Rivers, lakes, streams, and swamps in the rainforest

By Rhett A. Butler
April 1, 2019

Rivers, streams, and creeks in the rainforest

Tropical rainforests have some of the largest rivers in the world, like the Amazon, Madeira, Mekong, Negro, Orinoco, and Congo (Zaire), because of the tremendous amount of precipitation their watersheds receive. These mega-rivers are fed by countless smaller tributaries, streams, and creeks. For example, the Amazon alone has some 1,100 tributaries, 17 of which are over 1,000 miles long. Although large tropical rivers are fairly uniform in appearance and water composition, their tributaries vary greatly. Many tropical rivers and streams have extreme high and low water levels that occur at different parts of the year.

In addition to rivers, rainforests have conventional, free-standing lakes and so-called oxbow lakes, formed when a river changes course. These lakes are home to species adapted to the quiet, stagnant conditions.

Tropical waters, whether they be giant rivers, streams, or oxbow lakes, are almost as rich in animal species as the rainforests that surround them. But they, too, are increasingly threatened by human activities, including pollution, siltation resulting from deforestation, hydroelectric projects, and over-harvesting of resident species.

Types of rivers in the rainforest

First-time visitors to the Amazon or other large tropical rivers are often shocked to see the muddy brown, almost polluted-looking water. However, this color results not from sewage or pollution, but from the heavy sediment load of the water. Each day, tons of sediment are washed into rainforest rivers from the mountains and from run-off of surrounding forest areas due to heavy tropical rains. The sediment load is even greater where deforestation has left the soils unprotected and massive amounts of topsoil are eroded by the rains.

Despite their cafe-au-lait appearance, such tropical rivers are generally known as "whitewater" or brown-water rivers. Because whitewater rivers are often fed by a large number of acidic tributaries, they are relatively soft in terms of water hardness due to their relatively low mineral content, and they have a slightly acidic to neutral pH (6.3-7.0).

Whitewater tropical rivers are the typical form of large rivers in lowland tropical rainforests. Because such rainforests are generally flat with little elevation, large tropical rivers have little gradient and flow relatively lazily through them. The Amazon, for example, falls only 345 feet (105 m) from the Peruvian river port of Iquitos, a full 2,300 miles from the ocean. Thus the river descends at a rate of only 1.8 inches per mile (2.8 cm/km).

Sone of the best-adapted animals to the low visibility of the muddy whitewater are river dolphins, which are found in the Amazon, Ganges, and Indus among rainforest rivers. River dolphins have very poor eyesight, and like oceanic dolphins rely on sonar for navigation and location of prey. River dolphins are most abundant in the large open river channels, although during Amazonian floods they will range through the flooded forest areas.

Smaller tropical rainforest rivers are not so uniform in composition and in water flow as large tropical-forest rivers, which tend to be whitewater. There are two other water types found commonly in the tropical rainforest besides whitewater: blackwater and clear- or blue-water rivers.

More common in tropical lowland forests than clearwater rivers are blackwater rivers. The term blackwater describes the appearance of the water of such rivers, which is a dark coffee color. This color results from the leaching of tannins from the decaying leaves of adjoining vegetation. Blackwater rivers are also characterized by striking water clarity; so clear that visibility may exceed 30 feet (9 meters). However, after rainstorms, blackwater rivers can lose their typical clarity and color while sediment runs off from the surrounding forest. Within a few hours to a few days, the normal conditions return.

BLACKWATER

Chemically, blackwater rivers are very low in dissolved minerals and often have no measurable water hardness. The very acidic, almost sterile water, with a pH between 3.5-6, keeps parasite and bacterial populations to a minimum. For this reason, blackwater rivers are considered some of the cleanest natural waters in the world, most often compared to "slightly contaminated distilled water." The water chemistry of blackwater also inhibits the proliferation of insect larvae, so the forest around blackwater tends to be less "buggy" in terms of floor-dwelling mosquitoes.

