Fresh Water

Background information fact; by edgardowelelo@yahoo.com, master of the game

All life on land is dependent on FRESH WATER. Though in short supply, its influence is enormous. Not only does it shape the landscape, but where it collects, it harbours communities of unique and surprising life – forms. It is quite simply the most precious resource on Planet Earth. Fresh water is rare. Just 3 per cent of the Planet Earth’s water is fresh, the rest is SEAWATER. And of that tiny amount, around 70 per cent is locked up as ice and snow, mainly at the poles. In ANTARCTICA, the giant ice sheets nearly 5 km (3 miles) thick in places are virtually incapable of supporting life within their frozen depths. Another 30 per cent of fresh water is stored within rocks and in soil as ground water – only 0.3 per cent flows on the surface as rivers and lakes. Yet rivers and lakes provide some of the richest habitats on Planet Earth. Venture below their surface, and you will discover a world teeming with life of every size and shape. Forty per cent (40%) of all fish species live in fresh water, and most amphibians, countless insects, reptiles, plants, birds and mammals live in or on it. Science is slowly unlocking the secrets of this mysterious world, and finding new forms of life. The River AMAZON alone – home to 3300 species of fish, more than in the ATLANTIC –  is predicted to yield at least another 1700 new to science.

RUNNING WATER

The safari begins high in the MOUNTAINS. Humble streams flow down to join mighty rivers that travel hundreds of miles to their ultimate destination – the SEA. Here the cycle begins again, as water evaporates, precipitates over the land and eventually returns to the SEA through rivers. This is the global hydrological cycle. One of the most remote and isolated mountain plateaus, or tepuis, provides an insight into this rite of passage. Hidden in permanent cloud, the ancient tepuis of southern Venezuela rise 1,000m (3280 feet) above the rainforest. This imposing landscape was the inspiration for ARTHUR CONAN DOYLE’S The Lost World – an imagined prehistoric terrain cut off from the jungle below. The first explorer to fly over these plateaus, in the 1930s, was Jimmie Angle, an American aviator who set out in a small plane to look for signs of gold. He diced with death, and one safari ended dramatically when he crashed on top of the most famous tepuis, AUYAN, or DEVIL’S MOUNTAIN. He escaped with his life, but even today, flying over this tepuis is an unnerving experience. Fierce winds buffet the helicopter, and thick blankets of clouds close in, obscuring the way ahead. Though the shattered shards of Jimmie Angle’s plane have long ago been moved to a museum, the wreckage of other aircraft is strewn across the rocks as a chilling reminder of the dangers of venturing over this high plateau. The rewards of exploring this area, though, are enormous. Touching down on top of the KUKENON TEPUIS, you enter a strange world. Through the mist, weird forms appear – towers of sandstone, sculpted over the millennia by wind and torrential rain into shapes that eerily resemble figures. Underfoot in the bog is one of the greatest collections of insectivorous plants, the majority of which grow only here – pitcher plants and sundews that live on insects blown up from the jungle below. They thrive in the wet conditions created by an almost daily tropical downpour and an average annual rainfall of 400cm (155 inches).The source of this rain is moisture rising as water vapour from the sea, blown inland and forced up on reaching the mountains. As it cools, it condenses into cloud and finally rain. Sheets of water pour over the rocks and flow down gullies into streams that are channeled into small rivers. By helicopter, you can chart the courses of the golden streams that delineate the top of Devil’s Mountain until the water suddenly hurtles over a precipice – and your stomach falls into your feet. On a clear day, the full 9179m (3211 foot) drop of the longest continuous waterfall in the world can be seen. The descent creates a force so great that, long before the chute of water reaches its base in the Devil’s Canyon; it is blown by the wind into a fine mist. This waterfall – which eventually feeds into the ORINOCO, South America’s second largest river system – is called ANGEL FALLS after who else but the man who discovered it back in 1933, Jimmie Angel.

