Sweet Fresh Water The Living Planet

A strangely-shaped mountain

catching the clouds high above the jungles of Venezuela.

Its summit rocks are carved into a multitude of grotesque shapes.

The sculptor is an agent continuously at work

on the landscapes of our planet rainwater.

It washes over the rock,

eroding it chemically.

It permeates the cracks,

freezes and chips it off in flakes and splinters.

As the water flows down, it starts on a long journey

from the rainy mountains to the sea

and here, with a leap of over 3,000 ft,

it forms the Angel Falls.

Our journey begins not far from that towering waterfall,

on the high moorlands of Peru, 15,000 ft above the sea.

Water is a very extraordinary and very precious substance.

It's the only substance, apart from mercury,

which stays liquid at normal temperatures.

Because of that, it's an essential part of all living organisms,

animals and plants.

Without it, life would come to an end.

This particular water is a very rare kind.

97% of the water on earth is salty the sea.

But THIS was distilled from the surface of the sea by the sun,

rose into the sky and condensed to form clouds,

and fell again as rain and snow

to form streams of pure, fresh, sweet water.

This particular stream is on its way to the sea

a very long way away.

Because these are the Andes,

and this is one of the many streams feeding the biggest river on earth,

the Amazon.

The difficulties of living in this young, violent river are formidable.

Its waters are thick with rock and mud

but they contain few nutrients.

They rush down the valley at tremendous speed.

Anything living here has to be a prodigious swimmer.

And these are. They're torrent ducks.

They exploit the swirls and eddies with consummate skill,

paddling with their large webbed feet.

They head always upstream,

bracing themselves against rocks with their stiff-quilled tails

and using small horny spurs on the wrists of their wings

to give them purchase.

A pair owns a stretch of the river,

working up it until they reach the edge of their territory.

Then they are swept downstream to begin all over again.

Anchored firmly to the rocks

is a kind of moss.

Mosses are primitive ancient plants

that grew on earth long before flowering plants.

Torrent moss grows in young rivers and streams all over the world.

Wherever it grows, in the Andes or here in Europe,

it provides shelter for a multitude of insect larvae.

In summer, they are transformed,

briefly leaving the river to mate.

But most of their lives are spent underwater.

Some are streamlined against the current.

-Caddis fly

-larvae live in protective tubes:

hollow stems, or a construction of wood stuck together with silk.

Some weight themselves down so the current doesn't shift them

by building their shelters from heavy grains of sand.

The larva of the blackfly

holds on to a pebble with its back end

and grabs at passing food particles with the antennae on its head.

It grips the rock with a ring of hooks, but if it loses its hold,

all is not lost.

It has a lifeline of silk

which it has attached to its pebble.

Having hauled itself back,

it has to get a new grip.

It spins a pad of silk from a hole beneath its mouth

and fixes its hooks into that.

The nets it collects food with are modified antennae

and the larva brushes off its catch

with alternate flicks of its mouth parts.

Not all caddis larvae live in solid tubes.

This one lives in a construction

that is also a food-catching device.

It uses its silk

to produce a funnel-shaped scaffold of crossed threads.

Undulating its body helps its breathing,

speeding the flow of oxygen-bearing water

through the funnel.

It holds on with the hooks at the back...

..leaving its jaws and front legs free to do the construction work.

This blackfly larva

wasn't saved by its lifeline.

But the caddis-fly larva itself,

ferocious and artful trapper though it is,

is also at risk.

The dipper relishes it.

Dippers live in the rivers of North America and Europe.

Under water, their swimming is different from the torrent ducks'.

Its feet aren't webbed like a duck's

so it uses its wings to "fly" under water.

In similar cold, fast-flowing streams in North America

lives a giant newt, the hell-bender.

When it is young, it takes insect larvae.

But it can grow to over two feet long

and then it seeks much bigger prey.

A crayfish would suit it admirably.

A narrow escape.

The crayfish saved itself by a snap of its tail.

But the hell-bender doesn't give up easily.

Both animals try to keep out of the current

and habitually creep into crevices.

But that, sometimes, is a mistake.

Small streams tumbling down the sides of valleys to young rivers

have their own population.

The big-headed turtle clambers around the waterfalls

using its tail as a prop.

In West African waterfalls, and nowhere else,

lives the extraordinary hairy frog.

Its hairs are filaments of skin on its flanks

which act as gills, helping it absorb oxygen from the water.

And, almost as unusual,

it has claws that help it grip the slippery stones.

The many sources of the Amazon

began as numberless rivulets in the eastern Andes.

Now, 5,000 feet lower down,

each has grown beyond recognition and cut its own zig-zag valley.