Surrounding some pure blackwater rivers are blackwater forests, which are different than conventional rainforests. The acidity of the water limits the number of tree species that can grow in the area near the river. The low tree diversity is responsible for a lower variety of insect species, because insect species that would otherwise pollinate and feed upon other tree species have no opportunities in blackwater forests. This, coupled with the harsh water conditions of blackwater rivers, results in considerably lower overall insect diversity than in other forests. Accordingly, the blackwater forest areas support fewer numbers of other animal species.

The cause of the soft, acidic conditions of blackwater is the origin of most blackwater streams in the lowland tropical forest, where the ancient soils have no minerals to increase water hardness. Adding to the acidity of blackwater rivers are the tannins released from decaying leaves.

Because blackwater rivers are directly fed by run-off from surrounding rainforest, the soils of which are generally nutrient-deficient, these rivers are nutrient poor, and the surrounding floodplain areas are less suitable for cultivation than the floodplains of larger whitewater rivers like the Amazon. The nutrient deficiency of the soils along the shores of the Rio Negro makes the river known by Indians as the River of Hunger. Similarly, blackwater rivers support a lower bio-load than surrounding whitewater rivers, though they tend to have a tremendous diversity of fish species.

Fish have specially adapted to tolerate blackwater conditions. Many of the fish species that inhabit blackwater are best known outside the tropics for their popularity as aquarium fish. Discus, angelfish, arowana, elephantnose fish, many gouramis, and cardinal tetras are a few of the examples of blackwater species kept as aquarium subjects. The brilliant, iridescent colors of many small, schooling species like cardinal tetras, are believed to aid the school in recognition of its members in the dark waters.

The arowana, sometimes kept as an aquarium subject, is also know as the "monkey fish" for its leaping abilities—reportedly up to six vertical feet. It uses these abilities to catch its prey, which includes insects, small animals, and even young monkeys and sloths. The arowana is a large, elongated, even eel-like fish which swims along the surface looking for prey.

The Rio Negro in Brazil is one of the world's largest rivers (five miles at its mouth) and the most famous blackwater river. In contrast to the whitewater Amazon (the Rio Solimones at this point), which has its origins in the mountain valleys of the Andes, the tributaries of the Rio Negro rise in the ancient rock formations of the Guyana shield and flow through white-sand rainforests. The differences between the blackwater of the Rio Negro and the whitewater Amazon are readily apparent where the two rivers meet near Manaus, Brazil. The rivers run side by side, clearly distinct as separate white and black water, before blending together after several miles.

CLEARWATER

Clearwater or bluewater rivers are so named for their clear water. Such waters are fairly common as creeks and rivers flowing through ancient rock, but are not abundant in lowland tropical rainforest. Clearwater rivers are mostly found in the highlands like the Guyana and Brazilian shields of South America where mountain and cloud forests grow. Because of their elevation, and tendency to run over rock, clearwater rivers are often rapid or fast-flowing. The Xingu river is one of the biggest and best known clearwater rivers in the Amazon.

Clearwater rivers have a higher pH and tend to have some dissolved minerals, making the waters harder than both blackwater and whitewater rivers. There is not much suspended matter because the rock formations are ancient and no longer erode in the current.

Due of their clarity and mineral content, some clearwater rivers support abundant plant growth. Additionally, algae grow vigorously on the rocky substrate supporting a variety of sucker-mouth catfish, another popular aquarium subject commonly known as plecos.

Tropical Rivers, Streams, and Creeks

RIVERS

Overhead pictures of tropical rivers reveal another curious aspect: the meandering course. A river will twist and turn, sometimes turning almost 180 degrees back on itself. The lack of slope and the clay-like soils of many tropical regions allow rivers to have virtually free rein over their direction.

The volume of water flowing through tropical forests, coupled with the soils and varying water levels, can create great river cliffs over 100 feet high, even at regular water levels. These clay banks form an important part of the local ecology in parts of the Amazon. Macaws gather by the hundreds on some of these banks to ingest minerals that bind to and detoxify chemicals in the fruits they consume.

With their huge volume, large rivers like the Amazon transport tremendous quantities of wood and debris. It is common to see giant logs and trees passing, though sometimes natural meadow rafts, complete with trees and animals and sometimes a shack, are seen floating downstream. Often river navigation is complicated by massive logjams that form in river channels. The Rio Madeira, a sizeable tributary of the Amazon, gets its name from the large amounts of wood that pass down the river. These logjams, along with sunken wood, provide a critical habitat for fish and other aquatic animals.

