In winter, the exact opposite happens since the lakes are covered with ice. Most of the water under the ice is 39 Fahrenheit; however, there is a thin layer of water under the ice that is colder than 39 and therefore less dense.
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Surface Temperatures. —During the winter, from the latter part of December to the breaking up of the ice in the spring, the temperature of the water under the ice is 32 F. The water then warms at a uniform rate to 72 F.
As a result, close to freezing, colder water floats to the top and the warmer water sinks to the bottom. The density of water as a function of temperature can be seen in the plot on the right. Eventually, the coldest water, which has floated to the top of the lake in wintry conditions, freezes to form a layer of ice.
In water that is around the freezing point, a person is likely to survive only 15 to 45 minutes with flotation and possibly up to an hour or so with flotation and protective gear before the brain and heart stop (Table 1). The surface temperature of Lake Superior in early to mid-summer is about 40 to 50 F.
Health risksWinter swimming can be dangerous to people who are not used to swimming in very cold water. After immersion in cold water the cold shock response will occur, causing an uncontrollable gasp for air. This is followed by hyperventilation, a longer period of more rapid breathing.
Do fish die in frozen lakes or in lakes that are partially frozen? Since fish are cold-blooded animals, they can survive because they are able to regulate their body temperature to match their environment. However, they could die if a body of water freezes over completely and remains frozen for an extended period.
Great Lakes that have completely frozen include Lake Superior, Lake Huron, Lake Erie, and Lake Ontario. Lake Michigan is the only Great Lake to have never frozen entirely.
Most lakes and ponds don't completely freeze because the ice (and eventually snow) on the surface acts to insulate the water below. Our winters aren't long or cold enough to completely freeze most local water bodies. This process of lakes turning over is crtically important to the life in the lake.
Lake ice freezes first at the surface starting at the edges or shoreline for two reasons. Water near the shore is typically shallower and contains less heat than deeper water so it can reach the freezing point faster than deeper water.