White water tumbling down the valley wall

joins the brown water of a larger tributary,

heavy with mud and sediment.

And as it gets bigger and bigger,

it becomes more and more powerful.

It's the dry season now, and the river is comparatively low.

But during the rains when it's in spate, its waters rise

above here.

And the sheer volume and weight and force of them

can shift boulders the size of these.

The volume and speed of its waters

are not the river's only weapons.

It also has teeth,

and in this empty part of its bed, you can see them.

Sand and gravel.

Fragments of rock that were eroded from higher up in its course

which the river hurls with enormous force

at the rocks of its river bed.

With such tools, it can carve away the sides of mountains.

Young, vigorous rivers transform the land,

demolishing the mountains,

breaking down the debris into smaller particles

and carrying them downstream.

This river in China is always so turbid

that it's called the Huang Ho, the Yellow River.

It carries more sediment than any other river.

During floods, each cubic yard of water

contains over 2,000 pounds of soil and pulverised rock.

Rivers in the full vigour of their youth are terrifyingly strong.

They roll great boulders along their beds,

they cut away at the banks,

undermining trees which crash into the water and are swept away.

When a river encounters a band of unusually hard rock,

like an ancient flow of basalt lava,

its progress is temporarily slowed.

It spreads out over the barrier and tumbles over the edge.

So some of the loveliest cascades are formed.

These are the Falls of Iguazu on the Brazil-Paraguay border.

They can't compare in height with the Angel Falls,

but in terms of the volume of water passing over them,

they are incomparably bigger.

The falling waters pound away at the base of the Falls,

undercutting the basalt until blocks split off the face.

So the falls steadily work their way upriver,

leaving a deep gorge downstream.

And animals live even here, within the Falls themselves.

Swifts perch on the rock face, behind the cascade.

Every evening they congregate high above Iguazu.

After a day of hunting insects, they are ready to roost.

And where safer than behind a screen of falling water?

Some dive down with such speed

that they shoot right through the fall.

Now the river has left the mountains far behind,

and has changed its character considerably.

It's bigger, it's broader,

and its waters carry not only sand and gravel

but rich nutrients washed in from its vegetation-covered banks.

After it goes over its last rapids and tumbles over its last fall,

it becomes a very different river indeed.

It's middle-aged. Ampler, less violent, more sluggish,

and richer.

Beside the Amazon tributaries,

the jungle stands thick.

Birds like the sun-bittern stalk quietly in search of a meal.

Huge fish cruise through the slow waters.

The arapaima, one of the largest of freshwater fish,

grows over six feet long.

The Amazon contains over 3,000 different kinds of fish.

That's more than live in all the Atlantic.

Rays probably evolved in the sea,

but this species has made the change to fresh water

and lives high up the Amazon.

Many other fish have evolved here in fresh water,

adapting to all its variations of depth, speed and chemical content,

to muddy water and to clear,

to areas thick with plants

and places where there are none.

Their variety is enormous.

Take, for example, just one family, the catfish.

They are bottom-dwelling fish, with feelers on their snouts

that have sense organs on them,

so the fish can feel and taste their way

through the thick muddy water, or at night.

There are small ones and huge ones,

some that give electric shocks, others that swim upside down.

Those living in fast-flowing waters

have suckers on their chins or bellies

with which they cling to rocks.

In South America alone,

there are 1,200 different species of catfish.

In these crowded waters

many fish give special protection to their young

for the first weeks of their lives.

The discus fish goes even further.

It provides its fry with special food.

Parents exude a nutritious slime from their skin

and the young graze over their flanks, feeding on it.

In a week they're big enough to feed on small floating particles.

These are now a month old,

and have already assumed the disc-like shape of their parents.

They're becoming independent,

but they're too near the lair of an electric eel.

The eel has very poor eyesight

but it detects the presence of objects around it

with short electric discharges, like radar.

It rises for a gulp of air.

This time the young discus seem to have escaped detection.

But the eel can produce a major electric shock to stun its prey.

Astonishingly it releases its captive.

Perhaps it's too small to eat.

The young discus, apart from the eel's jaw marks on its flanks,

seems no worse off.

One Amazonian fish puts its eggs

beyond the reach of any water-living predator,

on leaves overhanging a river.

A pair of splashing tetras are courting.

Curving their bodies, for a moment

they leap clear of the water.

Sometimes a third fish joins in.

The bigger of the two is the male.

For a moment they hang on the leaf

supported by the suction of the male's floppy fins.

Again and again they jump.

In this moment the female lays her eggs and drops off

and the male fertilises them. Each time they leave 12 or so eggs.