STREAMS AND CREEKS

Tropical streams and creeks are even more variable than tropical rivers and can change from a virtually dry river bed to a raging torrent 30 feet deep in a matter of hours during a heavy rain. Smaller streams and creeks are often invisible by air because they flow beneath the rainforest canopy. Despite their inconspicuousness, these waterways house an astounding array of animal life. Creeks are common in the rainforest and provide an important niche for certain fish, amphibian, and insect species in addition to providing an important source of water for other forest floor dwellers.

Some of these creeks, especially in lowland Amazonia, can be surprisingly deep with a U-shaped riverbed. The clay substrate helps these creeks keep their form and seemingly defy the laws of physics.

THE GREAT RAINFOREST RIVERS OF THE WORLD

THE AMAZON [news]

The Amazon River is the most voluminous river on Earth, eleven times the volume of the Mississippi, and drains an area equivalent in size to the United States. During the high-water season, the river's mouth may be 300 miles wide, and up to 500 billion cubic feet per day (5,787,037 cubic feet/sec) may flow into the Atlantic. For reference, the Amazon's daily freshwater discharge into the Atlantic is enough to supply New York City's freshwater needs for nine years. The force of the current, from sheer water volume alone and virtually no gradient, causes the current to continue flowing 125 miles out to sea before mixing with Atlantic salt water. Early sailors could drink fresh water out of the ocean before sighting the South American continent.

The river current carries tons of suspended sediment, causing the characteristic muddy whitewater appearance. It is calculated that 106 million cubic feet of suspended sediment are swept into the ocean each day. The result from the silt deposited at the mouth of the Amazon, is Majaro island, the world's largest river island, about the size of Switzerland.

Despite its immense size, the Amazon today is being affected by human activities, including deforestation, which can affect sediment levels and water flows; dams, which disrupt nutrient cycles, water flow, and fish migration; and climate change, which is causing more extreme conditions including flood and drought.

THE CONGO

The Congo River (formerly Zaire River), is Africa's most powerful river and the second most voluminous river (not counting the Madeira and Negro which are considered part of the Amazon) in the world, with 1,500,000 cubic feet of water passing out of its mouth every second. It is the fifth longest river in the world, draining a basin of nearly 1.5 million square miles.

The river is best known for its role in history. Known as the heart of darkness by Joseph Conrad, the river and surrounding rainforest have long been known as the mysterious land of pygmies, brutal colonial violence, and unusual animals.

The river itself is as turbulent as its history, though it begins peacefully enough in the savannas just south of Lake Tanganyika. Gradually the river widens and picks up speed until it enters the "Gates of Hell," a 75-mile long canyon of impassable rapids. The river emerges again, surrounded by lush tropical rainforest, as the Lualaba or Upper Congo. During the course of its journey through the foreboding rainforest, the river crosses the equator twice. Because the watershed of the Congo drains from both the Northern and Southern Hemispheres, it does not have as great seasonal fluctuations in water level as other great rivers. Its flow is relatively stable because part of its watershed is always in the zone of rain. The Upper Congo abruptly ends with Stanley Falls, a 60-mile stretch of rapids.

Stanley Falls gives way to the Middle Congo, a 1,000-mile stretch of navigable river, nine miles wide in some parts. Along this quiet stretch of river is the city of Kinsangani, a city known for violence since Belgian colonial days. Near the end of the Middle Congo, the river slows to a virtual standstill for 20 miles, a section known as Stanley or Malebo Pool. Here the river is 15 miles wide and flanked by the capital cities of Kinshasa and Brazzaville. The peace of the pool is suddenly shattered by Livingstone Falls, a series of rapids and cataracts 220 miles long. There are some 32 cataracts, having as much power as all the river and falls in the United States combined. The final 100 miles to the Atlantic Ocean from the end of the falls is fully navigable.

