The Nile.
Draining over three million square kilometres of Africa,
then flowing through one of the harshest deserts in the world.
It's the world's longest river.
This river was the power house behind the world's first great civilisation.
Without the Nile's extraordinary fertility,
there would be no Tutankhamen,
no Cleopatra, no pyramids.
It changed the world forever.
Each year, the Nile performed a miracle -
a great flood that brought not only water to the parched desert,
but a staggering one hundred and forty million tonnes of fertile volcanic soil.
Somewhere far away,
powerful forces gathered
to swell the river and send it racing downstream.
But the Ancient Egyptians had no idea where this flood came from.
They believed it was the gods who sent these vital gifts so reliably each year.
Impassable rapids prevented
the Egyptians travelling far enough upstream to discover the truth.
They lived in isolation,
believing this one green valley contained their entire,
god-given Universe.
They knew nothing of what lay beyond.
But where does the Nile's great flood really come from,
and how is it able to perform its miracle?
The river has been reluctant to give up its secrets,
now we can unravel the mystery.
The answer can only be found at the end of an extraordinary journey,
far upstream, into the remotest
and strangest corners of Africa.
It's an epic journey...
in search of the great flood.
The Nile's influence as a lifeline in the desert
stretches a long way south from Egypt -
onwards into Nubia, now part of northern Sudan.
For over a thousand kilometres,
the Nile is the only source of life in a forbidding desert known
as "The Belly of Stones".
For millennia, there have been human settlements here
on this green strip clinging to the banks of the Nile.
But it's a man-made oasis,
extending only as far as farmers can pump the river's precious water.
Animals have also benefited
from this transformation of barren desert into lush paradise.
These, the rivers original inhabitants have witnessed
more than one civilization born of the Nile.
These pyramids are the tombs of the so-called
"black pharaohs" of the Nubian Kingdom of Kush.
They too harnessed the Nile's riches,
and were at one time powerful enough to rule their Egyptian neighbours,
a thousand kilometres to the north.
But unlike the Egyptians,
little is known about their mysterious culture and language.
A little way upstream from those enigmatic pyramids,
the Nile's story becomes more complicated.
Now there are two rivers
the Blue Nile and the White Nile.
Here they meet and merge in what Arab poets
call "The longest kiss in history".
The clearly different colours of the Blue
and White Nile suggest different characters and origins.
This meeting of the waters is the site of another great human settlement
Khartoum.
Khartoum is the capital of Sudan -
a country of nearly six hundred tribes and over a hundred languages.
Centuries old,
Khartoum remains to this day one of the most important crossroads
in the African landscape.
The Nile has seen Kushites, Christians
and Arabs each rule here in their time.
But Islam has emerged as the dominant voice in modern Khartoum.
Every Friday, Islamic mystics still gather
to affirm their beliefs.
A crowd urges them to spin.
They enter a state of trance believing in this way
that they can span the divide between heaven and earth.
These are the legendary "whirling dervishes".
Like the waters of the Blue and White Nile,
people have merged here.
In a human kaleidoscope,
Arabia meets Africa, and the old ways meet the new.
But throughout Khartoum's fluid history,
the Nile's yearly flood has remained the one true constant.
Beyond Khartoum, both branches of the Nile lead into a wilder Africa...
But which branch brings the great flood?
This, the larger branch, is the White Nile.
Just a few hundred kilometers upstream from a desert city,
the White Nile flows through a wholly different and stranger world.
This is the Sudd.
It's one of the largest swamps in the world.
Sudd means "The barrier".
Throughout history, anyone who tried
to navigate this shifting landscape found it well named, and it still is.
Arab and African worlds collide here.
Southern Sudan has seen many civil wars
the latest raging for 20 years.
Hostile geography and politics combine to keep
the Sudd hidden from the outside world.
The course of the White Nile becomes lost in an ocean of papyrus reeds.
The labyrinth of papyrus provides food
and shelter for enormous numbers of fish.
