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Lakes

Lakes

Lakes and ponds are bodies of standing fresh water impounded in basins and depressions in the earth's continental crust . Lakes are temporary catchment basins for flowing surface and groundwater . Freshwater reservoirs form behind natural and man-made dams, surface water collects in topographic lows, and groundwater discharges into ephemeral lakes, but eventually all continental runoff drains to the ocean. Lakes provide humans with fresh drinking water, recreation areas and, in the case of the world's largest lakes, navigable waterways for ship traffic. Regional climate strongly affects the chemical and hydrological properties of lakes, and lake sediments often provide high-resolution records of climatic fluctuations. Lake basins typically fill with interlayered coarse and fine sediments, and organic material. Many ancient lacustrine deposits contain petroleum reservoirs. Because ponds, lakes, and inland seas are smaller and less well-mixed than the oceans , they are particularly susceptible to pollution.

Tectonic motion created the crustal basins and sags that contain the world's largest lakes. Elongate, deep lakes fill the axes of incipient divergent plate tectonic boundaries, or rift zones. The lakes of the East African Rift systemLakes Turkana, Kiva, Tanganyika, and Malawifill the central grabens of the rift zone between the African and Somali Lithospheric Plates . Lake Baikal, the worlds deepest (5,370 ft,

or 1,637 m) and most voluminous (Lake Baikal contains about 20% of the earth's fresh surface water) lake, occupies a rift valley in southern Siberia. Lakes also fill broad, shallow intercra-tonic basins that form during the earliest stages of continental rifting . Lake Eyre in central Australia , and Lakes Victoria and Chad in Africa are examples of lakes in shallow extensional basins.

Many modern lakes, including the Great Lakes of North America , occupy basins created by Northern Hemisphere ice sheets of the Pleistocene Epoch . The weight of the Laurentide and Eurasian ice sheets depressed large regions of the continental crust into the mantle, a phenomenon called glacial isostasy . Since the ice sheets retreated about 20,000 years ago, meltwater and stream runoff have collected in these broad depressions. Large regions of the northern continentsthe Great Lakes region and the Scandinavian Peninsula for exampleare presently undergoing rapid uplift, known as isotatic rebound, as these glacially depressed regions continue to readjust. Small ponds and lakes are also common in glacial environments. Erosion by moving ice carves bedrock depressions where lakes form, and leaves sills that impound glacial streams. Glacial sedimentary landforms , including moraines , kame terraces, and eskers serve as natural dams for glacial lakes. Glacial terrains are dotted with small ponds that fill circular depressions called kettles that form when ice blocks buried in glacial till deposits melt.

Lake basins also form in a number of other geologic environments. Small lakes and ponds are common in continental fold belts where outcrops of resistant bedrock divert and dam perennial streams. Abandoned meanders along low-gradient streams form circular lakes called oxbow lakes. Groundwater discharge zone lakes form where the top of the saturated zone , the water table , intersects the land surface. In humid and temperate climates, where the water table is close to the land surface, discharge lakes typically have an outlet stream. In arid regions, ephemeral groundwater discharges into closed, saline playa lakes that fill and dry seasonally.

Man-made lakes are a significant component of the earth's present-day hydrologic cycle . Most of the world's rivers have been dammed, creating reservoirs for human water supplies, recreation, and generation of electrical power. While reservoirs provide many benefits to human populations, they also force numerous readjustments to natural and artificial systems. Ecosystems must compensate for the loss of drowned habitats, human populations are displaced, and water quality is often compromised. Streams that have been segmented by dams regrade their equilibrium profiles, creating new patterns of erosion and deposition throughout the stream system. In fact, natural stream processes act to remove obstacles like dams by eroding the streambed below them, and depositing sediment in the reservoir above them. Poorly constructed and maintained dams are thus a safety hazard for downstream inhabitants.

Climatic factors control the chemical and hydrological properties of lakes. Regional variations of temperature , precipitation , and winds determine water levels, circulation patterns, vertical stratification, and the concentration of dissolved materials in lake water. The quantity and seasonality of rainfall in a drainage basin controls the balance between recharge and discharge that maintains lake level. Lake salinity is a function of the relative concentrations of dissolved ions and diluting water. During a drought , lake levels fall, salinity increases, and a lake can change from a permanent freshwater reservoir to an ephemeral saline lake. The Great Salt Lake in Utah is all that remains of Lake Bonneville, a much larger freshwater lake that existed during the wet period at the end of the Pleistocene Epoch. A 25% decrease in freshwater flow to the Aral Sea in central Asia has led to a 50% decrease in surface area and a four-fold increase in salinity since the 1960s.

Seasonal temperature variations, changes in the balance between precipitation and evaporation , and wind patterns affect lake circulation and stratification. High-latitude lakes that are subject to large diurnal temperature variations and strong winds are typically so well mixed that the water column is unstratified. Warm, stagnant low-latitude lakes are often permanently stratified. Without mixing, the lower water column of these stagnant, oligomictic lakes becomes depleted in oxygen , and aquatic plants choke the ecosystem in a process called eutrophication. Human water pollutants that contain phosphatesdetergents for examplealso encourage eutrophication. Temperate lakes that experience large seasonal temperature fluctuations undergo seasonal overturns in which a layer of cold surface water circulates to the bottom of the lake or the pond. This process of periodic restratification oxygenates the base of the water column and infuses lake-bottom ecosystems with nutrients. Lakebed sediments record these seasonal patterns, and can be used to deduce and date the regional climate history. Lake stratigraphers, or limnologists, use features like preserved pollens and winter-summer couplets of thin sedimentary laminae, called varves, to recreate the geochronology of a lake basin.

See also Hydrogeology

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Lakes

234. Lakes

See also 356. RIVERS ; 360. SEA ; 414. WATER .

lacustrine
of or pertaining to lakes.
limnophobia
an abnormal fear of lakes.

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Lakes

LAKES

LAKES . Water, essential to life on earth, has occupied a preeminent place in religious thought and imagery, together with the land and sky. In many cultures it is considered to be procreative, a source of forms and of creative energy. The life-giving property of water has been projected in its almost universal perception as fons et origo, "spring and origin," the element that preceeds solid form and is the support of all earthly creation. In this context, from remote times to the present, among peoples who have perceived the world in terms of sacred and profane phenomena, springs, ponds, and lakes have figured importantly in the realm of water symbolism. In many regions of the world where lakes are major geographic features, they often have been the setting of cosmogonic myths and have been invested with many meanings, historical associations, and ritual functions.

The importance of sacred lakes in cultural context will be discussed by examining the ritualistic and mythic significance of two American lakes, Titicaca and Texcoco, associated with the Andean and Mexican civilizations respectively. The areas around both these lakes have been heavily populated in ancient, colonial, and modern times. Accounts of the ancient ceremonial pageantry, mythology, and human-made or natural sacred places in and around these lakes have been reported since the sixteenth-century Spanish conquest. Their meanings and functions in the evolution of native American civilizations continue to form an expanding field of inquiry in archaeology, art history, and ethnology, as well as in the history of religions.

Lake Titicaca

Lake Titicaca lies between southern Peru and northwestern Bolivia, where the Peruvian intermontane valleys and rugged cordilleras give way, at the 12,500-foot level, to the spacious Altiplano. This lake is impressive in its size and is the highest navigable body of water in the world. It sustained the agricultural and economic life of the surrounding areas. As the mainstay of complex societies, the religious meaning of the lake was most dramatically defined in the case of the civilizations of the Tiahuanaco (c. 100 to 1000 ce) and the Inca (c. 1400 to 1532 ce). Near Lake Titicaca, the principal archaeological ruins are those of Tiahuanaco, located in Bolivia a few miles inland from the southern shore. This was an important political and religious center whose influence spread over large section of Bolivia and southern Peru. The Titicaca basin came again within the orbit of an imperial state during the fifteenth century, when the Inca nation extended political control from the capital of Cuzco, some 200 miles to the north. At this time the Inca nation affirmed a spiritual and historical connection with the earlier Tiahuanaco state, and the Inca ruler Tupac Yupanqui visited the islands of Titicaca and Koati on the lake and commissioned shrines there. Inca interest in the lake was expressed in religious art and architecture, the location of major shrines, and the incorporation of ancient myths concerning the lake into their own mythology. By these means the lake's ancient significance continued to remain part of an imperial sacred geography.