THE NIGER RIVER

The 2,590-mile (4,170-km) course of Africa's third largest river was one of the great mysteries of Africa until the mid-nineteenth century. From its origins in Guinea less than 150 miles (240 km) from the Atlantic, the river heads north into the Sahara desert. At Timbuktu, the legendary city of gold, the river turns east, then abruptly south back towards the Gulf of Guinea. The river splits into 23 real mouths in the coastal mangrove forests of Nigeria, and some of these are only navigable by canoe. The Niger Delta is one of the world's largest wetlands, covering more than 7,700 square miles (20,000 square km), and houses Africa's largest mangrove forest.

It is the river's great arc and seeming lack of a mouth that made its course so elusive. It was long speculated that the river was a tributary of the Nile or Zaire (Congo) River.

The river was first thoroughly explored by the Scotsman Mungo Park, who drowned in a rapid during an expedition. The mouth of the river was discovered in 1830 and by the turn of the century had become the focus of European attention for its rich oil deposits. Today, Nigeria depends on these oil reserves to fuel its economy.

The forests of the Niger River delta and Nigeria overall are fast declining. According to the Food and Agriculture Organization of the United Nations (FAO), Nigeria has the world's highest deforestation rate of primary forests. Between 2000 and 2005 the country lost 55.7 percent of its primary forests—defined as forests with no visible signs of past or present human activities.

Flooding, Low Water, High Water

Seasonal flooding is characteristic of many tropical rivers, although few compare to the so-called igapo (swamp forest) and varzea (flooded forest) of the Amazon River Basin, where large tracts of rainforest are inundated to depths of 40 feet during seasonal flooding. The lowest flood stage occurs in August and September, while the highest stage occurs in April and May. Tributaries that drain the Guyana Shield flood in June, while the tributaries that drain the Brazilian Shield flood in March or April. Since the peak rainy seasons are out of phase, the peak discharges of left bank (Guyana shield) and right bank (Brazilian shield) rivers are somewhat offset, having the effect of moderating high and low water levels on the main stream, but tributaries can have extreme variations.

Rain and snow that fall in the Andes and other highland areas reach the Amazon through its tributaries and produce the high-water season. Deforestation of foothills and upper basin may have caused a shift in rain levels during certain times of the year resulting in irregular high and low river levels.

Flooding has important functions for the surrounding forests including eradicating pests, enriching soils with nutrients from whitewater rivers (especially varzea forests), and dispersing seeds.

LOW WATER

The contrasts between the low- and high-water season in some areas of the Amazon Basin are extreme. Low water leaves vast islands and sand bars exposed and river banks high above water level. Smaller tributaries may become so shallow that travel by dugout canoe is barely possible only when travelers push the canoe. Creeks and streams, which are raging torrents when rainstorms come, may dry up altogether.

Low water is a time of troubles for most Amazonian fish and a time of plenty for predators like arapaima, large catfish, dolphins, and jaguars. With the dramatic decrease in water area, fish become trapped in tiny lakes and river shallows and are easy targets for predators.

In the floodplains, which during highwater are a continuous stretch of water, bodies of water are reduced to floodplain lakes. These floodplain lakes are packed with fish and predators, and dissolved oxygen levels are sharply reduced. During a few weeks each year, massive die-offs are caused in these pools when cold Antarctic air passes over parts of the Amazon, cooling surface waters and causing them to sink to the bottom. The bottom of floodplain lakes is often a decaying anaerobic layer of organic sludge. As surface waters sink to the bottom, methane and hydrogen sulfide from the bottom pushes toward surface causing tremendous die-offs. Vultures crowd by thousands to feed on carcasses.

Many fish have adapted to lack of oxygen by developing structures that enable them to take atmospheric oxygen from the air. Most famous are the lungfish of South America, Africa, and Australia, but many catfish, labyrinth fish, and loaches also are able to directly use atmospheric oxygen.

The best-known predator of floodplain lakes is the arapaima or piracucu, one of the world's largest freshwater fish. The species attains a maximum of 16 feet, though today such large individuals are extremely rare because of overfishing. Today conservation efforts are focused on restoring this magnificent species.

The anaconda is also an apex predator in floodplain lakes.

