Original subtitles

Only 3 percent of the water on our planet is fresh.

Yet these precious waters are rich with surprise.

All life on land is ultimately dependent upon fresh water.

The mysterious tepuis of Venezuela -

isolated mountain plateaus rising high above the jungle.

This was the inspiration for Arthur Conan Doyle's 'Lost World,'

an imagined prehistoric land.

Here, strange towers of sandstone have been sculptured over the millennia

by battering wind and torrential rain.

Moisture rising as water vapor from the surface of the sea

is blown inland by wind.

On reaching mountains, the moisture is forced upwards

and as it cools, it condenses into cloud and finally rain -

the source of all fresh water.

There is a tropical downpour here almost every day of the year.

Fresh water's journey starts here, high in the mountains.

Growing from humble streams to mighty rivers

it will travel hundreds of miles to the sea.

Angel Falls, the highest waterfall in the world.

Its waters drop unbroken for almost a thousand meters.

Such is the height of these falls

that long before the water reaches the base in the Devil's Canyon

it's blown away as a fine mist.

In their upper reaches,

mountain streams are full of energy.

Streams join to form rivers,

building in power,

creating rapids.

The water here is cold.

Low in nutrients, but high in oxygen.

The few creatures that live in the torrent

have to hang on for dear life.

Invertebrates dominate these upper reaches.

The hellgrammite, its body flattened to reduce drag,

has bushy gills to extract oxygen from the current.

Black fly larvae anchor themselves with the ring of hooks,

but if these become unstuck,

they're still held by a silicon safety line.

There are advantages to life in the fast stream -

bamboo shrimps can just sit and sift out passing particles

with their fan-like forearms.

Usually, these mountain streams only provide enough food

for small animals to survive.

But with the spring melt here in Japan

monsters stir in their dens.

Giant salamanders, world's largest amphibian,

almost two meters long.

They're the only large predator in these icy waters.

They begin their hunt at night.

These salamanders have an exceptionally slow metabolism.

Living up to 80 years they grow into giants.

The fish they hunt are scarce

and salamanders have poor eyesight.

But sensory nodes on their head and body

detect the slightest changes in water pressure.

Free from competition,

these giants can dine alone.

Pickings are usually thin for the salamanders,

but every year some of the world's high rivers

are crowded by millions of visitors.

The salmon have arrived.

This is the world's largest fresh water fish migration.

Across the northern hemisphere

salmon, returning from the ocean to their spawning grounds,

battle their way for hundreds of miles upstream.

Up here, there are fewer predators to eat their eggs and fry.

A grizzly bear.

From famine to feast -

he's spoilt for choice.

This Canadian bear is very special -

he's learnt to dive for his dinner.

But catching salmon in deep water is not that easy

and the cubs have lots to learn.

The annual arrival of spawning salmon

brings huge quantities of food into these high rivers

that normally struggle to support much life.

Although relatively lifeless,

the power of the upland rivers to shape the landscape

is greater than any other stage in a river's life.

Driven by gravity,

they're the most erosive forces on the planet.

For the past 5 million years

Arizona's Colorado river has eaten away at the desert's sandstone

to create a gigantic canyon.

It's over a mile deep

and at its widest it's 17 miles across.

The Grand Canyon.

This river has cut the world's longest canyon system -

a 1,000 mile scar clearly visible from space.

As rivers leave the mountains behind,

they gradually warm and begin to support more life.

Indian rivers are home to the world's most social otter -

smooth-coated otters form family groups up to 17 strong.

Group rubbing not only refreshes their coats,

but strengthens social bonds.

When it comes to fishing

there is real strength in numbers.

Fishing practice begins when the cubs are four months old.

Only the adults have the speed and agility needed to make a catch.

Adults share their catches with their squabbling cubs.

Most otters are solitary,

but these rich warm waters can support large family groups

and even bigger predators.

Mugger crocodiles, four meters long, could easily take a single otter.

But, confident in their gangs,

the otters will actively harass these great reptiles.

Team play wins the day.

The Mara river,

snaking across the plains of East Africa.

As the land flattens out

rivers slow down and lose their destructive power.

Now they are carrying heavy loads of sediment

that stains their waters brown.

Lines of wildebeest are on their march.

Each year nearly two million animals migrate across the Serengeti plains

in search of fresh green pastures.

For these thirsty herds

the rivers are not only a vital source of drinking water,

but also dangerous obstacles.

This is one of the largest concentrations of Nile crocodiles in Africa,

giants that grow over five meters long.

From memory, the wildebeest are coming

and gather in anticipation.

The crocodile's jaws snap tight like a steel trap -

once they have a hold, they never let go.

It took over an hour to drown this full-grown bull.

To surprise their prey

crocodiles must strike with lightning speed.

Here, only the narrowest line separates life from death.

Most rivers drain into the sea,

but some end their journey in vast lakes.

Worldwide lakes hold twenty times more fresh water than all the rivers.

The East African Rift Valley holds three of the world's largest:

Malawi, Tanganyika, and Victoria.

Lake Malawi, the smallest of the three,

is still bigger than Wales.

Its tropical waters teem with more fish species

than any other lake.

There are 850 different cichlids alone,

all of which evolved from just one single ancestor

isolated here thousands of years ago.

These two-meter wide craters are fish-made.

Fastidiously maintained by the males,

these bowls are courtship arenas.

Cichlids are caring parents.

Brooding young in the mouth is a very effective way of protecting them.

This lake can be a dangerous place.