A few infertile eggs drop off the leaf and are eaten.

Eventually, 200 eggs are placed out of harm's way.

And the river can be an exceedingly dangerous place.

Piranha, the most savage of all the Amazon fish.

A swimming capybara realises their presence and tries to retreat

too late.

The splashing, the taste of blood

spreading through the water

attracts more of the shoal until there are hundreds of fish

possessed by a frenzy for flesh.

They're only a foot long, but their teeth cut clean through bone.

In minutes, there is little left.

As the river gets older, it slows down.

A minor obstacle in its path will deflect it.

Water flowing round the outside of a bend has to travel farther

and speeds up, eroding the bank.

Inside the bend the current is slow

and the water drops its heavy sediment to form a shoal.

So the bend gets more exaggerated

as the elderly river swings from side to side

with many loops and meanders.

One bend approaches another

until the land between them is so narrow it collapses.

The river takes the shorter course,

and the meander is left isolated as a curving lake.

There the water, at last, is still.

Plants no longer have to fight a current

and the lakes become clogged with vegetation.

These are the largest floating leaves of all,

the leaves of the famous giant Amazon lily.

Covering the water with huge leaves is a very aggressive act,

for it cuts out the light in the water below

making it hard for other plants to grow.

The upturned rims of the pads as they grow

thrust aside all other floating plants.

And to prevent these leaves from being eaten by fish,

they're protected by very effective ferocious spines underneath,

as you can see most clearly on this half-opened bud.

It grows from the size of a plate

to a huge disc six feet across in a few days,

growing by one square inch every minute.

The flowers develop as quickly.

They open first in the evening

and remain with petals spread, powerfully fragrant, all night.

By morning it's closed again.

But during the night it's taken prisoners.

Inside the flower

are beetles.

Sometimes up to 40 in a single bloom.

They're attracted by special sugary outgrowths

in the centre of the flower,

and while they are trapped

they will feed on those.

This evening the flower opens again

to release the beetles, which will fly off carrying pollen

to cross-pollinate another lily flower.

Then, after just two nights,

this bloom, by now turned purple,

will crumple and die.

The immense leaves, strengthened by air-filled ribs beneath,

can support a small child.

And water birds can walk over them with total confidence and safety.

The jacana has elongated toes that spread its weight so well

that it can tread on much flimsier leaves than those of this lily,

without submerging them.

It seeks insects, and there are plenty of them.

The pond skater sits on a leaf, but it can also sit on the water.

The surface is a springy platform that supports many small creatures.

Water molecules are bound to one another by a force like magnetism.

They're not attracted to molecules of air, above,

so surface molecules have their forces concentrated sideways,

giving the surface a very strong tension.

And the pond skater hunts on it.

It's lost its prey under the leaf.

This time there is no escape.

The pond skater stabs its victim and sucks it dry.

It's important for the pond skater to keep meticulously clean.

Its waxy body and hairy feet repel water,

but any dirt on them that is wettable

breaks the surface tension film.

They're aggressive insects,

each with its own territory among the lily pads.

Intruders are immediately chased away,

and fights between rivals are common.

The surface tension film is the pond skaters' platform

and their sounding board.

Sense organs on their feet detect the vibrations

caused by a struggling insect that has fallen on the surface.

By bouncing up and down, they communicate to one another,

warning off rivals

and sending come-hither signals to potential mates.

Whirligig beetles use surface vibrations

in a different way.

By gyrating, they create ripples,

and the returning echoes show the presence of other creatures

and obstacles around them.

They have excellent eyes which are partitioned

so that the lower half peers down

to see what's happening in the water beneath.

Hanging below the surface is another hunter.

Its tail has two tubes

which penetrate the surface film to collect air for it to breathe.

Its head has ferocious jaws to seize its prey.

This is the larva of the giant diving beetle

and it has caught a tadpole.

It must come to the surface, even as an adult, to collect air

to sustain it on its hunting forays into deeper waters.

The water boatman patrols the surface looking for prey,

not from above, like the pond skater, but from below.

The two kinds of insects, between them,

collect most of the creatures trapped in the surface film.

The camphor beetle lives on plants by the water.

It is the most versatile of all water walkers.

It can run over water like a pond skater.

It can produce a substance like camphor

which reduces the tension between water molecules.

In emergencies, it squirts this from its tail,

and with tension reduced behind,

but pulling hard at the front,

it shoots across the surface so fast

that you can only see it in slow motion.

And, like other beetles,

it can fly.

One particularly ferocious hunter

lives on the edge of lakes and ponds in Europe

the fishing spider.

