Lakes
Slovenska različica
Author: Boštjan Burger
Definition and Limnology
The definition of lakes or ponds, and even more so wetlands, can vary according to the historical period of definition, countries, and scientists. What constitutes a "lake" is still a matter of debate. According to some definitions, a lake would be composed of fresh water, unlike seas and oceans which are salty. This definition is incorrect because the Baltic Sea has less than 4 g/l of salt, while the Great Salt Lake contains approximately 250 g/l.
Classifying a water body as a lake or pond based on its local name in toponymy is also not reliable: the same water body is sometimes popularly called a pond, a lake, or even a sea. The boundary between the terms lake and pond is blurred. We also know of underground lakes (such as Križna jama in Slovenia) and even underwater lakes, which represent accumulations of hypersaline water on the deep sea floor.
In 1892, the Swiss François-Alphonse Forel, a pioneer of limnology (the science of inland waters), clearly defined the lake. He designated a "lake" as a mass of standing water with no direct connection to the sea, situated in a depression in the ground, enclosed on all sides. A lake is an inland water body representing an accumulation of water in a land depression and, unlike an inland sea, has no direct connection to the world ocean—meaning it has no inflow or outflow due to sea currents. The inflow and outflow are usually small compared to the total volume of water in the lake.
Unlike a river, a lake has no slope. The term "inland lake" is used to distinguish continental lakes from coastal ones (coastal lakes, coastal brackish lakes, or lakes formed by embankments), as well as to denote lakes in general. In limnological terms, a lake is typically much deeper than a pond, pool, or puddle, allowing a stable temperature stratification to develop over days or months. The frequency of their mixing is used to classify lakes, as it carries far-reaching ecological consequences.
Global Abundance and Volume
In this respect, shallow steppe lakes, such as Lake Neusiedl or Lake Balaton, are not considered "true" lakes (limnologically defined as "shallow lakes"). By more commonly used definitions, lakes are only standing waters with a depth of more than two meters. Lakes in the true sense are natural water bodies, though in a broader sense they also include artificial bodies such as reservoirs and mining lakes (e.g., Lake Velenje). However, the precise demarcation between lakes and pools/ponds is vague and subjective.
Therefore, some limnologists call any water-filled depression a lake. For their categorization, it would be irrelevant whether the lake is constantly or occasionally filled with water. Colloquially, the designation often depends on salinity, but this is not a scientific criterion. Although lakes usually contain fresh water, there are also large salt lakes such as the Caspian Sea, the Aral Sea, and the Dead Sea. In these cases, the term "sea" is merely historically/culturally descriptive. There are also soda lakes, such as those in the East African Rift Valley (e.g., Lake Nakuru), Lake Van in Anatolia, and some lakes around Lake Neusiedl.
Further definitions can be made by size. The minimum size for a lake is approximately one hectare. Through satellite observation, about 100 million surface lakes larger than one hectare have been identified on Earth. Combined, they cover more than 300 million hectares or about 2% of the land surface. The share of smaller lakes was long underestimated: in 2006, a global model based on Pareto’s Law doubled previous estimates to a total of 4.2 million square kilometers of land area. If ponds and small reservoirs are included, 4.6 million square kilometers (3%) of the Earth's inland surface would be covered by water.
The depths of the world's lakes are less known and vary significantly by hydrogeological context and season. For instance, Loch Ness in Scotland has an average depth of 132 meters—more than four times deeper than Lake Erie in North America, which is 4,500 times larger in area. Recent models inferring depth from surrounding topography predict a total global lake volume between 160,000 and 280,000 km³.
Cael, in collaboration with Adam Heathcote and David Seekell, developed a new model showing that the number of lakes in a geographical area relates to a mathematical power law: as lakes get smaller, they are more likely to be arranged in clusters. Larger lakes are rarer and more isolated. Small lakes (under 1 ha) are roughly 100 times more numerous than 10 ha lakes. This suggests that the number of small lakes was previously underestimated, leading to an upward revision of the total number of lakes and their surface area, but a downward revision of their average depth.
If these lakes are indeed shallower than thought, they likely emit more methane than expected. Their role as carbon sinks would be less significant, and their contribution to global warming greater. With warming and increased organic input due to human-induced eutrophication and erosion, this contribution could increase further. The latest estimate gives a total volume of 199,000 km³ and an average depth of only 42 meters.
Classification by Origin
In 1957, British ecologist George Evelyn Hutchinson published 'A Treatise on Limnology', which remains the definitive classification of lake types. Hutchinson recognized 11 main types:
1. Tectonic Lakes
Formed by the deformation and subsequent lateral and vertical movements of the Earth's crust (faulting, tilting, folding). Some of the world's largest lakes fill tectonic rifts, such as the East African Rift lakes (Lake Tanganyika) and Lake Baikal in Siberia. Others, like the Caspian Sea, were isolated from the ocean by tectonic uplift.
2. Volcanic Lakes
Formed in volcanic basins such as craters, maars, or calderas. Crater lakes form when precipitation fills a crater faster than it drains. Example: Crater Lake in Oregon. Others form when lava flows or lahars dam rivers.
3. Glacial Lakes
Created by the direct action of glaciers and ice sheets. These are the most numerous lakes globally, including proglacial, subglacial, finger, and epishelf lakes (found primarily in Antarctica).
4. Fluvial (River) Lakes
Formed by running water. The most common is the oxbow lake, created when a river cuts through a meander neck. They can also form where tributary sediment blocks a main river.
5. Karst (Solution) Lakes
Fill depressions formed by the surface dissolution of soluble rocks like limestone. These cavities often collapse to form sinkholes. In Slovenia, smaller ones in closed depressions are called "kali".
6. Landslide Lakes
Formed by the damming of a river valley by mudflows, landslides, or rockfalls. Although often large and deep, they are typically short-lived. Examples include Quake Lake (USA) and Lake Sarez (Tajikistan).
7. Aeolian Lakes
Produced by wind action in arid environments, including basins dammed by sand, interdunal lakes, and deflation basins. Example: Moses Lake in Washington (USA).
8. Shoreline Lakes
Formed by the blockage of estuaries or the uneven accumulation of beach ridges by longshore currents and other coastal processes.
9. Organic (Peat) Lakes
A form of organic lake created where the accumulation of partially decayed plant matter (peat) in wet environments leaves a vegetative surface below the water table for long periods. They are typically acidic.
10. Anthropogenic Lakes
Man-made lakes resulting from human activity, such as damming rivers for reservoirs or the filling of abandoned excavations (e.g., Lake Velenje).
11. Meteorite (Impact) Lakes
Formed by the catastrophic impact of extraterrestrial objects (meteorites or asteroids). Examples include Lonar Lake (India), Lake El'gygytgyn (Siberia), and Pingualuit (Canada). They contain valuable paleoclimatic records.
Adrian Cho (2017), World's lakes are shallower; B.B. Cael, MIT; Geophysical Research Letters, vol. 44; Boštjan Burger, Hydrology of Triglav National Park, 1999; Hutchinson, G. E. (1957). A Treatise on Limnology. Vol.1; Downing, J. A. (2006). The global abundance and size distribution of lakes; Veillette, Julie (2008). Arctic epishelf lakes; Desiage, Pierre-Arnaud (2015). Deglacial and postglacial evolution of the Pingualuit Crater Lake.