What happens to a lake when it freezes?

Once an ice film has formed on the surface of the lake and the air above the ice continues to be colder than the ice, the ice will thicken. The cold air above the ice causes heat to leave the lake water under the ice and from the ice itself. This causes the water below the ice to freeze into successively deeper layers.



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Additionally, if ice forms too thickly on the surface of the water, it can prevent oxygen from diffusing into the water and causing widespread fish deaths. Another problem that can occur when a lake freezes is that of entrapment. If there are cracks or holes in the ice, fish may swim into them and become trapped.

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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.

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Only the top layer of the lake or river freezes. Underneath the frozen upper layer, the water remains in its liquid form and does not freeze. Also, oxygen is trapped beneath the layer of ice. As a result, fish and other aquatic animals find it possible to live comfortably in the frozen lakes and ponds.

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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.

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The high concentration of salt in ocean water lowers its freezing point from 32° F (0° C) to 28° F (-2° C). As a result, the ambient temperature must reach a lower point in order to freeze the ocean than to freeze freshwater lakes.

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Therefore, below freezing temperatures are needed for a week or more to form ice on a large lake. As I have mentioned before, water is a unique substance in that the solid form (ice) is lighter than the liquid form (water).

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Once the surface water cools to approximately 32 degrees Fahrenheit, the water molecules crystallize into interlocking lattice-like patterns and ice is formed.

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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.

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Because of the time required to melt ice that has thickened over the winter, the clearing of lake ice occurs some time after average daily air temperatures rise above freezing. Typically the lag is on the order of one month at latitude 50° N and about six weeks at 70° N.

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If water were most dense as a solid, lakes would freeze from the bottom up, eventually freezing solid. In that case, little or nothing would survive in the lake. Most lakes and ponds don't completely freeze because the ice (and eventually snow) on the surface acts to insulate the water below.

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Health risks Winter 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.

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As a general rule of thumb, ice is safe to walk on when there is 4 inches or more of clear ice. It is important to understand that different types of ice varying in their strength. Clear blue or black ice is the strongest. This ice is formed from the lake water freezing.

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During some really cold winters, some lake ice can be as much as six feet thick! In winter most fish spend their time near bottom, where the food and warmest (39.2°F) water lies.

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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.

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If you can see the ground through the ice, then there's going to be warmer water under it. Out in the middle, the ice grows thicker over the cold dark depths. Also, ice always melts away from the shore first.

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Ice is seldom the same thickness over a single body of water; it can be two feet thick in one place and one inch thick a few yards away.

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