HIGH WATER

High water is the time of the flooded forest when water levels rise 30 to 40 feet and flood the surrounding forest and floodplains, linking river branches as one massive body of water. The higher water level makes the lower canopy accessible by boat. Many tree species depend on the floods for seed dispersal through animal or mechanical (floating downriver) means. It is a time of abundance for most herbivorous fish which can feed on the fruit and seeds that fall from fruiting trees. The Amazon is home to the vast majority of fish species dependent on fruits and seeds.

One famous fruit-eating fish is the tambaqui, a large fish that crushes fallen seeds with its strong jaws. The tambaqui waits beneath trees that are dropping seeds, congregating especially under its favorite, the rubber tree Hevea spruceana, which is widely scattered in the flooded forest. Humans take advantage of the tambaqui and other fish that wait for fallen seeds by imitating falling seeds using a pole with a seed attached by a line. When the fish is attracted within range, the hunter harpoons it. In Amazonian folklore, it is said that the jaguar hunts such seed-eating fish using its tail to mimic the "thud" of falling seeds.

The high-water season is a difficult time for fish predators. The increased water area gives potential prey a larger range and predators must rely on their fat stores from their heavy feeding during the dry season. Many omnivorous species eat mostly seeds and fruit during this period.

High water also means difficulty for ground-dwelling plant and animal species. Many ground dwellers migrate to more elevated areas, while some species move up into the trees. Understory plants and shrubs may spend 6-10 months underwater where they are thought to continue some form of photosynthesis.

Research published in 2005 found that flooding in the Amazon causes a sizable portion of South America to sink several inches because of the extra weight and then rise again as the waters recede. Scientists say that this annual rise and fall of earth's crust is the largest ever detected, and it may one day enable researches to calculate the total amount of water on Earth.

Floating Meadows in the Rainforest

Floating plants have advantages over submerged plants in that they always have access to sunlight and can readily use the nutrients of whitewater rivers. Submerged plants have difficulty capturing enough sunlight in the muddy waters to carry out sufficient photosynthesis.

In some areas giant floating meadows form unique ecosystems colonized by small trees, shrubs, and vines. Such meadows may exceed a square mile in area and are home to a multitude of vertebrate and invertebrate species. One of the most famous floating-meadow dwellers is the world's largest rodent, the cabybara. The cabybara, resembling a 50-kg guinea pig, is most commonly seen grazing grasses on floating meadows and along rivers. Although it looks nothing like a swimmer, the cabybara is a strong swimmer using its webbed feet. Cabybaras live in herds of 10 to 15 individuals and are most active at night. Their numbers have been somewhat reduced due to intense hunting by locals for their good-tasting meat, but their reproductive rates are high and the species is now being used in sustainable development schemes.

Another mammal found in the quiet waters surrounding floating meadows is the manatee. The manatee, thought to be descended from elephants, is a source for the legendary mermaids of ancient times. Although more than 15 feet long and stocky, their form was often mistaken by sailors for that of a beautiful maiden, half fish, half human. The manatee is found both in marine systems and freshwater habitats from Florida to the Orinoco to the Amazon Basin. The manatee is a slow, peaceful creature that spends most of its time sleeping and consuming huge quantities of grasses and aquatic vegetation. During the flood season, when aquatic plants and grasses are easily accessible, the manatee gorges itself with over 110 pounds (50 kg) every day. When the water drops, and food is scarce, the manatee depends on its fat reserves with help from its slow metabolic rate. Interestingly, the manatee has a well-developed system of tooth replacement because its teeth are rapidly worn down by the large quantities of silica in the vegetation on which it feeds. Because of their size, adult manatees have no natural predators, but nonetheless they are highly endangered today by hunting, habitat loss, boat traffic, and other human activities.