It uses the surface tension film as other spiders use their webs.

With its front legs resting on the surface,

it feels for vibrations.

But it also has excellent sight and can see prey below the surface.

The stickleback sees only the spider's feet.

That is a greatly slowed-down version of the kill.

In reality, the pounce is rapier-swift

and the stickleback had little chance once it came within range.

The lakes and ponds fed by streams

or cut off from the main course of the river

are comparatively small.

Where rivers flow into basins created by geological faults,

their water accumulates in immense lakes.

Lake Prespa in Yugoslavia is not the largest of lakes,

but even so, it's 20 miles long.

Rivers entering its still waters slow down and drop their sediment.

So such lakes are potentially very fertile.

Their animal inhabitants, no longer harassed by the current

or hemmed in by a shallow bottom or narrow banks, can proliferate.

Fish swarm in their waters.

And fish-eating birds, like pelicans and cormorants,

swarm correspondingly.

Land based creatures haunt its margins.

These are its most fertile parts,

for the lack of strong currents in a deep lake

starves the lower waters of oxygen.

But in the shallows, warmed by the summer sun,

algae and other plants flourish, small invertebrates proliferate

and there is food for even the least agile hunters.

In one way, these large lakes are very special.

This trout with distinctive red spots lives in Lake Ohrid,

a few miles away from Lake Prespa, but nowhere else in the world.

Isolated in a lake, fish become very inbred.

Small characteristics that get lost in bigger populations become fixed,

and the fish evolve into new species.

A similar thing has happened to the shrimps.

And among the many species of water snails,

several are now unique to Lake Ohrid.

In central Russia lies a stretch of fresh water so huge and ancient

that these processes have produced new species

on a scale unequalled anywhere else in the world.

Lake Baikal.

The lake lies in a great depression

formed by faulting in the earth's crust.

It's 400 miles long

and 5,000 feet deep, the deepest of all lakes.

In the depths of the lake, 1,000 feet down,

lives a unique salmon, the omul.

In summer they move to the shallows

and feed on caddis fly larvae and sand hoppers

and here they are caught in great numbers, for they're delicious.

This is only one of Baikal's special inhabitants.

Of the 1,200 different kinds of fish and other animals

and the 500 plants it contains,

over 80% are unique.

There are unique molluscs, unique flatworms,

and even one unique mammal, the Baikal seal.

This tiny seal is descended from the ringed seal of the Arctic sea.

Today the lake is over 1,000 miles away from that sea.

The ancestors of these seals arrived in the Ice Age,

when the journey was shorter and easier.

Cut off from other seals, they've developed in their own way.

The Amazon has no great lake on its course,

so even in its middle stretches

it carries mud from the Andes.

The Rio Negro, joining it here, is clear it comes from the northwest

where the rocks are hard and bare.

The two huge rivers flow alongside one another in the same bed,

scarcely mixing.

As well as sediment, they carry abundant nutrients

and life on their banks flourishes as never before.

Herds of capybara wade through the shallows

cropping the luxuriant plants.

They are excellent swimmers, with webs between their toes,

and they have that placing of eyes, ears and nose

so valuable to mammals that regularly swim on top of the head

so that the submerged animal can see, hear and smell

what is going on above water around them.

Giant otters have a similar head design,

and sometimes lift themselves above the surface

to get an even better view.

This Amazonian species is the biggest of all the world's otters,

six feet long and a powerful swimmer,

equipped with large webbed feet,

a flattened tail and sensitive whiskers.

A pair makes territorial patches on the river bank,

marking them with their own personal smell.

There are otters in many of the great rivers of the world,

and they are the most graceful of swimmers.

In India they share the fish harvest with the gavial.

Most adult members of the crocodile family feed largely on carrion

but the gavial eats only fish.

It has long, narrow jaws

studded with abundant teeth, for catching them underwater.

Birds also claim a share of the river fish.

The hooded merganser,

one of a group of ducks called sawbills.

Its beak, like the gavial's jaws, is long and narrow

so it is easily snapped together under water.

It has a notched edge

to grip slippery fish.

But their feathers trap so much air that the pair must work very hard

to get down to any depth.

Coming up again is easy enough.

But the meal is a mere mouthful

and the merganser must must look for another one.

On the bottom lurks more danger for a fish.

A worm, perhaps?

No, the deceiving tongue of a turtle.

And in the sky above the river, more trouble for a fish.

The kingfisher.

There's one left for next time.

The fish eagle is not a diver,

but a pouncer with marvellously co-ordinated action.

The aerial onslaught on fish goes on all day

and night.