 

LIFE IN THE FAST STREAM

Each spring, as temperatures start to rise, the melting ice and snow produce a surge of fresh water. These melt – water streams are full of energy as they cascade down the mountains, building pace and power as they go. As any canoeist hooked on the adrenalin of shooting rapids and plunging down waterfalls will testify, it is these upper reaches that a river is at its most dramatic. Mountain water is cold and low in nutrients but high in oxygen. Here invertebrates dominate, but to survive in the torrent requires adaptations. The predatory North American hellgrammite nymph has a flattened profile to reduce drag and a series of grappling hooks to prevent it being swept away. Black – fly larvae anchor themselves with a ring of hooks on their abdomen and deploy silken safety lines if they come unstuck. Caddisfly larvae build cases of stone, sand and sticks to protect themselves against both predators and being bashed against the rocks. Specialized fish, including the Tibetan stone loach, the world’s highest – altitude fish, feed by scraping the algae off rocks and hold on with enlarged mouth suckers. It may be a harsh existence, but there are advantages to life in the fast stream. In particular, predators are few. But there are, of course, the specialists. Torrent ducks, found high in the Andes of South America, and harlequin ducks, found from Alaska across to Siberia, have streamlined bodies and powerful webbed feet to battle against the river currents as they feed on aquatic invertebrates. The only large predators in the remote mountain rivers of China and Japan are the world’s largest amphibians, giant salamanders. These monstrous creatures, which can grow almost 2m (6.5 feet) long and live up to 80 years, have exceptionally slow metabolism – ideal for a cold environment. By day, they rest in communal underwater caves, but at night, they emerge to hunt fish such as trout. A giant salamander has poor eyesight, but sensory nodes on its body detect the slightest change in water pressure. It lies in ambush, and as prey nears, it strikes with astonishing speed. Free from competition, it dines alone.

THE GREAT SALMON RUN

New life reaches the high, cool temperate rivers of the northern hemisphere in the form of the world’s greatest freshwater fish migration. Each spring, at least 2 million Atlantic salmon invade around 2000 river mouths along the coasts of North America and Europe and battle upstream to their spawning grounds. It is a gruelling safari, in some cases more than 1600km (990 miles). Six species of Pacific salmon – sockeye(red), pink, chum, coho(silver), steelhead and king (Chinook) – pile into northern rivers from California to Korea. In the northwest Pacific alone, around 142 – 287 million salmon are thought to be on the move. Before commercial fisheries and habitat destruction took their toll, the amount was estimated to be up to 350 million salmon. A salmon may spend several years at sea, feeding and growing. It needs all its strength to embark on the last and most demanding safari of its life – to its birthplace high upriver. These fast – swimming and streamlined fish can swim up large rapids and even leap up waterfalls on their titanic endeavor. On the Canadian west coast, sockeyes break their safari in resting pools, where they conserve their energy while waiting for the next pulse of floodwater to raise the river level and aid their upstream safari. Hazards are everywhere, but at this stage, it is the grizzly bear that poses the greatest threat. For the bears, it is a time of plenty. Capable swimmers, some have even learnt to dive after salmon, pinning them to the riverbed with their massive clawed paws. Cubs also join in the fishing sorties, but catching salmon in deep water is not easy, and for much of the time, they rely on handouts from their mothers. On average, an adult bear will consume more than a ton of salmon over the six – week spawning period, and for the growing cubs, the amount of salmon eaten may make the difference between life and death in their first winter. The vast majority of salmon, though, escape to complete the final stage of their arduous safari. Their extraordinary homing and navigational systems guide them back to the streams where they were spawned years before. The physical transformation that began when they entered fresh water has now finished. They have changed from silver to bright red, and the males have launched up and developed a kipe, an upturned lower jaw, which they use for jousting to secure a female’s favour. Fertilized eggs are deposited in gravel scrapes, where they can develop bathed in highly oxygenated, relatively predator – free waters. Once Pacific salmon have spawned, they die. The riverbeds become mass graveyards. Even in death, salmon play a vital ecological role, with more than 137 species – from caddisfly larvae to armies of crayfish, from bald eagles to wolf cubs – feeding on their carcasses. It is a massive injection of energy into these high rivers, and even the neighbouring forests benefit, growing taller in soils fertilized by carcasses scattered by predators. But the future for salmon is looking bleak. In the North Atlantic, the number returning from the oceans has halved, possibly due to rising sea temperatures and a disruption in the food chain. Those that make it back to the rivers face another threat – escaped farmed salmon. In 2005, an estimated one million farmed fish found their way into the waters of Norway and Scotland alone. The escapees spread disease and mate with wild fish to create hybrids, which scientists fear will threaten the wild diversity and lead to genetic weakness and a drop in survival. Plans to farm transgenic or genetically modified salmon could pose more problems for the future.