In Andean religion the border between the notion of deities and the phenomena of nature was entirely open, with emphasis placed on direct communication with the elements of nature. The worship of huacas and major nature deities was a basic theme of Andean religion. A huaca was an object or phenomenon that was perceived to have unusual presence or power beyond the range of everyday life, where the sacred may have been manifested or where the memory of some past momentous event resided. It could be the locus of an oracle, a cave, or a curiously indented boulder where a people were thought to have emerged from the earth during the time of creation. This belief system was closely tied to the formation of sacred geographies and formed part of a cosmological religion with an array of gods associated with natural epiphanies.

The island of Titicaca, about seven miles long, serves as a good example of how a lake figured in Andean sacred geography. Adolph Bandelier's explorations and interpretive report of 1910 remain fundamental to an understanding of the island and its ruins. Toward the northern end of the island there is a construction, and it was across this isthmus that a precinct wall was built to separate sacred from proface space. Early Spanish accounts record that three gates were arranged in succession here and that confessions were required of all who sought to pass through. The religious and ritual focus of the site lay beyond the gates. The sacred feature was comprised of a great rock about 25 feet high and 190 feet long, with a broad plaza or assembly ground built in front. This was the chief huaca of the island, named in Aymara titi ("wild cat") kaka ("rock"), the latter word a substitution for kala ("stone"). The shrine rock was thus the source for the name of the lake itself. Also included within this sacred precinct were burial cysts with offerings and paraphernalia, storehouses, and residences for cult priests, officers, and aides. In this context it is important to mention the Pilco-Kayma building on Titicaca Island, which corresponds to another structure called Inyak-Uyu on Koati Island nearby. The design and siting of these two buildings reveal an important aspect of Andean religion. Both buildings stand near the eastern shores of their respective islands, and the principal apartments in each ruin, with the most elaborate entrances and prominent niches, open toward the majestic snowcapped peaks of Sorata and its neighbors across the lake on the Bolivian (eastern) side. These grand mountains even today continue to be worshiped by the people in their vicinity. Considered in relation to Lake Titacaca and the island of Titicaca with its huge rock huaca, the mountains complete the imago mundi of Altiplano peoples.

The Inca people paid homage to Titicaca Rock as the dominant and central feature associated with the lake. This is illustrated by Spanish written accounts of an annual pilgrimage made to the island across the straits from a shrine on nearby Copacabana Peninsula. In that festival, which centered around solar events, two principal idols were brought in reed boats from Copacabana: a statue of the sun father, Inti, and one of the moon mother, Mama Quilla. These two effigies were regarded as husband and wife, and they were transported with other idols dedicated to thunder and other natural forces. The sun was represented in the form of an Inca of gold embellished with much brilliant jewelry; the moon was represented as a queen of silver; and thunder was a man of silver, also very brilliant. Once landed, they were placed in splendid litters decorated with flowers, plumage, and plates of gold and silver, and they were carried to the sacred enclosure. The idols were set up in a plaza, almost certainly in front of the sacred rock. After having placed the idols, the attendant Inca priests and nobles prostrated themselves, first worshiping the effigy of the sun, then that of the moon, afterward that of the thunder, and then the others. The prostrations were concluded by blowing kisses to the images and to the huaca itself. Dances, banquets, and amusements were then held to close the festival.

Even today, Titicaca Island is known as the "island of the sun," while Koati is the "island of the moon." Yet it is clear that, although the cult of these celestial bodies was maintained upon the islands by the Inca, they remained subordinate to the primary cult of Titicaca Rock itself.

What then, was the meaning of the sacred rock, the dominant icon of the island? What was its relationship to the surrounding waters of the lake? The answers lie in mythology. Bandelier's compilation of myths recorded by the sixteenth-century Spanish chronicler Cieza de León include a text in which the Indians tell of an event that occurred before the Incas ruled in these lands. Long ago, they went without seeing the sun for a long time and suffered greatly, so they prayed to their gods, begging them for light. The sun then rose in great splendor from the island of Titicaca, within the great lake of the Collao (the ancient name of province), so that all were delighted. Then from the south there came a white man of large size who showed great authority and inspired veneration. This powerful man made heights out of level plains and flat plains out of great heights, and created springs in live rock. Recognizing in him such power, they called him Maker of All Created Things, Beginning Thereof, and Father of the Sun. They also said that he gave humans and animals their existence and that they derived from him great benefits. This being, called Ticciviracocha, was regarded as the supreme creator. Another myth, recorded by Juan de Betanzos before Ceiza, also connects two successive "creations" of the world by "Con Tici Viracocha" to Lake Titicaca (Bandelier, 1910, pp. 298299). In yet another version, the sun and moon were said to have risen from Titicaca itself. In this mythological context, "wild cat rock" must be seen as a cosmogonic place of origin.

Rising from the windswept sheet of reflecting water, the island hills and promontories are removed from the sphere of ordinary life. On the ridges, marine fossil strata underscore the theme of aquatic emergence at this unusual site. The placement of the huaca and the relationship of buildings to the distant mountains are joined with ritual and mythic imagery in a powerful metaphor of humanity and land. The sense of place, of being "at the center," is also linked to notions of history, for the ancient myths and the architectural features of the Pilco-Kayma building (designed in an archaic style) reminded the Inca of Tiahuanaco and established a succession to that old imperial tradition. In this respect, the Inca shrine incorporated a sense of the past and signaled territorial possession. Woven into these levels of meaning was a still more fundamental theme. Most of all, the setting was designed to bring to mind the time and place of the beginnings. The sacred lake was the primordial natural icon, a reminder of illud tempus. Passive in the mythic imagery, the lake formed the fluid cosmogonic field from which all forms came forth in darkness. Upholding the island birthplace of the sun and moon, Lake Titicaca, as the home of Viracocha, who gave form to mountains, plains, and people, was the element from which the world itself was made.

Lake Texcoco and the Valley of Mexico

Rimmed by mountains and snowcapped volcanoes, the Valley of Mexico is a spacious basin that formerly contained a system of shallow interconnected lakes. The central lake, known as Texcoco, was saline from evaporation, but the southern lakes of Chalco-Xochimilco were fed by abundant aquafers that issued from the base of the steep Ajusco Mountains. To the north, lakes Zumpango-Xaltocán depended more on seasonal rain, but there is evidence that in ancient times the surrounding hills and open fields were watered by abundant springs and streams. In a collective sense, the entire set of lakes may be referred to as Lake Texcoco.

By the first century ce, the city of Teotihuacan began to dominate the lesser settlements in the Texcoco lakeshore region. A powerful manufacturing, trading, and religious center of some one hundred thousand people, Teotihuacan became the center of a trade network that ramified to the most distant parts of Mesoamerica. With the violent eclipse of this metropolis in the seventh century, power was transferred to other capitals throughout the neighboring highlands. The old ascendancy of the Valley of Mexico was not restored until the fifteenth century with the rise of the Méxica-Aztec capital of Tenochtitlán and its allied neighbor, the city of Texcoco. Built on an island and reclaimed marshes near the western shore of Lake Texcoco, Tenochtitlán became the most feared and powerful city, the seat of the most powerful empire in Mesoamerican history.

An agricultural economy, supplemented by fishing and the gathering of natural products, remained fundamental to urban life throughout the long history of the valley, and the problem of maintaining fruitful relationships between humans and nature formed an underpinning of religious life. To express this relationship in symbolic form, monumental works of art and architecture were built as stage sets and memorials for seasonal rites as well as the important ceremonies of government and war. The ruined pyramids of Teotihuacan were the largest in Mesoamerica, and long after the city was destroyed they were visited in pilgrimage by the rulers of the later Méxica-Aztec state. In the middle of the Méxica-Aztec capital, Tenochtitlán, a new pyramid and attendant temples were built in a great quadrangular enclosure, with gates at the cardinal directions. The main pyramid, representing a symbolic mountain with dual shrines to the rain god, Tlaloc, and the Méxica national ancestor hero, Huitzilopochtli, established the vertical axis mundi of the cosmological design. A similar but smaller ritual center was constructed in the allied city of Texcoco.