After dark, predatory dolphin fish emerge from their daytime lairs among the rocks.

Like packs of sharks, they're on the prowl for sleeping cichlids.

In the darkness these electric fish hunt

by detecting distortions in the electric field they create around their bodies.

Any cichlid that trenches out will be snapped up.

The floor of Lake Malawi drops 700 meters into an abyss.

Here, in this dead zone

the larvae of lake fly midges hide out away from predators.

In the rainy season they balloon up to the surface

and undergo a magical transformation.

At dawn the first adult midges start to break out.

Soon, millions upon millions of newly hatched lake flies

are taking to the wing.

Early explorers told tales of lakes that smoked, as if on fire.

But these spiraling columns hundreds if meters high

are mating flies.

Once the flies have mated,

they will all drop to the water surface,

release their eggs and die.

Malawi may look like an inland sea,

but it's dwarfed by the world's largest lake -

Baikal in Eastern Siberia.

400 miles long and over a mile deep,

Baikal contains one fifth of all the fresh water

found in our planet's lakes and rivers.

For five months of the year it's sealed by an ice sheet over a meter thick.

Baikal is the oldest lake in the world

and, despite the harsh conditions, life flourishes here in isolation.

80 percent of its species are found nowhere else on Earth,

including the world's only fresh water seal.

With this seal

and its marine-like forests of sponges

Baikal seems more like an ocean than a lake.

There are shrimp-like crustaceans - giant amphipods - as large as mice.

They are the key scavengers in this lake.

The water here is just too cold for the bacteria that normally decompose the dead.

Most rivers do not end in lakes

but continue their journey to the sea.

The planet's indisputable super-river is the Amazon.

It carries as much water as the next top-ten biggest rivers combined.

Rising in the Peruvian Andes, its main trunk flows eastwards across Brazil.

On its way the system drains a third of South America.

Eventually, over 4,000 miles from its source,

it empties into the Atlantic Ocean.

The Amazon transports a billion tons of sediment a year,

sediment clearly visible at the mixing of the waters

where one massive tributary, the Rio Negro, flows into the main river.

Its waters are wonderfully rich.

To date over 3,000 species of their fish have been described -

more than in the whole of the Atlantic Ocean.

The Amazon is so large and rich in fish

that it can support fresh water dolphins.

These botos are huge - two and a half meters long.

In these murky waters they rely on sonar to navigate and hunt.

They work together to drive shoals of fish into the shallows.

Botos are highly social

and in the breeding season there is stiff competition for mates.

The males hold court in a unique way.

They pick up rocks in their jaws

and flaunt them to their attending females.

Maybe each male is trying to show how strong and dexterous he is

and that he therefore is the best father a female could have for her young.

Successful displays lead to mating.

Even for giant rivers like the Amazon

the journey to the sea is not always smooth or uninterrupted.

Iguassu Falls on the border of Brazil and Argentina

is one of the widest waterfalls in the world -

one and a half miles across.

In flood 30 million liters of water spill over every second.

All the world's great broad waterfalls:

Victoria, Niagara and here, Iguassu,

are only found in the lower courses of their rivers.

In their final stages

rivers broaden and flow wearily across their flat flood plains.

Each wet season here, in Brazil,

the Parana river overflows its banks

and floods an area the size of England.

The Pantanal -

the world's largest wetland.

In these slow-flowing waters aquatic plants flourish

like the Victoria giant water lily with leaves two meters across.

These underwater forests are nursery grounds for fish.

Over 300 species breed here, including red-bellied piranha

and other predators, like the spectacle caiman.

Ripening fig trees overhanging the water's edge

provide welcome food for shoals of hungry fish.

The commotion attracts dorado,

known locally as the river tiger.

They patrol the feeding shoals,

looking for a chance to strike.

And waiting in the wings,

ready to pick off any injured fish,

are the piranhas.

The feeding frenzy quickly develops.

Piranha can strip a fish to the bone in minutes.

Great numbers of fish sustain vast flocks of water birds.

The rose-eared spoonbill is just one of the 650 bird species found in the Pantanal.

They nest alongside wood stocks in colonies thousands strong.

Spectacle caiman linger below,

waiting for a meal to fall out of the sky.

When rivers finally reach the sea

they slow down, release their sediment and build deltas.

In Bangladesh the Ganges and Brahmaputra rivers join

to form the world's biggest.

Every year almost 2 thousand million tons of sediment

eroded from the Himalayas is delivered to the ocean.

At the delta's mouth - the largest mangrove forest in the world,

the Sundarbans.

These extraordinary forests spring up throughout the tropics

in these tidal zones where rivers meet the sea.

Crab-eating macaques are mangrove specials.

In Indonesia these monkeys have adopted a unique amphibious lifestyle -

they fish out fallen food.

The troop also uses the waters to cool off during the heat of the day.

But the channels are also the playground for restless young macaques.

Some of the young have even taken to underwater swimming.

They can stay down for more than 30 seconds

and appear to do this just for fun.

Yet these swimming skills acquired during play

will certainly be useful later in life in these flooded mangrove forests.

In cooler climes, mud, laid down in estuaries,

is colonized by salt marsh grasses

and form one of the most productive habitats on the planet.

400,000 greater snow geese flock to the estuaries along the Atlantic coast of the United States

to rest and refuel on their long migratory journeys.

This is the end of the river's journey.

Collectively they've worn down mountains

and carried them to the sea.

And all along the way,

their fresh water has brought life and abundance to planet Earth.

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