GORGES AND CANYONS

High – altitude rivers, fed by rain and melt water, continue to build in volume as they merge. Main tributaries, still powered by gravity, become the most erosive forces on the Planet Earth. They shape the land, carving out V- shaped river valleys, gorges and canyons. The deepest canyons are in the Himalayas – the world’s highest mountain range and subject to massive river erosion – with the Yarlung Tsangpo Gorge, in a remote region of TIBET, being the deepest of them all (only recognized as such in 1994). Up to 5.9km (3.7 miles) deep and 45km (28 miles) wide in places, it was created by the mighty Brahmaputra River. Other giant canyons in the Himalayan plateau include the 4.3km (2.7 mile) deep KALI GANDAKI CANYON in Nepal, and the 3.6km (2.3 mile) deep TIGER LEAPING GORGE in China. In the Americas, the deepest canyons include COLCA in the Peruvian Andes at 3.6km (2.3 miles), the COPPER CANYON in Mexico at 1.7km (5770 feet) and perhaps the most famous of them all, the GRAND CANYON in Arizona, USA. The Grand Canyon is just one part of the world’s longest canyon system – a 1600km (1000 mile) scar, clearly visible from space – created over 5 million years as Arizona’s Colorado River has eroded the sandstone. Today, the mighty Colorado River has been tamed by the HOOVER DAM, but its canyon legacy is testament to the land – shaping power of mighty rivers. At 355km (220 miles) long and more than 1.6km (a mile) deep and 27 km (17 miles) across at its widest, the Grand Canyons draws enormous crowds to marvel at its colossal size and grandeur.

RIVER DRAMAS

As rivers wind their way down from the mountains, they (rivers) start to slow down and gradually warm up, becoming richer in life as they do so. In southern India, the CAUVERY RIVER is laden with nutrients as it flows down from the Western Ghats. As its waters warm, they teem with fish, including 45 kg (100 pound) mahseer, prized by fishermen. Mahseer are food for mugger crocodiles – 4m (13 feet) or more long – which can be seen basking on the sandbanks. At dawn, great splashes reveal under water dramas as the mahseers attempt to escape the muggers’jaws. The rich waters provide enough food to support family groups up to 17 – strong of smooth – coated otters – the most sociable of all otters. In the dry season, before the monsoons arrive, when the water level falls and the boulder – strewn river divides into numerous channels and islands, it is possible to glimpse otters moving from one hunting pond to another. They leave their riverbank holts before dawn to begin their sorties across their huge, 10 km (6.2 mile) wide territories. When it comes to fishing, there is real strength in numbers. They often work together to drive and corral fish. Even the cubs may help, starting to learn to catch fish at about four (4) months old, but only the adults have the necessary speed and agility to land prey. They can remain under water for up to three minutes at a time and catch, on average, an eighth of their body weight in fish a day. The otters share the water with the mighty muggers, but the advantage the otters have is safety in numbers. Once in a while, there is an outburst of shrieking and primeval hissing as a crocodile appears to attack an otter. But watch closely and you will discover that it is the otters who are the aggressors, ganging together to hound the crocodiles off the sandbanks. Crocodiles are cold – blooded, and all 22 species usually live in lower – altitude subtropical and tropical rivers and estuaries. Large concentrations of giant Nile crocodiles live in the rivers that snake across East Africa’s Serengeti. These rivers are dangerous obstacles but also vital sources of water for thirsty herds of grazing animals crossing the parched grasslands in search of fresh green pastures. Once a year, a drama unfolds as 2 million wildebeest march across the plain. The crocodiles know the wildebeest are coming and gather in anticipation of their annual feast. As the thirsty wildebeest file warily down the riverbank of the Grumeti, crocodiles slide silently into the murky waters. Adapted to hunt in low visibility, they rely on stealth, crawling along the riverbed, homing in on the vibrations from the drinking wildebeest. Flattening themselves to remain hidden in only 30cm (12 inches) of water, they inch closer and closer until their snouts are almost touching those of the wildebeest above. For a film crew, the tension can be unbearable. The nervous wildebeest edge ever closer to the water. It can take hours – one step forward, one step back. Then suddenly, the edgy calm is shattered by an eruption as a crocodile launches out of the water. Chaos breaks out as wildebeest jump back and topple over each other in panic. Zebras alarm – call in terror. Only a state – of – the art camera can slow down time enough to reveal the narrow line separating life from death. Once a crocodile has a hold, it never lets go. A titanic tug – of – war can develop between a bull wildebeest and a crocodile, lasting more than an hour before the crocodile manages to pull the bull into water deep enough to drown it in.

