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Natural Lakes
Natural lakes fill depressions in the landscape caused by geologic processes. Many lakes in North America fill low spots scooped out by the ice in glaciers. These include very large lakes such as the Great Lakes and Great Slave Lake, as well as many of the smaller lakes in Wisconsin and Minnesota. Other lakes, some quite large, fill low spots in closed drainages that result from earth movement related to geological faulting. Examples of such lakes include the Great Salt Lake in Utah, Humboldt Sink in Nevada and Lake Baikal in Russia.
Lakes Behind Dams
An impounded lake is usually a man-made lake, such as the reservoir trapped behind a dam. Examples of impounded lakes created by dams include Lakes Powell and Mead on the Colorado River of the western U. S., the impoundment of the Three Gorges Dam in China and many smaller reservoirs around the world.
Dug Lakes
A second form of man-made impounded lake is the dug lake. Water in these lakes is impounded in a depression created by removing the soil and piling it around the edge of the lake as a berm. Several examples of dug lakes can be found in Ohio, including Grand Lake St. Marys, which is nine miles long.
Naturally Impounded Lakes
A few lakes are the result of natural impoundment. These are lakes created when a river is blocked by a landslide, lava flow or other natural barrier. Just like a man-made dam, water fills the valley behind this natural dam until it is high enough to flow over a low spot in the dam. A well-known example is Lower Slide Lake near Jackson Hole, Wyoming, which was created in 1925 when a massive landslide blocked the Gros Ventre River. During the Ice Ages, water impounded behind a ridge of ice filled a lake 600 meters (roughly 2,000 feet) deep covering a large portion of western Montana.