In addition to such urban monuments, other shrines and temples were scattered throughout the valley on mountaintops, in caves, by springs and rivers, and in the waters of the lake itself. These places were the shrines of nature deities whose cults were also represented in the temples of the city. These cults, in addition to those of conquered nations, were woven into the religious fabric of the city in an effort to form an embracing state religion. The many divinities were impersonated by ritual performers on festival occasions. The costumes often visually corresponded to the cult names themselves: For example, Chalchiuhtlicue, a female deity of water on the ground, that is, a lake, river, or spring, would appear with a green-painted skirt or a skirt sewn with pieces of jade. Chalchiuhtlicue means "jade skirt" in the Nahuatl language. Thus the costume was an ideogram, and the impersonator became a living, moving metaphor naming the element of nature that she represented.

An illustration of this custom is recorded by the sixteenth-century chronicler Alvarado Tezozomoc, who describes a ceremony that took place to inaugurate an aqueduct built from mainland springs to the island of Tenochtitlán. The emperor Ahuizotl instructed two high priests to be attired as Chalchiuhtlicue and go welcome the incoming water. As the water arrived, they sacrificed quail and burnt copal incense. After drinking, the chief priest spoke directly to the water: "Be very welcome, my lady, I come to receive you because you shall be coming to your home, to the middle of the reeds of Mexico-Tenochtitlan" (Alvarado Tezozomoc, 1975). The passage shows how a deity impersonator would also address the natural element whose symbolic form he represented. In this way of thought, the elements themselves were seen to have life-force and were considered inherently sacred. Lake Texcoco was spoken of as Tonanhueyatl, "our mother great water," a provider of moisture to agricultural fields who was teeming with edible algae, aquatic plants of various kinds, mosquito eggs (also edible), shrimp, a diversity of fish, as well as frogs, ducks, and other aquatic birds. As a sustainer of life, the lake was looked upon as the mother of Tenochtitlán.

Pilgrimages were made by the Aztec and their neighbors to sources of water at springs, streams, and lakes, as well as in hidden caves and ravines on cloudy mountaintops. At such places it was common practice to offer green stones and jewelry as well as sacrifices. A preoccupation with fertility was paramount among the reasons why water was so widely venerated. Nowhere was this more apparent than in an elaborate annual pilgrimage made by the ruler of Tenochtitlán and three allied rulers to shrines on the summit of Mount Tlaloc and in the middle of Lake Texcoco. The relationship between these two water shrines shows that no part of the natural setting could be considered in isolation, and that the imagery of sacred geography, based upon the ecological structure of the land, established fundamental integrating bonds between society and nature.

The bonds between humans and nature are evident with the sequence of rituals, beginning at Mount Tlaloc. The archaeological ruins of the Tlaloc temple are located below the summit of this mountain, close by a grassy vale where springs are still located. Here the ruins of a square courtyard enclosed by masonry are entered via a long narrow walkway that had a controlling function in ritual procedure. Within, there was a flat-roofed chamber housing the main Tlaloc effigy, around which were clustered a group of lesser idols. These were intended to represent the other mountains and cliffs surrounding Mount Tlaloc. Thus the arrangement was a microcosm of the land itself, the symbol of a geographical setting where rain and springs were seen to originate.

This shrine was visited in late April, at the height of the dry season, by the ruler of Tenochtitlán and the allied rulers of Texcoco, Tlacopán, and Xochimilco (the number four was a ritual requirement). The pilgrimage was the duty and privilege of royalty alone. The ceremonies opened with the sacrifice of a male child, followed by a hierarchical procession in which the kings approached the idol in order of rank (Tenochtitlán first). One by one, they proceeded to dress the idols with splendid headdresses, breechcloths, various mantles, jewelry, and so on, according to the status of each monarch.

The next phase again involved a procession in order of rank, as the rulers approached with food for a sumptuous repast. After the food was put before the images, a priest entered to sprinkle everything with blood from the sacrificed child. The blood offering at Mount Tlaloc had a contractual function. As chief ritualists of their respective nations, the rulers set in motion a vital principle that unified the rain and mountains with their people, circulating life and energy throughout the social and ecological orders. Correspondingly, the structure of political alliances was reinforced through sacramental rites.

While these events were taking place, another rite was unfolding in the main religious precinct of Tenochtitlán. A large tree was brought in and erected in the courtyard of the dominating pyramid, on the side of the Tlaloc shrine. This tree, called Tota ("father"), was surrounded by four smaller trees in a symbolic forest designed according to the center and the cardinal directions. A girl attired as Chalchiuhtlicue to represent the great lake and other springs and creeks was brought to sit within the forest. A long chant with drums was then begun around the seated figure, until news was finally received that the rulers had completed the Mount Tlaloc offerings and were now at the Texcocan lake-shore, ready to embark in their canoes. At this time, the Chalchiuhtlicue impersonator was placed in a canoe at Tenochtitlán, and the Tota tree was also taken up and bound upon a raft. Accompanied by music and chanting, a vast fleet of canoes filled with men, women, and children embarked with the symbolic figures to a sacred place within the lake called Pantitlan. This was the site of a great spring, an aquifer that welled up from the lake bottom with remarkable turbulence. At this site the two processions met and, as the rulers and population watched, the Tota tree was unbound and set up in the muddy lake bottom by the spring. The Chalchiuhtlicue child was then sacrificed and her blood was offered to the waters, along with as much jewelry as had been given on Mount Tlaloc. The theme of water as fons et origo was strikingly expressed, incorporating the renewal of vegetation and of life itself at the height of the dry season. The ceremonies were then concluded and everyone departed, leaving the Tota to stand along with others of previous years. Diego Durán (1971) remarks that the peasantry went on to the preparation of the fields, continuing to make offerings at local springs and rivulets.

The imagery of this long and remarkable sequence of ceremonies was directly based on the ecological structure of the highland basin. The relationship of mountains to rain, mountains to springs, and springs to the great lake was symbolically acknowledged in covenants and pleas for water, crops, and vegetation. This communal ceremony, in which the major rulers and lords of the valley participated, affirmed a topographic metaphor: atl tepetl (lit., "water-mountain"), which means "city" or "community." In the Nahuatl language, the habitat of humanity was defined in terms of landscape elements that made life possible. The structure of the ceremony and the metaphor it brought to mind represent a powerful integrating principle that was known and recognized by everyone. Rooted in what was seen and experienced in the land itself, the imagery of the Tlaloc-Chalchiuhtlicue rites represented a sense of order in the highland way of life and symbolically legitimized the governments with which it was identified.

Conclusion

In the Andes and Mexico, sacred lakes formed part of religious systems that grew out of landscapes. The patterns become evident upon considering the ethnohistoric texts, archaeological monuments, and new ethnological reports of religious practices in the context of topography. At the time of the Inca and Méxica-Aztec empires, lakes were seen as sources of life where the generative, procreative qualities of water were especially concentrated. The properties of lakes were acknowledged in myths and metaphors, as in Andean cosmogonic stories and the atl tepetl theme of highland Mexico; in the powerful imagery of ritual; and in the design and disposition of monuments in the city and the country. These symbolic forms of representation celebrated the dynamic relationship of lakes and other features of the natural and human-fashioned environment.

These concerns were fundamentally bound with fertility and agriculture, but the imagery of lakes was also creatively employed by ruling elites in building imperial domains. Myths, rites, and monuments affirmed territorial claims, consolidated alliances, and validated the larger interests and policies of state organizations. Rooted in cosmogony and the seasonal cycle, the symbolism of lakes was inseparably interwoven with the imagery of history. In New World Indian religions, the order of the cosmos and the structure of the state were inseparably bound.

See Also

Caves; Mountains; Water.

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Reinhardt, Johan. The Nazca Lines: A New Perspective on Their Origin and Meaning. Lima, 1985.

Rowe, John Howland. "Inca Culture at the Time of the Spanish Conquest." In Handbook of South American Indians, vol. 2, pp. 183330. Washington, D. C., 1946.

Sahagún, Bernardino de. Historia general de las cosas de la Neuva Espana (compiled 15691582; first published 1820). Translated by Arthur J. O. Anderson and Charles E. Dibble as Florentine Codex: General History of the Things of New Spain. 13 vols. in 12. Santa Fe, N. Mex., 19501982. See especially book 1, The Gods; book 2, The Ceremonies; and book 12, The Conquest.

