Lakes: Definition & Facts

Instructor: Maria Howard

Maria is a teacher and a learning specialist and has master's degrees in literature and education.

Learn about the different types of lakes, from glacier lakes to oxbow lakes, and how they are formed. Explore how lakes are part of every ecosystem and found on every continent.

What Is a Lake?

A lake is a body of water surrounded on all sides by land. Lake water is still or standing, meaning it doesn't flow from point A to point B in the same way a river's does.

Since they are often fed by rivers, springs or precipitation (a.k.a. rain and snow), lakes are primarily freshwater. However, some of the more famous lakes, like the Dead Sea and the Great Salt Lake, are saline lakes and contain only saltwater.

How Are Lakes Formed?

A lake's origins, unsurprisingly, affect its size and shape. Glacier lakes are found in large valleys or hollows carved out long ago by slow-moving glaciers, while smaller kettle lakes are made from depressions left by chunks of glacial ice. The Great Lakes in the American Midwest were formed by glaciers long ago, back when North America was covered in giant sheets of ice and snow.

A kettle lake in Greenland
Photo of a kettle lake

Some lakes, like the Caspian Sea, are created by shifts in the earth's crust where a large depression forms and fills with water. Likewise, crater lakes make their home in circular holes left at the tops of inactive volcanoes.

Crater Lake in Crater Lake National Park, Oregon.
Photo of Crater Lake in Oregon

Lakes that flow into rivers or other outside sources of water are called open lakes. Sometimes a portion of a river is blocked off and forms a closed lake that does not have a way to flow out, like when a beaver dams a part of a river. Occasionally a curved u-shape of a river is separated from the rest by deposited sediment, creating an oxbow lake. Over time, these closed lakes become increasingly salty, with the process of evaporation taking water away and leaving salt behind.

An oxbow lake in Hokkaido, Japan, was formed when sediment built up over time and separated this section from the rest of the river.
Photo of an oxbow lake in Hokkaido, Japan

Occasionally, we create our own lakes, like when we build dams across rivers to hold back freshwater. Man-made reserves of water are called, fittingly, reservoirs, and the water is used in the same way we use other lake water: for agricultural and household use and for recreation. Some reservoirs, like the Hoover Dam, were built to harness the power of water when it is released in large quantities from a great height.

The Gordon Dam in Australia blocks the river
Photo of the Gordon Dam in Australia

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