 

GIANT LAKES

Most rivers drain into the SEA, but the GRUMETI flows inland from Africa’s east coast into the world’s second – largest freshwater lake, Victoria (Nyanza). Lakes hold most of the Planet Earth’s fresh water – at least 20 times more than the rivers. From space they appear as immense patches of silvery blue, large enough to be mistaken for seas. Some are so wide and deep that they have created their own weather systems. Others contain marine – like communities of seals, jellyfish and sponges. Each supports large numbers of species found nowhere else. The world’s largest lake is the CASPIAN SEA in southwest ASIA, covering 370,000 sq km (143,000 square miles). Its waters are saline, giving it characteristics of a marine environment.

The largest freshwater lake is Lake Superior, part of the GREAT LAKES OF NORTH AMERICA, which form the biggest continuous mass of fresh water on Planet Earth, covering an expanse larger than UK. But the mother of all Lakes is Lake BAIKAL, in eastern SIBERIA, a huge chasm more than 1.6km (a mile) deep, which holds a fifth of all the world’s surface fresh water. For centuries, the deepest lakes have fuelled imagination, inspiring tales of prehistoric creatures surviving in bottomless, underwater worlds. Yet few lakes exceed 500m (1640 feet) in depth – Scotland’s Loch Ness, famous for its “monster”, is just 230m (755 feet) deep – and only LAKE BAIKAL sustains life beyond depths of 200m (655 feet). But one lake revered for its monsters does deserve its reputation. Lake Nicaragua in Central America is home to the dangerous bull shark, which grows to more than 3m (10 feet) in length. So numerous were its numbers that a shark – fishing industry operated until the 1970s, decimating the population. Bull sharks are now known to make their way up and down the connecting San Juan River, which drains into the CARIBBEAN SEA. They are one of the few shark species that can tolerate the salinity change between salt and fresh water.