Soldi, Ana Maria. "El agua en el pensamiento andino." Boletín de Lima (1980).

Sullivan, Thelma D. "Tlaloc: A New Etymological Interpretation of the God's Name, and What It Reveals of His Essence and Nature." In Proceedings of the Fortieth International Congress of Americanists, vol. 2, pp. 213219. Rome, 1974.

Townsend, Richard F. "Pyramid and Sacred Mountain." In Ethnoastronomy and Archaeoastronomy in the American Tropics, edited by Anthony F. Aveni and Gary Urton, pp. 3762. New York, 1982.

Townsend, Richard F. "Deciphering the Nazca World." Museum Studies 11 (1985): 116139.

New Sources

Demarest, Arthur A. Ideology and Pre-Columbian Civilizations. Santa Fe, 1992.

Los Misteros del Lago Sagrado (Mysteries of the sacred lake). Marco A. Ninamango Jurado, editor. Lima, 1998.

Palao Berastain, Juan. La Religión del Titikaka: Revelaciones del Yatiri. Puno, Peru, 2001.

Salles-Reese, Veronica. From Viracocha to the Virgin of Copacabana: Representation of the Sacred Lakes at Lake Titicaca. Austin, Tex., 1997.

Smith, Michael E. The Aztecs: The Peoples of America. Cambridge, Mass., 1998.

Stuart, Gene S. The Mighty Aztecs. Washington, D.C., 1981.

Townsend, Richard F. The Aztecs. New York, 1992.

Richard F. Townsend (1987)

Revised Bibliography

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Lakes

Lakes

Introduction

A lake is a body of water that is entirely enclosed by land or, in the case of an artificially constructed lake, a barrier such as the wall of a dam. How large the body of water has to be to be considered a lake is subjective. A very small enclosed body of water is typically referred to as a pond. Lakes can range in size from less than a mile long to Lake Superior—one of the Great Lakes that at 350 mi (563 km) in length is the largest freshwater lake in the world. Lake Superior and the other four Great Lakes are so large that they are also referred to as inland seas.

As with Lake Superior, almost all lakes contain freshwater. But there are saltwater lakes. Examples include Utah's Great Salt Lake, Qinghai Lake in the Tibetan region of China, and the Dead Sea located between Israel and Jordon.

Lakes can be indicators of climate change. In an obvious example, a prolonged drought can cause a lake to shrink in size or dry up completely. As well, the chemistry of lake water can change, as has occurred with many lakes in the Northern Hemisphere that have been affected by acidic precipitation (acid rain) due to atmospheric pollution.

Evidence is accumulating that the deterioration of lakes is being influenced, perhaps determined, by global climate change.

Historical Background and Scientific Foundations

Whereas Earth is more than 4 billion years old, the oldest lakes that have formed naturally date back only 25 to 30 million years. This is because their formation was due to erosion—the gradual wearing away of rock to create the pocket in which water would eventually collect. Many lakes in the mid-to-upper latitudes of the Northern Hemisphere are even younger. They were scoured out by the southward movement of ice sheets during the last ice age that began about 70,000 years ago and ended about 10,000 years ago. With the melting of the glaciers in the warming climate of that time, the scoured out areas filled with water. The Great Lakes were formed in this way.

Lakes can exist underneath ice sheets. The best-known example is Lake Vostok in Antarctica. The lake water is able to remain in a liquid form because the overlying ice insulates the lake, retaining the heat energy released into the water from, as one example, underground (geothermal) heat sources.

Lakes also formed when portions of Earth's crust shifted upward to create mountain ranges such as the Rocky Mountains in the western United States and Canada. A bowl-shaped depression formed during the upheaval could also fill with water. The crater of an inactive or dormant volcano can also fill to create a lake, such as Crater Lake in southern Oregon.

Lakes can be created artificially by physically blocking the flow of water in a river. The barrier causes water to accumulate upstream. This is how a dam is created. The controlled release of water through the barrier can then be used to generate electricity. The Three Gorges Dam, which is already partially operational, spans the Yangtze River in China. This dam is huge—1.5 mi (2.4 km) wide and more than 600 ft (183 m) high (approximately the height of a 50-story building). The water that has accumulated behind the Three Gorges Dam has widened the Yangtze River, creating a lake that is 375 mi (604 km) in length; as of 2007, nearly 1.5 million people have been displaced from their homes due to the rising water.

WORDS TO KNOW

ACID RAIN: A form of precipitation that is significantly more acidic than neutral water, often produced as the result of industrial processes.

DROUGHT: A prolonged and abnormal shortage of rain.

EROSION: Processes (mechanical and chemical) responsible for the wearing away, loosening, and dissolving of materials of Earth's crust.

EUTROPHICATION: The process whereby a body of water becomes rich in dissolved nutrients through natural or human-made processes. This often results in a deficiency of dissolved oxygen, producing an environment that favors plant over animal life.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

ICE AGE: Period of glacial advance.

ICE SHEET: Glacial ice that covers at least 19,500 square mi (50,000 square km) of land and that flows in all directions, covering and obscuring the landscape below it.

INDUSTRIAL REVOLUTION: The period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.

REFLECTANCE SPECTROSCOPY: The study of the spectral (wavelength or frequency) properties of light that has been reflected or scattered from a solid, liquid, or gas. It has many uses in Earth sciences; for example, reflectance spectroscopy of near-infrared light is a rapid, inexpensive way of characterizing organic substances and has been used to infer past climate changes by examining the content of lake sediment layers.

TROPHIC: Relating to feeding. Communities of living things can be imagined as forming a pyramid defined by trophic levels, with each level feeding on the one below it.

WETLANDS: Areas that are wet or covered with water for at least part of the year.

Very often a lake is open-ended. That is, water enters one part of the lake via a river, stream, or runoff from the surrounding land, and exits the lake at a lower altitude, usually also via a stream or river. The time for the volume of water of such a lake to be fully replaced varies considerably, depending on the lake volume and speed of the water flow in the lake. Smaller lakes may replace the volume of water within several months, while the mammoth Lake Superior requires approximately 190 years to turn over the volume of water.

Other lakes are not open-ended. Water enters via an underground source and/or surface runoff and gradually evaporates. As long as the quantity of water entering the lake exceeds the amount that evaporates, the lake's volume will stay about the same. However, decreased entry of water, due to a drought for example, or increased evaporation, will cause the lake to shrink or even dry up.

Once formed, a lake can often support life. (Lakes that are very acidic or contain high levels of toxic compounds will not support most types of life.) This ability is dependent on the types and amount of food in the water. Depending on the nutrient richness, lakes are classified as oligotrophic (nutrient-poor, few plants, very clear water); mesotrophic (average level of nutrients, clear water); eutrophic (nutrient-rich, abundant plant growth, can develop explosive growth of algae); and hypertrophic. The last type of lake is in poor shape; it contains excessive amounts of nutrients that can cause the overgrowth of organisms to the point where the oxygen is depleted from the water, killing most of the living things in the lake. At this point the lake is effectively dead; recovery, if it occurs, can take decades. Hypertrophic lakes are a hallmark of human activity, in particular the runoff of fertilizer.

As the nutrient level increases in a lake, promoting more abundant growth of algae, the water becomes more murky. The clarity of the water can thus be used as a means of assessing the health of a lake. A Secchi disk—a plate-shaped object that has alternating black and white colored zones—is lowered into the water using a rope that is measured out in feet or meters. The depth that the disk can no longer be seen, known as the Secchi depth, is determined and related to a table to indicate the trophic status of the particular lake.

Impacts and Issues

Eutrophication of lakes is a reflection of the local climate. This is particularly true for lakes that are within a suburban area. Fertilizer runoff can be a death knell for a lake. In many municipalities, homeowners whose properties border a lake are required to leave the immediate lakefront portion of the property untouched. This natural zone acts as a buffer to runoff, decreasing the amount of noxious chemicals that enter the lake.