An owl goes fishing in Africa.

Its legs are bare.

Feathers would drag in the water.

Spines under its toes give it a firm grasp on fish.

In the last phase of their life,

great rivers often flow out of control.

Their mountain tributaries, fed by the storms of the rainy season,

pour so much water into them

that they burst their banks.

The Amazon rises every year,

to flood tens of thousands of miles of forest,

in some parts as much as 40 feet deep.

Some of these trees are flooded for eight to 10 months every year.

They need only two months annually out of water

for them to grow and for their seeds to germinate and sprout.

We still don't know how they manage it.

As the floods well out over the land,

fish from the river travel with them.

This is going to be their best feeding time of the year.

As it is for other creatures too.

Among the fallen leaves on the bottom lies the mata-mata turtle,

marvellously camouflaged, waiting for a decent-sized fish.

And there are plenty already here,

sheltering, like the turtle, among the still-unrotted leaves.

Piranha are here, too not the flesh-eating kind,

Their teeth are used for something different.

Fruit.

As the river becomes older and older,

its riches increase still further.

All over the world, as rivers approach their end,

they deposit the sand and mud

that they have gathered from so far and carried for so long.

In many parts of the world,

reeds grow thickly on these banks.

Their stems collect more sediment as the waters swirl through them.

Living in the dense reed beds requires great skill.

The little bittern somehow finds its nest

hidden out of sight in this seemingly uniform stretch of reeds.

It regurgitates from its crop

ample supplies of fish and frogs for its young.

Their world is an infinity of vertical stems,

but they are nimble climbers from an early age

and they leave the nest within a few days of hatching.

There they wait, almost invisible,

for their parents to return with re-stocked crops.

The reed-clogged waters of a river delta are full of potential riches,

not only for birds but for humans.

The reeds are used for many purposes, but it's not an easy life.

Firm land on which to live is hard to find.

In the Danube delta, the few solid sandbanks are packed with houses.

Earth is carefully conserved with piles

to stop a change in the current from washing it away.

There is the threat of a rise in the water level

caused by heavy rains upstream

or a very high tide, backed by a storm sweeping up from the sea,

which can cause devastating floods.

In the joined deltas of the Tigris and Euphrates in Iraq,

the Marsh Arabs have become specialists in an amphibian life.

Their houses seem to have solid enough foundations.

In fact, they are floating on rafts of reeds.

Some are the most elaborate constructions,

yet these soaring arches and roofs are made from bundles of reeds.

And reeds provide food for the livestock.

So gathering them is a daily and never-ending chore.

The herds have to be

as much at home in the water

as they are on their floating platforms.

The rewards of this precarious existence are the abundant fish

which live all around the houses and even underneath them.

So the fish, the Marsh Arabs,

and the pelicans all flourish in one integrated community.

The river has finally delivered

the minerals from the mountains and the nutrients from the forests.

They sustain plants, the food for small animals,

which are eaten by bigger fish,

which are ultimately gathered by the great flocks of birds

that are the glories of the deltas.

A blizzard of snow geese in northern Canada.

Across the world in the tropics, in Papua New Guinea,

magpie geese.

In Australia, brolga cranes.

Scarlet ibis in Venezuela.

Plovers all over the world.

And equally widespread, stilts.

Flamingos in Africa.

And spoonbills.

Of all the deltas in the world, none is greater than the Amazon.

For hundreds of miles along its lower course,

the river has been so wide

that you can't see from one side to the other.

Now, instead of receiving water,

it splits into a tangle of several channels.

On the last firm land on its banks stands a great thriving port,

for the river is so wide and deep

that cargo ships from overseas

can use it as a highway

that takes them 1,000 miles into the heart of South America.

The Amazon's vital statistics are astounding.

At any one time, two thirds of all the river water in the world

is flowing between its banks.

Here at its mouth, at Belem, it's 200 miles across,

a maze of channels and islands,

one of which is bigger than the whole of Switzerland.

The river maintains its identity far out into the sea.

That's how it was discovered.

In 1499, a Spanish sea-captain

sailing well beyond the sight of land

became aware that the water he was crossing was fresh and not salty,

and he turned west and discovered this immense river.

Indeed, it's not until 100 miles beyond the edge of the continent

that particles of water that fell on the Andes

complete their 4,000 mile long journey

and mingle with the salt water of the ocean.

Farther along the coast, the thrust of the river's flood is not so great,

and there is a halfway house.

The water is neither fresh nor salt, but brackish.

It's neither land nor sea, but banks of mud and sand

that are half the time submerged and half exposed.

And that intermediate, ever-changing territory

is where we will be next time.