Found throughout the Amazon and other tropical waters are giant water lilies, which in clusters form a sort of miniature floating meadow. The most magnificent water lily is Victoria amazonica, the Amazon water lily. Measuring up to four feet in diameter, it is capable of supporting the weight of a small child. The Amazon water lily has a remarkable pollination cycle. Giant white flowers, some the size of a plate, open at dusk with a speed readily seen. The flowers generate a strong butterscotch odor and trigger a stimulus that causes the temperature of the central blossom to rise 11¡ above that of the surroundings. The fragrance combined with the heat attracts scarab beetles, which gather at the flower's center. As night falls the flowers close, trapping the beetles. By dawn the flowers have turned pink and the beetles are gorging themselves on the inner parts of the flower. By the late afternoon the flowers, which have turned a deep reddish purple, open and the beetles, coated in pollen, fly off to find another lily flower. In doing so, they carry the pollen of the first flower and fertilize the second.

A common avian resident of the water-lily meadows are jacanas, which have the ability to run on the water surface or on floating vegetation using their extremely long toes, which distribute their weight sufficiently so they do not sink. Jacanas make nests in floating vegetation, and when the eggs or young are threatened, adult birds feign broken wings, pretending that they cannot fly in order to distract predators.

Both during high and low water the emerged and submerged parts of floating plants provide food and breeding habitats for many vertebrate and invertebrate species. The submerged root zone of one square meter of floating meadow will support over 50,000 invertebrate individuals. These include insects (especially larvae), mollusks worms, arachnids, and crustaceans.

Riverside wildlife in the Rainforest

Few riverine (bank) plant species will be found in the forest. These tend to resemble gap-colonizers and edge species that grow well in the strong sunlight of open areas. There is not a great diversity of riverside plant species, and rivers are often bordered by walls of vines which cover trees because of the access to bright tropical sunlight. The presence of this thick vegetation is largely why early explorers in the Amazon referred to the rainforest as an "impenetrable jungle."

Periodically inundated floodplain forests are quite different from terra firme forests found on well-drained soils. Since floodplain forests have high turnover rates, they are characterized by lower tree diversity and a less-developed canopy. Island forests typically consist of this type of forest. In the Amazon, floodplain forests have high densities of fruiting trees which attract large numbers of mammals.

ANIMAL LIFE

Many terrestrial animals depend on rainforest waters for survival, and some are found almost exclusively along rivers and lakes.

Reptiles are abundant along rainforest waterways. Lizards, including water dragons (Australia), monitors (Africa, Asia, Australia), iguanas (New World), and other species are especially common. One of the most interesting is the basilisk lizard of the New World. This lizard, popularly known as the Jesus Christ lizard, has the ability to run across water for considerable distances by using its long toes and tail. Any visitor to the rainforest can get countless hours of enjoyment by chasing these lizards off beaches and watching them use their quick stepping to support their weight as they escape across the water.

The giant snakes of the world, pythons of the Old World and boas and the Anaconda of the New, are found along waterways where they feed on large prey that comes down to drink at the river. Crocodiles, in various forms, are found worldwide, but perhaps the most impressive is the saltwater crocodile of Australia and New Guinea, which is famous for occasional attacks on humans. These giant crocodiles, which are found in marine, brackish, and freshwater habitats, may exceed 20 feet in length.

Bird life along rivers is prolific because of the availability of foods in the form of fish, insects, and fruits produced by riverside vine species. One interesting phenomenon is the gathering of macaws along the clay river banks of the upper Amazon where they lick the mineral-rich clay that binds to and detoxifies the harsh chemical content of the fruits they consume. River areas are among the best places for rainforest bird watchers, because they are open and bird life is abundant.

Mammals also are found along rivers. Jaguars are most often seen around rivers where they hunt and fish. Another predator found along rivers is the giant river otter of South America. This otter, which has no natural predators and is an efficient hunter, has been rendered rare by over-hunting, and can only found in scattered areas today where human presence is limited.

Importance of Rainforest Rivers to People

Tropical rivers have always played an essential role in the ecology of the rainforest, but they have also been important in the lives of forest and non-forest peoples. Before the arrival of the Europeans, sprawling civilizations and smaller societies formed along major waterways which served as a means of transport and communication, a route for trade, and a source of fish and fresh water. However, because of their location along major rivers, such settlements were the first to disappear, either directly affected by warfare or indirectly affected by the onslaught of European diseases.

