Lake

• 1. Introduction

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  A lake is a naturally occurring body of water sitting in a basin, surrounded by dry land, and cut off from the ocean even when a river links the two. Picture all of them together. Every lake and pond on Earth covers roughly 2.5 million square kilometers, which is less than 2 percent of the planet's surface. Most hold fresh water. They account for almost all of the world's surface freshwater. Yet a few are saltier than the sea itself. There is no agreed line between a lake and a pond, no scientific border that everyone honors. So how do you decide what counts? Why does almost all the lake water on Earth sit inside fewer than a hundred giant basins? And what does it mean that every lake, no matter how vast, is temporary? These questions run beneath the still surface, waiting.

• 2. What Counts As A Lake

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  The word lake travels back through Middle English lake, Old English lacu meaning pond or pool or stream, and the Proto-Germanic lakō, all the way to a Proto-Indo-European root meaning to leak or drain. The English words leak and leach share that ancestry. So does the German Lache, a puddle, and the Icelandic lækur, a slow flowing stream. The vocabulary itself blurs the boundary between a lake and a trickle. That blur runs deep. Limnologists, the scientists who study inland waters, have called a lake simply a larger pond, one big enough for waves to strike the shore or for wind to stir the whole water column. None of these definitions cleanly excludes a pond, and all are hard to measure. Size-based rules try to settle it, ranging from a minimum of 2 hectares up to 8. Charles Elton, a pioneering animal ecologist, drew his own line at 40 hectares. Naming defies all of it. As one textbook puts it, "In Newfoundland, for example, almost every lake is called a pond, whereas in Wisconsin, almost every pond is called a lake." One hydrology book tries to pin the term down with five characteristics. A lake fills one or more connected basins, holds essentially the same water level throughout, takes no regular intrusion of seawater, traps a good share of its suspended sediment, and exceeds some chosen threshold of area such as a single hectare. Other hydrologists accepted or expanded on every one of these except the rule about seawater.

• 3. Where The Water Sits

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  Most of the world's lakes are freshwater, and most lie in the Northern Hemisphere at higher latitudes. Canada alone, shaped by a deranged drainage system, holds an estimated 31,752 lakes larger than 3 square kilometers, with a total count thought to reach at least 2 million. Finland counts 168,000 lakes of 500 square meters or more, and 57,000 of those rank as large. The full tally of lakes on Earth stays unknown, because most are too small to appear on any map or satellite image. One widely cited study estimated 304 million lakes and ponds worldwide. It found that 91 percent of them measure 1 hectare or less. Tiny ponds vastly outnumber large lakes. Despite that, nearly all of Earth's lake water sits in fewer than 100 large lakes, because lake volume scales superlinearly with area. A small jump in surface area means a far larger jump in the water held below. Lakes maintain themselves through balance. Most have at least one natural outflow, a river or stream that drains excess water and holds the average level steady. Some have no outlet at all and lose water only to evaporation or underground seepage. These are called endorheic lakes. Look beyond Earth and the catalog continues. Saturn's moon Titan carries lakes whose shapes closely resemble those here. Mars once held lakes on its surface, now reduced to dry beds.

• 4. The Hutchinson Classification

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  In 1957, G. Evelyn Hutchinson published a monograph called A Treatise on Limnology, treated ever since as the landmark classification of lake types by origin. His scheme recognizes 11 major types divided into 76 subtypes. The first is the tectonic lake, born from the Earth's crust faulting, tilting, folding, and warping. Some of the largest lakes on the planet are rift lakes lying in rift valleys, including the Central African Rift lakes and Lake Baikal. Crustal extension can carve alternating grabens and horsts, elongate basins beside mountain ranges, and in arid zones these trap salt lakes such as Great Salt Lake and the Dead Sea. Volcanic lakes fill craters, maars, and the larger calderas left by eruptions. Crater Lake in Oregon sits in the caldera of Mount Mazama, which collapsed in a massive eruption around 4860 BC. Among all lake types, volcanic crater lakes come closest to a perfect circle. Glacial lakes, carved by glaciers and ice sheets, are the most numerous lakes in the world, and most lakes across northern Europe and North America were shaped by the latest glaciation. Epishelf lakes, found mostly in Antarctica, trap a layer of meltwater behind an ice shelf. Fluvial lakes come from running water, the most common being the crescent-shaped oxbow lake, formed when a meandering river cuts through the neck of a horseshoe bend and abandons it. Solution lakes occupy basins where precipitation has dissolved bedrock into sinkholes, abundant along the Dalmatian coast of Croatia and across much of Florida. The remaining types stretch the definition further: landslide, aeolian, shoreline, organic, anthropogenic, and meteorite lakes.