On a much larger scale, the health of lakes is influenced by the global climate. As one example, a study published in 2003 in the journal Science documented how Lake Tanganyika, a lake in equatorial Africa that is one of the oldest and deepest in the world, is being adversely affected by global warming. Lake Tanganyika requires thousands of years to completely replace the volume of water. By sampling the lake for more than a century, scientists have been able to gauge the long-term changes in the water, since pollutants that have collected over that time and that do not degrade will likely still be present. The data reveal a lake whose health has been in decline beginning in the mid-twentieth century. Furthermore, greenhouse gases have appeared in the water, first at the surface and up to 150 ft (46 m) deep in the water. Given the large volume of water in the lake, finding gases at such a depth means that there are very high levels of greenhouse gases present.

In China, water sources that supply the Yellow and Yangtze rivers are declining. Here, as elsewhere in the world, the glaciers that feed the rivers have shrunk considerably, in this case by 30%. Wetlands and lakes have also shrunk in size. The cause has been attributed to global warming.

Research conducted in Canada has established that the Canadian high arctic is also being affected by global warming. Using a technique called reflectance spectroscopy, scientists have measured lake sediment for a compound called chlorophyll a, which is produced by plants. Because plants require warmth to grow, chlorophyll a can be used to gauge the length of growing seasons over time in the sediment that piles up at the lake bottom over years, decades, and centuries. Analyzing a core of sediment provides a glimpse back in time. Scientists have demonstrated increasing levels of chlorophyll a, indicative of longer growing periods of warmth, beginning about 150 years ago, coincident with the Industrial Revolution, when humans began to have an appreciable impact on the atmosphere.

These and other studies from lakes all over the world are providing convincing evidence that the climate-induced changes to lakes are a global phenomenon. The most likely reason for this, according to a majority of scientists, is global warming.

Even the geographically immense Great Lakes may not be spared. The majority of Canadian and U.S. scientists who study the Great lakes agree that the water level in the entire Great Lakes system could decline by at least several feet over the next 100 years. If so, coastal regions, especially the wetlands that are vital for maintaining healthy ecosystems, would be drastically affected. Commercial shipping through this important transportation corridor to the Midwest could also be hampered, as could the use of the lakes to generate hydroelectric power.

See Also Great Lakes; Oceans and Seas; Rivers; Wetlands.

BIBLIOGRAPHY

Books

Dennis, Jerry. The Living Great Lakes: Searching for the Heart of the Inland Seas. New York: St. Martin's Griffin, 2004.

Grady, Wayne. The Great Lakes: The Natural History of a Changing Region. Vancouver: Greystone Books, 2007.

Periodical

Verburg, Piet, Robert E. Hecky, and Hedy King.“Ecological Consequences of a Century of Warming in Lake Tanganyika.” Science 301, no. 5632 (July 25, 2003): 505–507.

Web Sites

“Climate Change Shrinking Chinese Rivers, Scientists Say.” Canadian Broadcasting Corporation, July 16, 2007. <http://www.cbc.ca/technology/story/2007/07/16/china-rivers.html> (accessed November 9, 2007).

“Climate Change Transforming Alaska's Landscape.” National Research Council of Canada, September 28, 2005. <http://cisti-icist.nrc-cnrc.gc.ca/media/press/alaska_e.html> (accessed November 9, 2007).

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Lakes

Lakes

Introduction

Lakes are bodies of water that are completely surrounded by land. Lakes are not directly connected with the ocean, although a lake may drain to the ocean via a river. They range in size from small bodies that may be only a few hundred yards at their widest point to bodies of water such as the Great Lakes that are dozens of miles in width with depths that easily accommodate the passage of huge cargo ships.

The estimated number of lakes around the globe exceeds 300 million. The 122 largest lakes, each with an area of 386 square mi (1,000 square km) or more, contains almost 30% of Earth’s total inland water.

Almost all lakes contain freshwater. Exceptions include Utah’s Great Salt Lake, Caspian Sea (at over 134,750 square mi [349,000 square km] the largest lake in the world), Aral Sea, Dead Sea (which separates Israel and Jordan, and, at 1,378 ft (420 m) below sea level, represents the lowest point of land on the planet), and Bras d’Or Lake in Cape Breton Island, Canada (which empties directly to the ocean at its northern end). Most lakes are located in the Northern Hemisphere, particularly at higher latitudes. Canada claims over 60% of the planet’s lakes.

Lakes are particularly susceptible to pollution, as runoff from the surrounding land empties into the water. If the rate at which the volume of water in the lake is completely replaced is long, the slow turnover of the total volume of water may support the growth of potentially harmful microorganisms, and chemicals that can be hazardous to health even at low concentrations will be present in the water for a longer time. Many municipalities that have small lakes within their boundaries will regularly check the water quality during the summer months, when they are used for recreation. Tests that detect unacceptable levels of indicators of water quality can lead to the closure of the affected lake to swimming until the source of the problem has been identified and corrected.

Runoff of chemicals such as phosphate and nitrogen, which are used as a food source by some microorganisms in the water, can lead to an explosive increase in the number of the microbes with the subsequent depletion of oxygen in the water, which can be lethal to fish and other life in the lake. This phenomenon, which is called eutrophication, can occur even in large lakes; an example is Lake Erie, which in the 1960s was in danger of becoming incapable of supporting life.

Lakes can also be affected by human activities, both directly (such as by excessive withdraw of water) and indirectly (drying up due to hotter and drier weather that is thought to be due at least in part to global warming).

Historical Background and Scientific Foundations

Lakes are formed naturally in a number of ways. As a portion of Earth’s surface heaves upward, or as two adjacent plates of Earth’s pull apart, depressions that are created will fill with water over time. Lake Baikal located in southern Siberia and Lake Tanganyika located in central Africa were both formed by tectonic activity. These lakes are over 20 million years old and are the deepest lakes known. Lake Baikal is the world’s deepest lake at 5,371 ft (1,637 m).

In present-day Canada, Scandinavia, and Russia, where glaciers have advanced southward and retreated northward periodically, the moving walls of ice scoured out depressions, which subsequently filled with water. A notable example of glacier-created lakes are North America’s Great Lakes. The system of five lakes that make up the Great Lakes are among the 13 largest lakes on Earth’s surface.

Lakes with a high content of salt can form as a consequence of the underlying geology of the region, or when a lake has little if any outlet to drain off water and so becomes elevated in salt that is present in the runoff entering the lake.

The depression that is left behind in an extinct volcano can also fill with water over time to create a lake. One example is Crater Lake in Oregon.

A type of lake known as an oxbow lake can also form from the changing flow of rivers that can occur with time. Rivers tend to adopt a more winding path as they age. Sometimes this meandering can become so pronounced that a loop forms and erosion eventually joins the sections of the river near the bottom of the loop. When this occurs, water flows straight through the newly created path, progressively isolating the looped portion, which is eventually closed off to form the crescent-shaped oxbow lake.

Lakes can also be artificially created. Most commonly, this occurs following the construction of a dam, with the impeded water collecting behind the retaining wall. Excavation of land can also be done to create small lakes; this is typically done to provide for water recreation at resorts, in planned communities, or to provide a scenic challenge in golf courses. As an artificial lake has no natural outflow, an exit for water movement must be created or the water quality of the lake must be monitored and maintained to prevent eutrophication.

A eutrophic lake contains an abundance of nutrients, which encourages the growth of microorganisms

WORDS TO KNOW

COLIFORMS: Bacteria that live in the intestinal tract of warm-blooded animals. The presence of coliforms in water indicates fecal pollution.

EUTROPHICATION: The process whereby a body of water becomes rich in dissolved nutrients through natural or man-made processes. This often results in a deficiency of dissolved oxygen, producing an environment that favors plant over animal life.

HYDROLOGY: The study of the distribution, movement, and physical-chemical properties of water in Earth’s atmosphere, surface, and near-surface crust.

RUNOFF: Water that falls as precipitation and then runs over the surface of the land rather than sinking into the ground.

WATERSHED: The expanse of terrain from which water flows into a wetland, water body, or stream.

such as some bacteria and algae. The opposite type of lake, in which nutrients are scarce, is known as an oligotrophic lake. The crystal clear appearance of an oligotrophic lake can be misinterpreted as a sign that the water body is healthy. In fact, the lake is plant-free and has few fish. Lakes that have been affected by acidic precipitation (acid rain) are oligotrophic. The mid-ground between oligotrophic and eutrophic is mesotrophic. A mesotrophic lake is acceptably clear and has good but not over-abundant plant and fish life. Finally, a lake that has received a great deal of nutrients and has become very murky is known as a hypertrophic lake. This type of lake is also depleted of oxygen and is considered to be dead.