LITTLE CICHLIDS AND BIG SWARMS

East Africa’s Great Rift Valley, running south from the end of the RED SEA to TANZANIA, is a great split in the Planet Earth’s Crust. In this trench are some of the world’s largest, deepest and most fascinating lakes.  Lake Victoria (Nyanza) covers 70,000 sq km (27,000 square miles) but is relatively shallow and swamps. Scientists have so far identified more than 350 species of fish unique to the lake, mostly in the eye – catching cichlid family. Lakes Tanganyika and Malawi (Nyasa), though smaller in area than Victoria (Nyanza) – 33,000 sq km and 30,000 sq km (12,700 and 11,580 square miles) respectively – have even more fish diversity. Set against breathtaking mountainous backdrops, these tropical waters provide a freshwater diving experience as close as it gets to a coral – reef one, their crystal – clear rocky shallows displaying schools of colourful cichlids. But the most fish of any lake – 850 species of cichlid alone – are found in Lake Malawi (Nyasa), the smallest of the three lakes but still bigger than the country of Wales. Some have evolved into algal scrapers, others into sediment feeders and some into predators of the fry of others. Almost all the cichlids protect their young using mouth – brooding – at the first sign of a predator, the parent signals its fry to swim into its cavernous mouth – and it will continue to guard its babies until they grow too large to fit in. At night, though, cichlids are prey to the mormyrid, an electric fish that emerges from its lair to sweep the rocks for fish, detecting them by distortions in the electric fields around their bodies. Away from the shallow shore, the lake – bed plummets into an abyss without light, warmth or oxygen. Both Lake Malawi (Nyasa) and Lake Tanganyika have a thermocline – a 200 m (655 foot) permanent barrier that seals off the upper warm waters from the colder waters below, preventing any mixing of nutrients and oxygen and creating a bottom dead zone. In Lake Tanganyika, the second deepest lake in the world at 1500m (4920 feet), 90 per cent of its water is uninhabited. In Lake Malawi (Nyasa), little survives in the dead zone apart from the larvae of the lake fly, or phantom midge.   During the day, these larvae appear to seek refuge from predators in the oxygen – starved depths. At night, they balloon up to the surface to feed on PLANKTON. Come the rainy season, they undergo a transformation. At dawn the first adult midges appear on the surface. Soon millions upon millions of newly hatched lake flies are taking flight – spiralling orange tornadoes of making insects rising high in the air and at times so dense that fishermen are rumoured to have choked to death in the swarms. The spellbinding display lasts just a few hours. Once the flies have mated, they drop to the water surface, release their eggs and die. By the afternoon, all that is left is the orange stain of millions of dead insects on the water surface – soon cleared by cichlids.

 

THE OLDEST LAKE IN THE WORLD

LAKE BAIKAL, which means sacred sea, holds a fifth of the Planet Earth’s surfaces fresh water, and at 25-30 million years old, is the oldest lake in the world. Like Lake Tanganyika, BAIKAL owes its origin to movements of the TECTONIC PLATES that make up the EARTH’S CRUST, and is contained within a long, steep – sided trough, 650km (400 miles) long and nearly 2km (more than a mile) deep. Life in eastern Siberia is harsh. Winter temperatures average -20°C (-4°F), and the lake is FROZEN SHUT for more than five (5) months of the year by an ICE SHEET 1.2m (nearly 4 feet) thick-strong enough to support the 2- ton trucks that use it as a highway from one side to the other. Yet if you saw through the ICE and dive into the world below, you will find yourself in a winter wonderland. Below the ICE, the landscape is one of strange, translucent ice sculptures, and there is life in profusion. In its isolation, this ancient lake has undergone an evolutionary explosion and now holds more than 1200 species of animals and 1000 species of plant, 80 per cent of which are found nowhere else. Many resemble marine creatures, such as the sponges that carpet the lake’s shallows. There are 147 species of snails and 255 species of amphipods – shrimp – like crustaceans, some as large as mice. BAIKAL holds 40 per cent of the world’s amphipods, which play a vital role scavenging on the dead.  Amphipods can do the job in the dark, cold depths, where temperatures are not high enough for even decomposition bacteria to function properly. BAIKAL is home to the world’s only freshwater SEAL, the NERPA, which is thought to have migrated to the lake 22 million years ago from the ARCTIC OCEAN, along rivers that have long since disappeared. About 50,000 NERPA may live here. Smaller than most SEALS, they grow to just 1.2 – 1.4m (4-4.6 feet) in length. The females give birth during the winter in ice lairs. They maintain breathing holes in the ice by scraping the edges with their sharp claws or by gnawing. As spring progresses and the ice starts to melt, the lairs become exposed and the pups become vulnerable to wandering bears or even crows. As adults, NERPA can dive down to 300m (985 feet) and remain under water for more than 70 minutes at a time. Here they feed mainly on the translucent golomyanka, or oil fish, also found only in Lake BAIKAL. Golomyanka contain 35 per cent oil and are the commonest fish in the lake. They can cope with the crushing water pressure 1400m (4590 feet) down, but if they are taken out of the lake, they literally melt. Extraordinary discoveries have been made by deep – water submersibles. The water is constantly well mixed, and so oxygen is always plentiful, right to the very bottom, 1637m (5370 feet) down – which enables animals to live at any depth. At a depth of 400m (1312 feet), scientists were amazed to find HYDROTHERMAL VENTS, previously only thought to exist in the deep oceans. Thriving around these superheated vents are unique communities of sponges, bacterial mats, snails, fish and transparent shrimps. Descending to depths of more than 1km onto the abyssal plain of the lake, which itself lies on top of a 7 km (4.3 mile thick layer of sediment, they have found a giant deep – water flatworm that can grow to 40cm (16 inches) and preys on fish – just one of 80 species of flatworm in BAIKAL.