• 5. Lakes Born From Catastrophe

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  Quake Lake came into being through the 1959 Hebgen Lake earthquake, which loosed a landslide across a river valley. Landslide lakes form when mudflows, rockslides, or screes dam a valley, most often in mountains, and though they can grow large and deep, they tend to be short-lived. The danger does not end when they form. A landslide dam can burst suddenly long after, threatening everyone downstream as the water drains. In 1911, an earthquake triggered a landslide that blocked a deep valley in the Pamir Mountains of Tajikistan, creating Sarez Lake. The Usoi Dam at the base of that valley has held for more than 100 years, but the terrain below remains in danger of catastrophic flooding should a future earthquake break it. Meteorite lakes record a different kind of impact, gouged out when meteorites or asteroids strike the Earth. Lonar Lake in India, Lake El'gygytgyn in northeast Siberia, and the Pingualuit crater lake in Quebec all formed this way. At El'gygytgyn and Pingualuit, the basins hold sedimentary deposits that preserve long records of past climate. Lakes can also vanish in moments. On the 3rd of June 2005, in Nizhny Novgorod Oblast, Russia, a lake called Lake Beloye disappeared within minutes. Officials theorized that a shift in the soil beneath it had opened channels that drained the water toward the Oka River.

• 6. The Hidden Layers

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  Water reaches its maximum density at 4 degrees Celsius, and that single fact governs how a lake arranges itself. Professor F.-A. Forel, remembered as the Father of limnology, was the first to classify lakes by their thermal stratification, a system later refined by Hutchinson and Löffler. In a stratified lake, cold dense water forms the bottom layer, the hypolimnion. Above it lies a transition zone, the metalimnion. At the surface sits the warmer, lighter epilimnion. Lakes split into two camps. Holomictic lakes share a uniform temperature top to bottom at some point in the year, allowing their waters to fully mix. Meromictic lakes keep layers that never intermix, and their deepest water holds no dissolved oxygen, so no aerobic life survives there. That stillness lets undisturbed sediment build into lacustrine deposits, a slow archive at the lakebed. A paired black and white band in varved sediment marks a single year, the dark layer from winter die-off and the white from sparse summer deposition. Stratification carries a hidden hazard in deep tropical lakes, whose surface water never cools to maximum density and so never mixes. Carbon dioxide and other gases can saturate the depths until an earthquake or landslide stirs them loose. The deep water rises, pressure drops, and a vast cloud of gas erupts from solution. This is a limnic eruption. The disaster at Lake Nyos in Cameroon shows the cost, since carbon dioxide is heavier than air and can flow down a valley to settlements, causing mass asphyxiation.

• 7. Reading The Water

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  Limnology divides every lake into three zones: the littoral close to land, the photic open-water zone bathed in sunlight, and the dim profundal or benthic zone far below. How deep light reaches depends on turbidity, set by the density and size of suspended particles. Those particles color the water. Decaying plant matter can turn it yellow or brown, algae lend a greenish cast, and in very shallow water iron oxides stain it reddish brown. The Secchi disk measures all of this, a 20-centimeter disk of alternating white and black quadrants lowered until it vanishes from sight. That vanishing point, the Secchi depth, is a standard test for eutrophication. Nutrients sort lakes along their own scale. Oligotrophic lakes are nutrient-poor and clear, mesotrophic ones sit in the middle, and eutrophic lakes brim with nutrients that fuel plant growth and algal blooms. Beyond them lie hypertrophic lakes, so overloaded, often by heavy fertilizer use in the catchment, that clarity collapses and oxygen falls. A lake also steadies the land around it, because water has a very high specific heat capacity of 4,186 joules per kilogram per kelvin. By day a lake cools the shore with a sea breeze, and by night it warms the shore with a land breeze.

• 8. Records And Disappearances

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  The Caspian Sea is the largest lake by surface area at 371,000 square kilometers, counted as a lake in geography despite its name. The deepest is Lake Baikal in Siberia, its bottom reaching 1,637 meters and its mean depth of 749 meters the greatest in the world. Baikal is also the oldest lake and the largest freshwater lake by volume at 23,600 cubic kilometers. The longest is Lake Tanganyika in Tanzania, stretching about 660 kilometers along its center line. The lowest is the Dead Sea at 418 meters below sea level, one of the saltiest. The world's highest commercially navigable lake is Lake Titicaca in Peru and Bolivia at 3,812 meters, also the largest in South America. Only about 20 ancient lakes have lasted over a million years. The rest are passing. A lake fills with sediment and becomes a wetland, a swamp or a marsh, and reeds speed the closing as they decay into peat that chokes the shallows. Trees can eventually root in and turn the wetland to forest. Permafrost holds some lakes in place, and its thawing may explain the shrinking of hundreds of large Arctic lakes across western Siberia, as warming soil lets the water drain into the ground. Human hands accelerate the loss. The shrinking Aral Sea has been described as "murdered" by the diversion of its feeder rivers for irrigation. Between 1990 and 2020, more than half of the world's large lakes decreased in size, partly because of climate change.