The volume of water in a lake is a balance between the amount of water entering the lake (input or inflow) and the amount of water leaving the lake (output or inflow). If the inflow exceeds the outflow, the volume of the lake will increase and the level will rise. If the outflow is greater than the inflow, the volume of the lake will decrease and the lake level will drop. Precipitation, runoff, surface watercourses, and underground water emptying into the lake are sources of inflowing water. Outflow is due to evaporation, exit via underground routes or watercourses such as streams or rivers, and the deliberate removal of water (such as occurs to provide water for drinking or crop irrigation).

Lakes that are shallow enough for sunlight to penetrate close to or down to the bottom (lake bed) will tend to have a fairly uniform temperature throughout the volume. However, in deeper lakes the sunlit portion will be warmer than the deeper waters. Because warmer water tends to more biologically active, the upper zone of such lakes will be enriched in oxygen more so than the deeper regions.

Lakes can become smaller in size, become marshland, and finally disappear over time. Normally, this process is not noticeable as it can take millions of years to complete. However, the process can be accelerated in small and slow draining lakes that receive a high input of nutrients or sediment. A lake in the process of converting from an open body of water to a more marshy area often is ringed by floating plants and rushes along the shoreline. Rarely, the process can occur much more quickly. An example is the draining of Lake Beloye in Russia, which occurred in minutes on June 3, 2005, presumably due to activity on the lakebed that created an opening to an adjacent river. Lakes can also become depleted due to the excessive removal of water.

Impacts and Issues

Lakes can become polluted by natural activities (such as runoff from rocks containing increased levels of harmful compounds such as lead or arsenic) and, more commonly, from human activities. For example, lakes that undergo eutrophication have usually received runoff of fertilizer components including phosphorus and nitrogen, which are food sources for microorganisms.

Even large lakes can become contaminated. For example, the Great Lakes are known to contain over 360 chemical compounds such as DDT, mercury, and mirex. Some chemicals do not readily degrade and so can remain toxic for a long time. These can be ingested by

fish and, in turn, by creatures including humans who eat the fish. Contaminated lakes lose their biodiversity. In the Great Lakes, for example, populations of 7 of the 10 most commercially valuable fish have greatly lessened.

Efforts to restore the quality of affected lakes is ongoing at the municipal, state, national, and international level. An example of the latter is the Great Lakes Water Quality Agreement, which was first signed by the United States and Canada in 1972, and which has been modified several times since then. The agreement sets reduction targets for industrial pollutants and noxious chemicals including airborne pollutants.

Lakes are also bellwethers of climate change. In regions that are becoming drier and hotter, increased evaporation and decreased input of water is causing decreases in lake sizes. This is occurring even in northern regions such as the Kenai Peninsula of Alaska, where the average temperature from 1980 to 2005 rose by almost 1.8°F (1°C). In another example, the average temperature of Lake Baikal—the world’s largest and deepest lake, that also holds 20% of the world’s freshwater—has risen by 2°F (1.2°C) between 1946 and 2008. Scientists who have studied the lake are concerned that its extensive biodiversity is at risk. The findings at Lake Baikal imply that lakes with less volume could also be at risk in an increasingly warming world.

See Also Aquifers; Bays and Estuaries; Inland Fisheries; Surface Water; Water Pollution

BIBLIOGRAPHY

Books

Bernstein, Aaron. Sustaining Life: How Human Health Depends on Biodiversity. New York: Oxford USA, 2008.

Flannery, Tim. The Great Lakes: The Natural History of a Changing Region. Vancouver, British Columbia, Canada: Greystone Books, 2007.

Grady, Wayne. The Weather Makers: How Man Is Changing the Climate and What It Means for Life on Earth. Jackson TN: Atlantic Monthly Press, 2006.

Web Sites

International Lake Environment Committee. “World Lake Database.” http://www.ilec.or.jp/database/database.html (accessed May 4, 2008).

Brian D. Hoyle

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Lakes

Lakes

Lakes are large inland bodies of fresh or saline (salty) water. Lakes form in places where water collects in low areas or behind natural or man-made dams (barriers constructed to contain the flow of water). Some lakes are fed by streams (natural bodies of flowing freshwater), and some form where groundwater (water flowing in rock layers beneath the land surface) discharges onto the land surface. Water leaves lakes by flowing into outlet streams, infiltrating (soaking in) into groundwater reservoirs called aquifers, and by evaporating into the atmosphere (mass of air surrounding Earth). Lakes vary in size from large lakes such as the Great Lakes of North America, to small mountain lakes. Lakes are larger than ponds, which are small bodies of fresh water that are shallow enough for rooted plants to grow. The study of ecology (relationships between living organisms and their environment) in lakes, inland seas, and wetlands is called limnology.

Lakes store only a tiny percentage of Earth's fresh water. They are, however, an extremely important water resource for humans. Freshwater lakes provide water for agricultural irrigation (watering), industrial processes, municipal uses, and residential water supplies. People who live in the continental interiors use lakes for fishing and recreation, and very large lakes have important shipping and transportation routes. Humans also construct artificial lakes, called reservoirs, by building dams across rivers. In addition to providing the benefits of natural lakes, reservoirs also store water for specific communities, control floods, and generate hydroelectricity (electricity generated from water power).

How lakes form and disappear

Earth scientists who study water on the continents (hydrologists and hydrogeologists) see lakes as temporary reservoirs within stream and groundwater systems. All water that falls as precipitation (rain, snow, sleet, hail) on the land surface of continents eventually makes its way to the ocean or evaporates back into the atmosphere. Water collects in lakes because it enters more rapidly than it escapes, but it is never permanently trapped there. As in a tub with a running faucet and an open drain, individual water molecules (smallest unit of water, each containing two hydrogen atoms and one oxygen atom) are constantly entering and escaping. After arriving in a lake, an average water molecule spends about one hundred years before moving to a new reservoir. (The time that an average water molecule spends in a reservoir is called it residence time. Water resides for about two weeks in rivers, forty years in glaciers (slow moving mass of ice), and between two hundred and ten thousand years in groundwater reservoirs.)

To geologists (Earth scientists), lakes are temporary features. Stream-fed lakes within stream systems are destined for destruction. Every stream seeks to create a constant slope, called a graded profile, between where the stream's waters begin and ends by eroding (wearing away) and depositing sediment (particles of sand, silt, and clay) along its course. When a natural or man-made obstruction blocks a stream, such as a river, streams deposit sediment in the lake or reservoir behind the obstruction and erode away in front of it. Eventually, the dam will collapse, and the lake will empty. Lakes that fill depressions and have no outlets fill when the regional climate becomes wetter or when warm periods melt mountain snows and glacial ice. They evaporate away during periods of dry weather and dryer climate. It may take thousands, or even tens of thousands of years, but lakes eventually drain, collapse, or dry up.

Lake layers and overturns

Contrary to their common image as evenly mixed pools of unmoving water, lakes are complex, dynamic bodies of moving surface water. Lake water varies within the lake; its temperature, chemical content, light infiltration, and biological habitats vary from top to bottom and side to side. Furthermore, the vertical layering (stratification), horizontal variations, and circulation patterns within lakes change over time. Waves, currents (a moving mass of water), and even tides affect circulation of water within lakes.

Lakes are thermally stratified (layered according to temperature); they have layers of warm and cool water that are separated by layers where the temperature changes (thermoclines). Like the oceans, many lakes have a thin layer of warm surface water, and a thicker layer of cool deep water that is separated by a thermocline layer. Wind generates currents on lake surfaces and creates some mixing. Unlike the oceans, however, many lakes have seasonal overturns that mix their waters. water is denser than solid water (ice). Water reaches its maximum density at 39°F (4°C). Because of this odd property, the warm less-dense water rises, the cool denser water sinks, ice floats, and lakes overturn. Lakes that are ice-covered for part of the year undergo overturns that partially or completely mix their waters.

