Lakes and rivers

A lake is a large body of water surrounded by land. Lake waters move, partly due to rivers that enter and leave them. In addition, lake water circulates vertically, especially near its surface, where sunlight heats the water and lowers its density. Seasonal warming and cooling of water, evaporation, and other factors contribute to annual cycles of vertical lake water movement.

Most lakes form in hollows dug out of the ground by glaciers. Some huge lakes are really inland seas, such as the North American Great Lakes. Lake water is supplied by rivers and accumulated from rain draining off nearby land. River water contains minerals obtained by running over rocks, and rain holds airborne chemicals. Continued water flow through a lake keeps it fresh, enabling animal and plant survival. When water does not flow out of a lake, minerals are trapped and concentrated, as solar heat evaporates water. In time salt accumulation turns such lakes into bodies of salt water that few organisms can live in, such as the Dead Sea, which is ten times saltier than the oceans.

Animals Found in Lakes

Lakes provide rich habitats for many kinds of wildlife. They are also excellent examples of ecosystems, ecological units in which the plants and animals present depend on each other for survival. Removing or damaging one plant or animal species can cause a chain of events that may adversely affect or even ruin the ecosystem.

Land beside a lake is usually marshy. Rushes, reeds, willow and alder trees, and water lilies grow in the shallows around a lake edge. The plant roots prevent lakeshore erosion. Farther from shore are submerged and floating plants that produce oxygen and provide food for small underwater animals such as insects, insect larvae, snails, shellfish, and worms. These small animals are eaten by fish, which are then eaten by birds. Land mammals, such as martens and bears, also eat the fish. Overall, lake plants and animals make up a food chain that supports the lake ecosystem.

A great many animals inhabit lakes and the areas around them. The particular animals present at a given lake vary greatly depending on the lake’s geographic location. Birds, which usually eat fish and aquatic insects, inhabit lakes throughout the world; lake-dwelling bird species range from ducks, geese, and loons in Canada’s cold lakes to ducks, kingfishers, and herons throughout the United States to the flamingos of the southern United States, Central and South America, and Africa. African lakes are also home to marabou storks and other carrion eaters.

Flamingos may be among the most dramatic of lake-dwelling birds. There are six species of these gorgeous water birds, which have long legs for wading, webbed feet, swanlike necks, and red, flame-colored, pink, or white feathers. Their bills bend abruptly in the middle, and the upper mandible fits tightly into the lower one. Flamingos dip their heads upside down under water and scoop backward to feed. Their bill edges have ridges, and when the tongue pushes against the inner bill, the water runs out, leaving behind shellfish or other food. Flamingos live along lakes in nests that are made of mud, hollow on top, and able to hold one egg. The largest species, the greater flamingo, has two subspecies, one bright red and the other pale red. Males reach five feet tall. The smallest, most abundant species is the lesser flamingo.

Reptiles such as snakes and salamanders, amphibians such as frogs, and many turtle species are also present in lakes. Depending on size, they eat insects or each other and may themselves be eaten by birds. Many different fish species are also present, from minnow to sunfish to trout. They are parts of food chains, also eating insects, plants, small reptiles, and each other, depending on size.

River Animals

Only animals adapted to cold, fast-flowing water survive in the upper courses of rivers, which are fine habitats, clean and full of oxygen. Some plants survive, but limpets, snails, and hardy water insects are predominant. They become foods for the carnivorous fish, such as trout and salmon, found in rivers’ upper courses.

A river's current slows along its middle course, and sediment settles to the river bed in the form of earth and rock particles that contain minerals on which plants thrive. These plants, from microscopic algae up, are used as food by wildlife inhabiting river banks, such as otters, martens, and raccoons, as well as fish and other animals which live in river waters. Small fish and insects, such as water striders, dragonflies, and aquatic beetles, are plentiful in the water. Reeds and trees are also homes and nests for birds of many kinds, as well as small to medium-sized mammals, such as rodents and skunks, snakes such as water moccasins, and other reptiles.