Today, large tropical rivers remain important forms of infrastructure providing a cheap, reliable, and easy means for transportation and communication. Major rivers ports like Iquitos, Peru, and Manaus, Brazil, are trade centers where forest products are exchanged for outside goods. Often, the only access to such ports, isolated by the surrounding forest, is by water or air. For example, there are no roads leading into to Iquitos and all construction materials, automobiles, and other essentials must be shipped into the city by river or air.

Besides providing a means for commerce and communication, tropical rivers are a key source of protein, in the form of fish, for the population. However in some areas, fishermen report smaller catches as regional overfishing takes its toll. The river also provides services for city dwellers by fertilizing the surrounding soils every flood season and by taking away the tons of human waste and pollutants that city dwellers dump into the river.

Tropical fish provide an important source of income for many city dwellers, especially in the Amazon, which is home to some 3,000 species of fish. Many of the fish seen in temperate-zone aquarium shops are imported from Brazil and Peru.

Tropical lakes

The vast majority of natural lakes in lowland rainforests are oxbow lakes. An oxbow lake is a crescent-shaped lake formed when a river changes course. In lowland rainforests like the parts of the Amazon where soft alluvial soils dominate, meandering rivers gradually shift due to erosion and sediment deposition. Oxbow lakes typically form when loops in the river become so extreme that the main channel erodes a new straighter route, leaving the river bend apart from the river. As time passes, the oxbow lake becomes increasingly distant from the main channel. Water conditions change as the water stagnates.

The characteristics of an oxbow lake generally depend on its age. Younger oxbow lakes may be "flushed" by river currents, especially during the flood season. These lakes tend to be slightly more turbid (less transparent) and less acidic than older lakes more distant from the river channel.

Oxbow lakes are key habitats for many species of fish and aquatic animals. A number of popular tropical aquarium fish species live exclusively in oxbow lakes, which also serve as the primary habitat for giant river otters in South America.

Free-standing lakes in the rainforest also exist, though they are not as abundant as tropical rivers. Like oxbow lakes, the tend to become more acidic with time as vegetation decays.

Threats to Tropical Rivers and Lakes

Tropical rainforest waters are highly threatened today by hydroelectric projects, erosion from deforestation, overfishing, and pollution from industrial activities, including oil spills and mining waste. The effects from the degradation of these waters are widespread, inflicting damage on the global economy, the environment, and local peoples.

Dams

Increasing demands for energy are putting the world's rivers at risk, with hundreds of dams planned in the Amazon Basin, Borneo, the Mekong watershed, Central America, and Central Africa. Hydroelectric projects have been responsible for flooding vast areas of rainforest, with significant detrimental impacts for climate, wildlife, and river-dependent people.

Dams in the tropics have two principle greenhouse gas emissions sources: carbon released from soil carbon stocks and dying vegetation when the reservoir is flooded and methane formed where organic matter decays under low oxygen conditions at the bottom of the reservoir. Methane emissions are facilitated by a dam's turbines, which usually draw from the bottom of the reservoir and spray methane-dense water into the air upon release. Emissions from rotting vegetation occur on an ongoing basis when the levels of the reservoir fluctuate: during the dry season weeds, emerge from the muddy drop-down zone, only to rot again when waters return. The effect turns a typical tropical dam into what Philip Fearnside, an expert on the Amazon, calls a "methane factory".

Dams can also be enablers of deforestation, spawning roads that facilitate new clearing and generating electricity for industrial farms, mines, and aluminum manufacturing. For example, the controversial Belo Monte dam on the Xingu River in the state of Pará, Brazil, illustrates many of the concerns with dam-building in the Amazon, where 146 dams are planned in coming decades. The dam will flood a quarter of the city of Altamira and divert 80 percent of the Xingu's flow from the main stem of the river, leaving 60 miles (100 km) of one of the Amazon's largest tributaries nearly dry. Communities of that stretch of the river will lose their primary source of livelihoods — fishing. The dam will also impede migration of some of the river's largest fish species, potentially affecting fish populations elsewhere in the river basin. (A number of Amazon fish species move upriver or downriver to spawn). Other communities will suffer from inundation. In these areas, people are being removed by force, according to Fearnside.