Many lakes in temperate (moderate temperatures) regions like the northern United States overturn and mix completely twice a year (dimictic lakes). During the warm summer months, these lakes have a usual temperature profile with warm surface waters, a thermocline, and cool bottom water. In the fall, when the surface water cools down to 39°F (4°C), it becomes denser than the water underneath it and the surface layer sinks to the bottom. The bottom water rises to the surface, and the lake overturns. Over the winter, the bottom water is the warmest, and the frozen surface water is the coldest. (Plants and animals survive the winter on the lake floor in the chilly, but not frozen, bottom water.) In the spring, when the ice melts, and the water warms to 39°F (4°C), it sinks, and the lake overturns again.

Limnologists classify lakes according to the number of mixing events they undergo each year. Lake type classifications have the root term mictic, meaning "to mix," and include:

  • Oligomictic: Warm, ice-free lakes that rarely mix. They are warmest at the top, and coolest at the bottom. Tropical, oligomictic lakes have warm bottom water and very warm surface water. They rarely overturn because their water does not near 39°F (4°C).
  • Meromictic: Warm, ice-free lakes that mix incompletely. These are deep lakes that are warmest at the top, and coolest at the bottom.
  • Monomictic/dimictic/polymictic: Lakes with seasonal ice covers that overturn and mix completely once (monomictic), twice (dimictic), or many times per year (polymictic). Lake overturns are the norm in temperate regions, but local conditions affect the timing and number of overturns in specific lakes.
  • Amictic: Lakes that never overturn because they are icecovered throughout the year. These lakes exist near the North and South Poles and atop very high mountains. They have cold bottom water that hovers near 39°F (4°C) and frozen surface water.

Lake chemistry: saline lakes

Many of Earth's largest and most important lakes contain salt water. All surface water contains some dissolved chemicals, called salts. Groundwater, streams, and freshwater lakes all contain the chemical components of rocks and minerals. Humans can drink fresh water because our bodies can use or at least tolerate the types and concentrations of dissolved chemicals it contains. Salt water, on the other hand, has a very high concentration of dissolved salts, and is undrinkable. The Dead Sea, on the border between Israel and Jordan, is Earth's saltiest body of water. It is truly a dead sea because it is too salty to support life.

Saline lakes generally form in arid (dry) regions where surface water evaporates quickly. When water evaporates, the salts stay behind. Over time, the lake water becomes saltier. Some saline lakes, such as the Great Salt Lake, are all that remains of a much larger fresh water lake that has evaporated over time. Others, like the Caspian Sea in central Asia, began as saltwater-filled ocean basins that have since closed.

Saline lakes are often temporary features that fill during periods of wetter climate and then dry up when stream flow or groundwater discharge slows. Playa lakes are flat desert basins that occasionally fill with water. Desert oases (watering holes) form and disappear with such regularity that thirsty travelers think they imagined them. The Great Salt Lake, Caspian Sea, Aral Sea, and Dead Sea are all presently evaporating. Over time, the dissolved chemicals become so concentrated in drying lakes that they bond together and form solid salt crystals. Thick layers of salt cover dry lake beds.

Lake biology

Lakes support rich communities of plants and animals (ecosystems) that have adapted to live within ever-changing conditions on lake beds, within the water column (water running from the surface to the lake floor, often showing differences in temperature, nutrients, etc.), and along lake shores. Lakes, like islands, are often closed systems that only rarely gain new species or individuals from other lakes. Many lakes host groups of rare species that have evolved (changed over time) together in their specific lake. These ecosystems are rich and unique, but fragile. They have little defense against foreign predators or diseases. Human alterations and water pollution have threatened many lake species. Environmental groups and government agencies are presently attempting to protect and revive threatened lake species such as cichlids (rare double-jawed fish) that inhabit the lakes of the Great Rift Valley in east Africa.

Lake organisms live in zones that are determined by the physical structure of their lakes such as the amount of available light, water depth, and distribution of nutrients. Most lake

Dying Lakes: Great Salt Lake and Aral Sea

Utah's Great Salt Lake and the Aral Sea in central Asia are drying up. These large, hypersaline (very salty) bodies of surface water fill terminal basins (large depressions that have no outlets). They filled with water hundreds or even thousands of years ago when the climate was wetter and when rivers supplied them with more water. Today, the amount of water entering the lakes from streams, springs, and rainstorms is much less than the amount of water leaving by evaporation into the atmosphere or infiltration into underground rocks and soils.

Like the oceans, inland lakes and seas are salty because evaporating water leaves behind dissolved minerals. The water that flows into lakes contains the dissolved chemical components of rocks and minerals. When water evaporates into the atmosphere, the salts stay behind. Terminal lakes that only lose water by evaporation and receive very little fresh water from rivers become very salty as minerals accumulate. The waters of the Great Salt Lake and Aral Sea are much saltier than the oceans, and the dry portions of their lakebeds are blanketed with thick layers of salt crystals.

Today, the Great Salt Lake is about three to five times saltier than the ocean and supports only a few species of salt-loving fish and shrimp. Over the long term, the lake is drying, but the amount of water entering the lake from streams varies, so the lake level rises and falls from year to year. The lake fills a broad, very shallow basin, so the positions of its shorelines change drastically as lake levels fluctuate. Industries that depend upon the Great Salt Lake such as salt mines, petroleum fields, and brine shrimp fisheries have learned to adjust to moving shorelines, while recreational facilities such as lakeshore resorts and marinas have not fared as well.

The Aral Sea is a very large saline lake in a terminal basin in the central Asian nations of Kazakhstan and Uzbekistan. Like the Great Salt Lake, it is a remnant of a much larger prehistoric lake that has been shrinking slowly over thousands of years. Unlike the Great Salt Lake, however, the Aral Sea is drying from processes other than long-term climate change.

Forty years ago the Aral Sea was the fourth-largest inland sea in the world. It supported a thriving fishing industry and an economically pros-perous region of several hundred thousand residents who depended on the lake. In 1973, the former Soviet Union diverted the flows of the Amu-Darya and Syr-Darya Rivers to irrigate huge plantations of water-intensive crops in the dry grasslands of central Asia. Without replenishment from these rivers, the Aral Sea shrank to half its original size and lost about 60% of its water.

The rapid retreat of the Aral Sea has caused an environmental and economic disaster in central Asia. Retreating shorelines have hurt ports and marinas. Rising salt concentrations destroyed the lake ecosystem and killed the fish. Dry winds have blown salt and dust particles from the dry lakebed far across the continent where they have polluted soil and water. The local climate has even changed; less rain falls each year, the winters are colder, and the summers are hotter. Blowing salt and droughts have turned once-fertile agricultural lands into unusable desert. Air pollution causes respiratory ailments, and salt in the soil pollutes surface and groundwater. At present, international organizations such as the United Nations and World Bank have abandoned their original plans to revitalize the region by restoring its natural environment and have refocused on meeting the basic needs of the five million people left in peril by the Aral Sea disaster.

plants and animals live in shallow, well-lit surface waters called the euphotic zone. Most plants depend on the Sun's energy to produce food by the chemical process of photosynthesis, and they cannot grow in water that is too deep or too cloudy for light to penetrate. Lake animals such as fish need oxygen that plants give off during photosynthesis, so they live mostly in the euphotic zone as well. Plants with roots grow in shallow water along edges of lakes where light reaches the lake floor (littoral zone) and floating plants perform photosynthesis in the open surface waters (limnetic zone). Oxygen-consuming bacteria inhabit the deepest, darkest parts of lakes (benthic zone) where dead plant and animal materials accumulate.

Limnologists also classify lakes by the balance of organisms and nutrients in their waters. Types include lakes that are described as oligotrophic, eutrophic, and mesotrophic.

