What usually happens to lakes over time?

All lakes, even the largest, slowly disappear as their basins fill with sediment and plant material. The natural aging of a lake happens very slowly, over the course of hundreds and even thousands of years. But with human influence, it can take only decades. A lake's plants and algae slowly die.



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The middle layer, the thermocline, mixes and turns over throughout the year. It turns over due to climate, nutrient variations, and geologic activity such as earthquakes. However, major lake turnover happens during the fall and spring, when the lake's cold and warm waters mix and readjust.

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Twice a year, unseen forces churn water from the depths of our deeper lakes and deliver oxygen and nutrients essential to aquatic life. This temperature-driven process of lake “turnover” allows aquatic life to inhabit the entirety of the lake as oxygen becomes more available.

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A lake has turned over when water temperatures are the same from the surface to the bottom. The process can take days or even months to complete, depending on lake shape and depth, and air and water temperatures.

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Most lakes have at least one natural outflow in the form of a river or stream, which maintain a lake's average level by allowing the drainage of excess water. Some lakes do not have a natural outflow and lose water solely by evaporation or underground seepage, or both.

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Over a very, very long time, the lake will accumulate sediment and move from being a young oligotrophic lake, to middle-aged and mesotrophic, to old and eutrophic, and finally the lake will 'die' once it has completely filled in.

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Still Water Lakes and reservoirs are examples of non-moving waters, or rather, less fast moving water. Even lakes and reservoirs have places where water enters and exits. Most lakes formed naturally thousands of years ago as glaciers gouged depressions in the landscape, which later filled with water.

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Lakes lifespans are limited, as rivers dump their sediment into them and dead plant material builds up on the lake bottom. Most lakes are less than 10,000 years old.

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Dead zones are generally caused by significant nutrient pollution, and are primarily a problem for bays, lakes and coastal waters since they receive excess nutrients from upstream sources. Excess nitrogen and phosphorus cause an overgrowth of algae in a short period of time, also called algae blooms.

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It may surprise you to know there is no technical difference between lakes and ponds, according to the National Park Service(Opens in a new window). However, a general rule of a thumb is if a body of water is large and deep it's a lake and if it's short and shallow it's a pond.

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Turnover is like being at the mall … or at a fair. Turnover is a natural way the lake cleans up harmful bacteria and algae. It carries dead algae down into the depths of the lake where there is less sunlight, helping to prevent algae growth.

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The turnover can cause fish kills in some areas, but it is more common on smaller lakes with little moving water or in backwater creek areas where fish can't escape the water that is lacking oxygen.

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