Lake

A lake is a relatively large, fixed body of water located on or near the Earth's surface.

These bodies of water are localized within a basin or a series of connected basins surrounded by dry land. While lakes are separate from the ocean, they may be linked to it via rivers. Lakes play a vital role in the water cycle, which is the continuous movement of water around our planet. Most lakes contain freshwater, accounting for nearly all the world's surface freshwater. However, some are salt lakes with salinity levels even higher than seawater.

In total, lakes cover approximately 2.5 x 10^6 km², which is less than 2% of the Earth's surface.

Defining the exact difference between a lake and a pond can be difficult. There are no official scientific criteria or international definitions used by all disciplines. Some limnologists, who study inland waters, view lakes as simply larger versions of ponds. They might distinguish them by wind-induced turbulence that mixes the water column or wave action on the shore. Because of this uncertainty, many researchers use size-based definitions. These can range from 2 hectares to 8 hectares for a minimum lake size. The ecologist Charles Elton once suggested a lake must be at least 40 hectares.

In some regions, like Newfoundland, almost every lake is called a pond. In other places, like Wisconsin, the names are often reversed.

Lakes form through many distinct geological processes. Tectonic lakes are created by the movement of the Earth's crust, such as faulting, tilting, or folding. Some of the largest lakes, like Lake Baikal, occupy rift valleys. Volcanic lakes occur when water fills depressions like craters or calderas. For example, Crater Lake in Oregon formed in a caldera after Mount Mazama subsided.

Glacial lakes are the most numerous type in the world. They are created by the direct action of glaciers and continental ice sheets. These include subglacial lakes or epishelf lakes, which are found in Antarctica.

Fluvial lakes are produced by running water. A common type is the oxbow lake, which has a crescent shape. These form when a meandering river cuts through its own bend, leaving a horseshoe-shaped body of water behind.

In 1957, G. Evelyn Hutchinson published a landmark monograph titled *A Treatise on Limnology*. This work provided a comprehensive classification of lake types based on their origin and characteristics. Hutchinson identified 11 major lake types, which are further divided into 76 subtypes. His work remains a fundamental reference for understanding how lakes are distributed and how they function. This scientific foundation helps us categorize the vast variety of water bodies found across the globe.

Water management and movement also define different lake categories. Most lakes have at least one natural outflow, such as a river, to maintain their water levels. However, some lakes are endorheic, meaning they have no natural outflow.

These lakes lose water only through evaporation or underground seepage. This process can lead to very high salt concentrations in the water. Humans also create artificial lakes, which are known as reservoirs.

These are constructed for many purposes, including hydroelectric power, agriculture, and domestic drinking water supplies.

Lakes are distributed widely across the planet, with many located in the Northern Hemisphere. Canada is home to an estimated 2 million lakes, including over 31,752 that are larger than 3 square kilometers. Finland also has a high density of lakes, with 168,000 lakes measuring 500 square meters or larger.

While large lakes hold most of the world's lake water, small ponds are much more abundant. One study estimated there are 304 million lakes and ponds on Earth, with 91% being 1 hectare or less. This is because lake volume increases much faster than surface area.

Interestingly, lakes are not unique to Earth. Extraterrestrial lakes exist on Titan, a moon of Saturn.

The shape of these lakes is very similar to those found on our planet. Additionally, scientists have found evidence that lakes once existed on the surface of Mars, though they are now dry lake beds. Despite their importance, all lakes are temporary over long geological periods. They will eventually fill with sediment or spill out of their basins, changing the landscape once again.