Do lakes have a bottom?

Young lakes have rocky bottoms. They're deep and cold, the banks are steep and the shoreline vegetation is mostly pines and other conifers. Lake Superior is an example of an 'oligotrophic' lake. Middle-age lakes have sandy bottoms with very few aquatic weeds.



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The material at the bottom of a lake, or lake bed, may be composed of a wide variety of inorganics, such as silt or sand, and organic material, such as decaying plant or animal matter.

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Three distinct layers develop: The top layer stays warm at around 65–75 degrees F (18.8–24.5 degrees C). The middle layer drops dramatically, usually to 45–65 degrees F (7.4–18.8 degrees C). The bottom layer is the coldest, staying at around 39–45 degrees F (4.0–7.4 degrees C).

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Lakes formed by the erosive force of ancient glaciers, such as the Great Lakes, can be thousands of feet deep. Some very large lakes may be only a few dozen feet deep — Lake Pontchartrain in Louisiana has a maximum depth of only about 15 feet.

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Because warm water sinks in very cold freshwater, fish in these water bodies often gather in groups near the bottom. Some species, like koi and gobies, may burrow into soft sediments and go dormant like frogs and other amphibians, but most fish simply school in the deepest pools and take a winter rest.

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Technically, a dead zone is hypoxic (water with low levels of dissolved oxygen) or anoxic (water that does not contain dissolved oxygen) areas without enough dissolved oxygen to support most aquatic life. Dead zones can form after waters become stratified in the summer and surface and bottom waters do not mix.

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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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Underwater Hazards Unlike controlled swimming pools, lakes often harbor hidden hazards beneath the surface. Submerged rocks, fallen trees, sharp branches, glass products, and other trash, or uneven terrain can cause injuries if swimmers are not cautious or are unaware of their presence.

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Keep it safe Particularly if you're a beginning cold water swimmer, have someone with you. The cold water can increase the likelihood of heart problems or cramps that even experienced pool swimmers aren't expecting. If you're not a confident swimmer, it's better to start in warmer and predictably shallow waters.

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Our bodies adjust best to the cold water if you slowly wade in. A quick entry into especially cold water can be a shock that leaves you gasping and could lead to drowning. A slow entry is key. Once you're up to your shoulders, dunk in all the way a few times and you're good to go.

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