 

GIANT RIVERS

Most great rivers don’t end their journeys or safaris in lakes but run great distances to the SEA. From space, they (most great rivers) appear as long ribbons of silver snaking across the continents. The top ten (10), ranked by how much water they carry, are headed by the AMAZON – by far the largest, with more than four times the amount of water than the next biggest, the CONGO. The third biggest is the ORINOCO, followed by the YANGTZE, PARANA, BRAHMAPUTRA, YELLOW, YENISEI, GANGES and MISSISSIPPI. The Nile in Africa is the world’s longest river – one major tributary, is the BLUE NILE, rises in the Ethiopian Highlands and the other, the WHITE NILE, in Lake Victoria (Nyanza) – which empties 6695 km (4150 miles) later into the Mediterranean. Born in the HIMALAYAS, the world’s highest mountain range, are many other great rivers. The GANGES and BRAHMAPUTRA flow south through INDIA, join forces to form the PADMA – then effectively, the world’s third largest river in volume of water and the first in terms of the amount of sediment it carries – and emerge in the BAY OF BENGAL to form the world’s largest delta. Arising in TIBET and flowing across the north of China is the YELLOW RIVER, which is second only to the PADMA in terms of sediment. Further south in China is the YANGTZE, the longest river in ASIA at 5150km (3190 miles) and probably the deepest in the world when it flows through the Three Gorges area.

 