But the dam has bigger problems. As currently designed, Belo Monte will suffer from the Xingu's seasonal variation in water flows. The most expensive parts of the dam to operate — turbines and transmission lines — would need to be idled for four months each year due to low water. That issue makes it likely that Brazil will push forth with a plan to build additional dams upstream from Belo Monte (up to six dams were planned on the Xingu until 2008). These dams would capture water, ensuring more consistent flow for Belo Monte. However greater water level fluctuation upstream means these dams will have even higher methane emissions relative to their generating capacity.

Erosion

Erosion is a conspicuous impact of deforestation with serious consequences for river commerce and river life. Sediments build up creating sandbars and shallows and interfering with river transportation. Similarly, sediment build-up reduces the effectiveness of existing hydroelectric projects. Erosion and the resulting decline in water clarity can cause downstream mayhem for offshore coral reefs. River inhabitants also suffer due to the reduced water clarity. Species that rely primarily on sight decline the most, while the increased amount of suspended particles interferes with fish gills. Erosion inhibits plant growth and can hinder the development of fish eggs.

Overfishing

Overfishing is a problem that plagues the world's oceans (35-60 percent are overfished worldwide [overfishing news]) and freshwater habitats. Regional declines in catch have been reported throughout the Amazon. The loss of certain species responsible for seed dispersal will have a negative effect on the renewal of the rainforest.

Pollution

Poisoning from spills and pollution from industrial processes mining, and sewage impact the diversity of rainforest waters, in addition to affecting human populations. Chemical spills are usually associated with oil development and mining.

REVIEW QUESTIONS

Review questions - Part I

• Why are some of the world's largest rivers found in tropical regions?

Review questions - Part II

• What are three major water types in the rainforest?

Review questions - Part III

• What is the world's largest river by volume?

Review questions - Part IV

• How do changes in water level affect the Amazon?

Review questions - Part V

• What is the capybara?

Review questions - Part VI

• How are floodplain forests different from upland forests?

Review questions - Part VII

• Why is the Amazon River important to local people

Review questions - Part VIIa

• What is an oxbow lake?

Review questions - Part VIII

• How are rainforest rivers under threat?
• How do dams contribute to global warming?
• Why are dams usually bad for native fish species?

CITATIONS

Citations - Part I

• Richard Spruce's opening quotation (made around 1850) is found in One River (New York: Touchstone, 1996) by Wade Davis.

Citations - Part II

• The River of Hunger is a name used by Davis, W. in One River (New York: Touchstone, 1996) for the Rio Negro in Brazil. The name originated from indigenous people living around this blackwater river. Davis goes on to contrast the blackwater Rio Negro with the whitewater Amazon.

Citations - Part III

• Forbath, P. conveys the greatness and rich history of the Congo (Zaire) River in The River Congo, Boston: Houghtin Mifflin Company, 1977.

Citations - Part IV

• The flowering and pollination of the Amazonian water lily is described in Attenborough, D. (The Private Life Of Plants, Princeton, New Jersey: Princeton University Press, 1995); Goulding, M. (Amazon-The Flooded Forest New York: Sterling Publishing Co., Inc., 1990); and Davis, W. (One River, New York: Touchstone, 1996).
• Goulding, M. (Amazon-The Flooded Forest New York: Sterling Publishing Co., Inc., 1990) is the source for the number of species and individuals in a floating meadow.
• The ecology of the tambaqui is discussed in Amazon-The Flooded Forest (New York: Sterling Publishing Co., Inc., 1990) by M. Goulding.
• Goulding, M. (The Fishes and the Forest. Berkeley, CA: University of California Press, 1980) finds that numerous fish species are important seed dispersers in the flooded forest and warns that clearing of vàrzea forests could reduce their populations. He also reports that over three-quarters of the fish important in commerce and subsistence depend directly or indirectly on flood-plain forests for food.

Citations - Part VIII

• Oil operations in the Niger River Delta are examined in Moffat, D. and Lindén, O., "Perception and Reality: Assessing Priorities for Sustainable Development in the Niger River Delta," Ambio Vol. 24 No. 7-8 (527-538).
• Pakenham, T. (The Scramble for Africa, New York: Avon Books, 1991) provides the history of the European discovery of the Niger River.