  • Oligotrophic: Nutrient poor lakes that support very few plants and animals. Oligotrophic lakes are typically cool, deep, and have very clear water. Very little organic (relating to or from living organisms) mud accumulates in oligotropic lakes, and they often have sand and gravel beds.
  • Eutrophic: Lakes rich in plant nutrients that support abundant plant life in their surface waters. Their water is often clouded by microscopic plants, and their beds covered with thick layers of decaying plant material. Bacteria that live on the organic mud use up oxygen, and eutrophic lakes often have oxygen-poor deep water. Plants and bacteria eventually take over eutrophic lakes. They become oxygen-poor bogs and marshes where fish cannot live. Some chemicals that humans use, including fertilizers and detergents, cause a process called eutrophication when they run off into lakes, which causes the population of plants to increase to such an extent that eventually oxygen-starved fish die.
  • Mesotrophic: Lakes with moderate amounts of nutrients and healthy, balanced communities of plants, animals and bacteria. Mesotrophic lakes receive adequate amounts of fresh water and nutrients, and seasonal overturns allow nutrient-poor and nutrient-rich layers to mix. Mesotrophic lakes are intermediate between crystal-clear, lifeless oligothrophic lakes and cloudy, muddy eutrophic lakes.

Where lakes form: lake basins

Lakes form where water collects in depressions, or basins. Many lakes fill low areas created by plate tectonic movements (tectonic basins) and volcanic activity. (Plate tectonics is the movement of large, rigid pieces of Earth's outer rock shell called the lithosphere.) Retreating glaciers and ice sheets leave behind large basins and small depressions that fill with meltwater. Though flowing streams and rivers generally act to fill in and drain lake basins, other sedimentary processes can create landscape depressions and natural dams that confine water in lakes.

Lakes in tectonic basins Rift valley lakes fill long, linear valleys within rift zones. (Rifts are areas where the continental lithosphere is stretching and beginning to break into pieces. They are the precursors of ocean basins.) A chain of large lakes including Tanganyika, Naiveté, and Malawi follows the Great Rift Valley through eastern Africa. Lake Victoria, the world's second-largest lake, lies between two branches of the rift valley. The Red Sea, Sea of Galilee, Dead Sea, and Gulf of Ababa fill the northern branches of the rift where it crosses the Arabian Peninsula in the Middle East. Russia's Lake Baikal, the world's deepest lake, fills an ancient, inactive rift valley in central Asia.

Lakes also form in places where continents are moving toward each other. The Black Sea, Caspian Sea, and Mediterranean Seas fill a closing ocean basin between Africa and Europe. When continents collide, water fills depressions in the landscape over folded and broken (faulted) rock layers that were caught between the land masses. Slopes that are too steep collapse and block rivers with natural dams. Blocks of uplifted, erosion-resistant rock form bedrock that holds back mountain lakes.

Volcanic lakes Volcanoes are mountains that form from eruptions of molten rock (lava) on the land surface. When a volcanic peak collapses into its emptied magma chamber (a pool or room of magna held under tremendous pressure within a volcano prior to a volcanic eruption), it forms a large circular basin called a caldera. (Craters, the small basins near the top of active volcanoes, sometimes also contain small lakes, but most significant volcanic lakes, including inaccurately-named Crater Lake, fill calderas.) Yellowstone Lake in Wyoming and Crater Lake in Oregon are examples of caldera lakes. Volcanic ash, mud, and lava flows also create natural dams in river valleys. A dam of volcanic rock confines Lake Tahoe in a high valley of the Sierra Mountains on the California-Nevada border.

Glacial lakes The thick continental ice sheets that covered northern North America, Europe, and Asia during the Pleistocene ice ages (a division of geologic time that lasted from two million to ten thousand years ago) left behind thousands of lake and ponds when they retreated about twenty thousand years ago. The weight of the ice sheets pushed down on the continents, leaving broad basins that filled with melt water when they retreated. The Great Lakes of North America (Superior, Huron, Michigan, Erie, and Ontario) formed this

The Great Lakes

The Great Lakes of North America: Superior, Michigan, Huron, Erie, and Ontario, together make up the largest system of fresh surface water on Earth, and contain almost 90% of North America's fresh surface water. The lakes fill a broad depression that was created by the massive ice sheet that pressed down on the North American continent during the last Pleistocene ice age. The basin first filled with melted water from ice sheets that retreated thousands of years ago, and is today replenished by rivers, rainfall, and groundwater discharge. Water flows from west to east through the lakes and short connecting rivers. It eventually empties from Lake Ontario into the Atlantic Ocean via the St. Lawrence Seaway. The lakes and their surrounding wetlands, forests, and plains support thriving communities of plants and animals.

Humans have lived in the Great Lakes region for thousands of years. When French fur trappers and traders first explored the banks of Lakes Huron and Superior in the early 1600s, they found economically sufficient, prosperous Native American communities that had lived in the region for centuries. (Chippewa, Huron, Iroquois, Ottawa, Potawatomi, and Sioux are a few of the over 120 native tribes of the Great Lakes region).

Today, 25 million Americans in eight states (Minnesota, Wisconsin, Illinois, Indiana, Michigan, Ohio, Pennsylvania, and New York) and 8.5 million Canadians in the province of Ontario live in the Great Lakes region. They depend on the lakes for water, transportation, shipping and recreation. Major cities like Chicago, Detroit, and Toronto use lake water for drinking, household use, city supplies, industry, and recreation. Farmers draw water to irrigate fertile agricultural lands in the American and Canadian plains. Ships carry industrial materials and products far inland to the western end of Lake Huron. Man-made channels and locks (chambers the can be filled and drained of water so that boats can be raised or lowered as needed) like the Erie Canal and Soo Locks allow huge ocean-going ships to detour around impassible sections of the system, including Niagara Falls between Lakes Erie and Ontario.

In spite of their immense size and active circulation, the Great Lakes are very sensitive to environmental threats from human activities. Water moving through the Great Lakes system does little to remove or dilute substances such as the water that flows from agricultural lands, mining waste, industrial chemicals, acid rain and sewage. Pollutants tend to collect in the lakes where they threaten fragile natural ecosystems as well as the quality of the human water supply.

Human industry, agriculture, and urbanization in the first half of the twentieth century severely damaged the Great Lakes. By June 1969, the Cuyahoga River that connects Lakes Erie and Ontario had become so polluted with petroleum and industrial chemicals that it caught on fire near Cleveland, Ohio. This event and others like it eventually led to regulations such as the Clean Water Act and Great Lakes Water Quality Act in the 1970s. Today, government regulations, clean-up projects, and scientifically-guided water management by U.S. and Canadian groups have successfully restored the Great Lakes to a semblance of environmental health, though some problems remain to be solved.

way. Hundreds of lakes, such as the Winnipeg, Athabasca, Great Slave, and Great Bear cover the central and eastern provinces and territories of Canada that are still rebounding from their heavy ice load.

Advancing glaciers also pile tall ridges of sediment, called moraines, at their toes (the end of extensions of glaciers along the ground). When glaciers retreat, moraines hold back meltwater. Small lakes and ponds also form in glacial depressions called kettles that form when blocks of ice buried in glacial sediment melt. Melting mountain glaciers feed many mountain lakes and glacial sediment traps streams and meltwater.

Groundwater discharge lakes Water moving through pore spaces in rock and soil layers discharges on the land surface in places where the water table (level below which pore spaces are saturated with water) intersects the land surface. In regions with wet climates, the water table is near the land surface and ground water discharges in low spots. Groundwater chemically erodes limestone and other rocks and creates caves, cavities, sink holes, and collapse basins called karst features. Florida's many lakes, including Lake Okeechobee, are groundwater-filled karst features.

Laurie Duncan, Ph.D.

For More Information

Books

Rowland-Entwistle, Theodore. Rivers and Lakes. Morristown, NJ: Silver Burdett Press, 1987.

Sayre, April Pulley. Lake and Pond. New York: Twenty-First Century Books, 1996.

Websites

"Earth's Water: Lakes and Reservoirs." U.S.G.S. Water Science for Schools.http://ga.water.usgs.gov/edu/earthlakes.html (accessed on August 16, 2004).

"Great Lakes." U.S. Environmental Protection Agency.http://www.epa.gov/glnpo/ (accessed on August 16, 2004).

"Lake Classification Systems." Michigan Lake and Stream Associations.http://www.mlswa.org/lkclassif1.htm (accessed on August 16, 2004).

"Lake Ecology Primer." Water on the Web.http://www.waterontheweb.org/under/lakeecology/index.html (accessed on August 16, 2004).

"Ponds and Lakes." Missouri Botanical Garden.http://mbgnet.mobot.org/fresh/lakes/ (accessed on August 16, 2004).

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