Depending on location, the size and danger associated with middle-course river animal life varies. However, food chains in rivers also assure continuation of the balance of nature, as small plants and animals are eaten by medium-sized animals, which are in turn eaten by bigger animals. The largest, most dangerous river animals are found in the hot southwestern United States, Central and South America, Africa, and Asia.

Crocodiles, alligators, and caimans are among the most dangerous river animals. They are all crocodilians, differing in size, position of the fourth mandibular tooth, and snout shape. Crocodiles have lizardlike bodies up to twenty-five feet long, with short limbs, huge teeth, eyes atop their heads, long thick tails that facilitate swimming, and short, broad snouts in which the fourth mandibular teeth are visible. They are cold-blooded and inhabit salty, brackish, and fresh water in Africa, Australia, Asia, the Indies, and the Americas. They are carnivores, eating birds, amphibians, fish, mollusks, crustaceans, small mammals, and large mammals ambushed while drinking. Crocodiles mate in the water. After two-month gestation periods, females lay eggs on land. The eggs hatch in three months. Crocodiles can live for up to one hundred years.

Alligators are physically and reproductively similar to crocodiles, but they only grow up to sixteen feet long. Their fourth mandibular teeth are hidden by their upper lips, and they are more comfortable on land than crocodiles. During the day, alligators sun themselves on river banks in the southeastern United States, China, and Central and South America. Caimans of Central and South America are small alligators that typically grow only up to eight feet long, although the black caiman can be twice as large. They inhabit rivers and swamps. Alligators and caimans live for forty-five to sixty-five years.

The largest river mammal is the herbivorous African hippopotamus, which can reach a length of fifteen feet and a body weight of four tons. All sorts of fish live in rivers, including minnows, catfish, sunfish, bass, trout, and river dolphins. Most such fish are not very dangerous to organisms other than their specific prey. However, the carnivorous piranha of South American rivers eat any animals that stray into their territory.

Many wild animals found in a river’s middle course are even more plentiful downriver in its lower course, where the current is quite slow. However, in estuaries, which are salty due to the admixture of ocean water with the river’s freshwater, few plants and animals can survive. There, worms, crustaceans, and gastropods such as snails inhabit mud banks and provide food for birds such as herons, gulls, and kingfishers.

River Ecosystems

Rivers contain series of ecosystems whose compositions vary with conditions in a given portion of the upper, middle, or lower course. In river ecosystems, like those in and around lakes, all plants and animals depend on each other for survival, and removing or harming one species can ruin the ecosystem. Beyond this, the existence of multiple ecosystems on land and their connections to aquatic ecosystems indicate that damage to any living organism or its local ecosystem will have ripple effects, which can spread out to damage much wider areas than at first perceived. Hence, reasonable actions of humans toward all other living organisms should be attempted and achieved.

Principal Terms

carnivore: an animal that eats only the flesh of other animals

course: the pathway of a river from its source to its entry into an ocean

ecosystem: an ecological community, which together with its environment is perceived as a unit

erosion: the processes, including weathering, dissolution, and abrasion, by which earth or rock is removed from a part of earth’s surface

estuary: part of river’s lower course, near entry to an ocean, where slow river flow and tidal action forms mud flats and sand banks

hydrologic cycle: Earth’s cycle of evaporation and condensation of water, which produces rain and maintains oceans, rivers, and lakes

lake: a large area of water surrounded by land

Bibliography

Bramwell, Martyn. Rivers and Lakes. Updated ed., Franklin Watts, 1994.

Dodds, Walter K., and Matt R. Whiles. Freshwater Ecology: Concepts & Environmental Applications of Limnology. 2nd ed., Academic Press, 2010.

Lamar, William W. The World’s Most Spectacular Reptiles & Amphibians. World Publications, 1997.

Ogilvie, Malcolm, and Carol Ogilvie. Flamingos. A. Sutton, 1986.

Pringle, Laurence. Rivers and Lakes. Time-Life Books, 1985.

Romanowski, Nick. Living Waters: Ecology of Animals in Swamps, Rivers, Lakes and Dams. CSIRO Publishing, 2013.

Wade, Nicholas, editor. The Science Times Book of Birds. Lyons Press, 1997.