THE AMAZON QUEEN

In the Americas, the Mississippi and its tributaries drain almost the whole of the US. But the queen of all rivers is the AMAZON. Carrying almost a fifth of the world’s flowing freshwater – the amount carried by the next ten biggest rivers combined – its river basin is larger than any other, draining a third of South America. It transports a billion tons of sediment a year, visible from space as a brown stain hundreds of miles out to sea. Rising in the PERUVIAN ANDES, its main trunk flows eastwards across BRAZIL. In places this monster of a river is 40km (25 miles) wide when in flood. Even 3700km (2295 miles) from the sea, it is navigable by ocean – going ships. Eventually, more than 6450km (4000 miles) from its source, the world’s second longest river drains into the Atlantic Ocean. The Amazon’s waters are wonderfully rich. To date, in excess of 3000 different species of fish have been identified – more than in the entire Atlantic Ocean – and scientists predict the count could eventually rise to more than 5000. In the depths lurk creatures that communicate through electricity. By altering the properties of the electric fields around their bodies, they can “talk” to each other in the gloom, and the electric eel can deliver up to 600 volts – enough to render a human unconscious. Monster fish including pirarucu, or arapaima – the longest fish in South America at 2.5m (more than 8 feet) – also skulk in the gloom. Other oversized predators include the world’s largest freshwater dolphin, otter and river turtle and the green anaconda, which can grow to more than 6m (20 feet) long. The boto, or pink river dolphin, can weigh up to 100kg (220 pounds) and grow to more than 2.5m (8 feet) long. It is highly social and may live in groups of up to 20 individuals. In the breeding season, there is stiff competition for mates, and males will fight, inflicting serious bites. But they also display in front of watching females, rising up out of the water with rocks in their jaws, perhaps to prove how strong and dexterous they are. The Amazon provides food aplenty for botos. Migrating fish pass through their territories, including shoals of the dorado catfish. This species makes the longest migration of any freshwater fish – from the Amazon’s delta to the Andean foothills, probably to spawn there – a journey of more than 4830km (3000 miles). During the piracema, the water literally boils with fish so densely packed that individuals may die through lack of oxygen.  The botos follow the shoals and corral them in shallower water, where capture is easier. The Amazon is also home to the tucuxi, a porpoise known locally as a “river dolphin”. The tucuxi and boto are the only freshwater cetaceans not critically endangered. The four other species, all found in the largest rivers in the world, tend to live in muddy water, have reduced eyesight or are virtually blind and rely on highly developed sonar to catch fish. The two species on the Indian subcontinent are the Ganges dolphin, found from the foothills of the Himalayas to the Ganges – Brahmaputra delta, and the Indus river dolphin of Pakistan and India. The Yangtze River, or baiji, of China is the most threatened of them all. The development of the Three Gorges Dam, along with increased pollution, netting and river traffic, has pushed it to imminent extinction, with a handful of individuals left. A river’s final passage to the sea may still encounter obstacles. In the lower courses of the IGUACU RIVER on the border of Brazil and Argentina, where the waters have left hard rock and flowed over a softer layer, they have eroded a huge waterfall 2.5km (1.6 miles) across. You can hear the roar of the mighty IGUACU FALLS (Iguacu means “great water”) 15km (more than 9 miles) away. The best way to view the head of the falls is by helicopter, where the river plunges over its semi – circular bowl down into the mist – filled chasm known as the Devil’s Throat, but the spray and vicious updraft make it a hair – raising experience.

SWAMP AND MUD

In their final stages, rivers broaden and flow across their low – lying floodplains, and when they break their banks, they create WETLANDS. These cover 6 per cent of the Planet Earth’s surface and include bogs, marshes and swamps. Notable waterlogged areas are the Everglades in Florida, the Okavango in Botswana, Kakadu in northern Australia and the Amazon in Brazil. Important havens for wildlife, wetlands also act as the kidneys of the land, filtering out pollutants and rejuvenating the system. But agriculture and urban development continue to suck dry the remaining areas. Each wet season in southwest Brazil, the Parana River undergoes a dramatic change. Fed by rainfall far away, the river swells and overflows its banks to flood an area the size of England – approximately 130,000 sq km (50,000 square miles). This creates the largest wetland in the world, called the PANTANAL from the Portuguese for swamp. For the next six months, the vast tracts of dry grassland and forest are transformed into a temporary water world of swamps, pools and water channels. The PANTANAL supports diverse forms of animals and plants. Its slow – flowing, nutrient – rich river water nurtures underwater forests of aquatic plants such as water hyacinth and Victoria giant water lily. This environment is the perfect nursery for fish. More than 300 species breed here, including freshwater stingrays and piranhas. Yellow anaconda and spectacled caiman up to 3m (10 feet) long also thrive here. Snorkelling in these clear, shallow waters in like being in a giant tropical aquarium. Carried along in the gentle current, you sweep past swathes of aquatic plants and schools of exotic – looking fish. Fig trees overhanging the water’s edge provide welcome food for shoals of hungry fish. The competition for falling fruits is intense, and schools of pacu and piraputanga spend hours moving up and down the river channels in search of them. So keen are they that they appear to have learned to shadow brown capuchin monkeys, waiting for them to dislodge fruit. Darting pacu and piraputanga cause a commotion, which in turn attracts dorado, the largest predatory fish in the PANTANAL, known as river tigers because of their sharp teeth and powerful jaws. Ready to pick off their leftovers are the red – bellied piranhas, which pour out of the thick vegetation en masse and can strip a fish to the bone in minutes. By April the rains cease, and the PANTANAL begins to dry out, concentrating fish into a series of pools. These “fish larders” supply the abundant waterbirds that breed at this time. There are more than 650 bird species in the PANTANAL. The largest is the JABIRU STORK at 1.5m (5 feet) high, and herons, wood storks and roseate spoonbills form colonies 10,000 strong. The large numbers of growing chicks come under continuous attack from avian predators such as caracaras and turkey vultures, and any flightless chick that becomes dislodged from its nest is likely to fall into the waiting jaws of the spectacled caimans lingering below. As the dry season heat intensifies, fish desperately attempt to escape from the drying pools into the deeper river channels, and caiman are forced to march away in search of fresh water, and do so in long lines. Then the rains finally break once more, the fish eggs lying dormant in the soil like seeds hatch out, and the richest wetland in the world rejuvenates.

