A lake is any relatively large body of slow-moving or standing water that occupies an inland basin. Lakes are found mostly in high northern latitudes and in mountain regions. Many lakes are also found in lowlands near the sea, especially in wet climates. The study of lakes is called limnology.

Lake formation

Most of the world's existing lakes were formed by glacier action. During the Ice Age great sheets of glacial ice crept slowly southward across northern North America, Europe, and Asia, carrying masses of dirt and debris from the rocky surface below. With the force of their movement, the glaciers dug several basins in the weaker areas of rock. Some basins were also formed at places where glaciers left behind some of their debris. This dammed up former river valleys. Most lakes in central Canada, Minnesota, Michigan, Wisconsin, and similar glaciated regions lie mainly in these types of basins, called drift basins.

Some lakes are a result of both massive erosion and great deposits left by former glaciers. The Finger Lakes of western New York lie in old river valleys hollowed by ice and dammed by glacial deposits. The Great Lakes of North America lie in ancient river valleys or lowlands that were dug deeper by the glaciers. Glacial deposits, called moraines, built the rims of the lakes. The Great Lakes cover about 95,000 square miles (245,000 square kilometers) and form the largest expanse of freshwater in the world. Lake Superior is the largest of all freshwater lakes in surface area. Only the Caspian Sea, a saltwater body of water, is larger.

Some lakes have been formed by volcanic action. The craters of extinct or dormant volcanoes commonly hold lakes. One of the best-known examples is that of Crater Lake in Oregon, which is 1,932 feet (589 meters) deep.

More than four fifths of the Earth's freshwater is in the form of glaciers, polar ice sheets, and groundwater. A lake may contain either freshwater or salt water. Freshwater lakes constitute about 0.009 percent of the Earth's free surface water, while saltwater lakes account for a somewhat smaller percentage.

A lake's life span

Lakes can be very short-lived. Inflowing streams carry sediment into a lake that starts filling the basin. If the basin is filled enough for the lake to overflow, the outflowing stream can erode an edge of the basin and thereby drain away the water. Finally, the organic deposits from vegetation that accumulate at the bottom of lakes may also cause shallow lakes to become bogs or swamps, and, ultimately, dry land.

Lakes as a resource

Freshwater lakes are used for various purposes. One is channeling water for drinking and bathing. Lakes are also used for power generation, irrigation, and recreation.

The enormous demands made on freshwater lakes by modern industrial societies have led to pollution. The discharge of harmful chemicals or other industrial waste disturbs the normal biological activity in lakes. In turn, the use of polluted water is a threat to human health.

When lakes are used by modern power plants, it results in thermal pollution or the heating of lake water. In large amounts, chemical and thermal pollution can destroy all animal and plant life in a lake.