pushing the water it has displaced to the surface, where it too can cool. Well, it is wonderful for lakes and the life in them, not so for the roads on which we drive - more on that later. Something strange and wonderful happens when water reaches a temperature of approximately 39 degrees Fahrenheit (40 Celsius). at least to a point.Īnd that point is where the oddity owing to water's special chemistry comes into play, the one that keeps your favorite lake - and your favorite gamefish - healthy and happy. Able to exist in all three forms, water, as a liquid, contracts and becomes more dense as it gets colder. This principal holds for solids, liquids and gases. In general, as things get colder, they contract and become more dense/heavier as they get warmer, they expand and become less dense/lighter. ![]()
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