 

THE FINAL ACT

Down on the coastal plains, the gradient is now so shallow that the rivers begin to lay down their sediment. The growing banks of mud split the rivers into numerous channels, and where sediments build out into the sea, DELTAS are created. The largest DELTAS are, of course, created by the world’s biggest rivers. They can take different forms – the bird’s foot of the Mississippi delta or the fan of the NILE DELTA, for example. Though the AMAZON appears more like an estuary in shape, its vast delta is under water, and it carries fresh water far out under the sea – fresh enough to drink more than 160km (100 miles) out. The world’s biggest delta is where the BRAHMAPUTRA and the GANGES pour into the Bay of Bengal.  They deliver every year more than a billion tons of sediment eroded from the HIMALAYAS – more sediment than in any other river system. The resulting delta stretches 75,000 sq km (29,000 square miles) across WEST BENGAL and BANGLADESH. Along its coastal fringes, the channels and mud banks have been colonized by Mangroves to create the world’s largest mangrove forest, the SUNDARBANS – visible from space. The INDO – WEST PACIFIC REGION has the most extensive mangrove forests, but mangroves also take anchor across the tropical belt on protected coastlines, wherever river mud can accumulate. There are around 70 species, ranging from bush – sized plants to trees 25m (82 feet) tall. Though they can survive immersion by the tide, they also require fresh water, and for gas exchange, many have aerial roots that grow out of the waterlogged mud. At high tide, the underwater maze of mangrove roots provides shelter and creates important nursery grounds for fish. And when the tide retreats, another group of creatures emerges onto the exposed mud. These include mudskippers, armies of fiddler crabs and crab – eating macaques.  Not all rivers build DELTAS. In river mouths where a rising sea level has flooded the valley, the semi – enclosed area is called an ESTUARY. The candles mudflats of estuaries may look like wastelands but are among the most productive environments on Planet Earth. The mud dumped by the rivers is a nutrient – rich soup, in which an army of tiny creatures, from bacteria and protozoa to nematodes, breaks down the organic matter. Large animals such as POLYCHAETES, CLAMS and RAZOR SHELLS live in burrows or in permanent tubes. These in turn support great numbers of predators. Estuaries are vital breeding grounds for fish such as anchovies and mullet. And at low tide, huge flocks of wading birds, from OYSTER CATCHERS, GODWITS and PLOVERS TO KNOTS and SANDPIPERS  come to feed, each species with a beak fashioned to prise out or open the shellfish and other tasty creatures that inhabit the mud. In temperate and subarctic estuaries, salt – marshes grasses replace mangroves in the mud. Along the North American Atlantic seaboard, salt marshes were once extensive. Now just pockets remain. Though hemmed in by an urban sprawl that stretches from Washington and New York, the tidal salt marshes around Chesapeake Bay and the Delaware estuary still provide rich feeding grounds – vital stopovers for the 400,000 greater snow geese that arrive each autumn to rest and refuel on their long migratory journeys south. This is the end of the rivers’ journeys. Collectively, they have worn down mountains and carried parts of them to the sea. And all along the way, their fresh water has brought life in abundance. All the great rivers also provide drinking water, food, irrigation, transport and now power to much of the world’s population – though at a great cost to wildlife. Many predict that future world conflicts will be waged not over oil, but fresh water, arguably the world’s most precious resource.