These waters are clearly fertile,
but did they nourish the far away fields of Ancient Egypt?
The world of papyrus is always on the move.
Air sacs in the stem give it great buoyancy.
Huge papyrus rafts drift on the current.
These restless islands of reeds constantly
redefine the channels of the waterways.
It's no wonder explorers found the Sudd impenetrable.
A healthy population of dangerous animals
also helps maintain the Sudd's inhospitable reputation.
At this time of the year the huge swamp is, in fact, shrinking.
Fish become more densely concentrated, making them easier prey.
But it's not just the water that's becoming overcrowded...
White-eared kob retreat into the dwindling grasslands
of the Sudd during the dry season -
when they were last counted there were thought to be a million of them -
one of the greatest concentrations of large animals on Earth -
one thousand per square kilometre.
But the over-crowding has encourages strange behaviour.
The males are forced to fight over females by protecting tiny,
merely symbolic, territories.
But they aren't the only ones
for which the White Nile is a lifeline in the dry season.
Over a million people and their cattle live in the Sudd.
Among them - the Dinka.
Their cattle are at the centre of their world -
they depend on them for survival.
The relationship is certainly personal and spiritual,
but above all it's practical.
Rarely killed and eaten,
each cow is a precious source of milk or blood,
forming much of the Dinka's diet.
Cow dung is burned for fuel -
the smoke and the ash deters the hordes of biting insects.
When they can,
the Dinka catch fish from the swamp.
How can the Sudd sustain such huge numbers of people,
antelope and cattle?
It's all about to change...
As the rain sets in, the entire swamp begins to swell...
The Dinka are forced to move
or the rising waters will trap them.
The kob too.
Their migration is massive -
It rivals that of the wildebeest in East Africa.
In a year they will travel thousands of kilometres,
keeping one step ahead of the floodwaters.
Since the time of the Ancient Egyptians
these people have moved in rhythm with the Nile.
Unable to settle,
they couldn't build great cities or monuments
like those of Egypt or Nubia.
Distant tributaries bring yet more water to the Sudd.
The Nile bursts its banks.
Grasslands become lakes.
As the rising waters reach all corners of the swamp,
the transformation continues. Strange beasts begin to appear.
This is a fish - a lungfish.
It's been cocooned in mud and mucus underground for months,
waiting for the waters to rise.
Primitive lungs allow it to breathe air
as it searches for the nearest pool.
But reaching water doesn't mean safety.
The shoebill stork is a giant of the swamp - well over a metre tall.
Its bizarre beak is perfectly designed
for feeding in muddy water where prey is difficult to target.
A huge beak increases the chance of success.
It's surprising what a muddy pool can hide...
As the flood reaches its height, the Sudd has doubled in size.
Now covering thirty two thousand square kilometres,
it's one of the world's biggest wetlands.
For a few months, this water-world is left to nature.
For some, it's paradise.
Fish can now colonise areas that were recently grassland.
Forjust a few weeks
they can take advantage of the huge new supplies of food.
Spot-necked Otters are very social.
They live in close-knit family groups, often hunting together.
Intelligent and agile,
they hunt using speed and cunning.
Webbed feet give propulsion and a long tail helps them steer.
Although they prefer fish,
these otters will sometimes eat snails, frogs or even insects.
But there's one swamp creature that really will eat anything...!
With a muscular body one
and a half metres long and a powerful, poisonous bite,
the Nile Monitor is understandably avoided by all its neighbours.
Relatively clumsy on land,
underwater it's transformed into an agile hunter.
But the huge lizard is just as happy eating others' rotting
leftovers on the bottom of the swamp.
In fact, it will eat and digest almost anything
it can fit in its mouth.
The Nile monitor is one of the most adaptable of all the Sudd's animals.
In this constantly changing water world
it's a winning strategy.
Soon it's all change again...
The tropical sun burns off the Nile's shallow waters,
leaving a shifting mosaic of pools and new grassland.
Now it's time for the Dinka to return.
But others fare less well as the land re-emerges.
This catfish could easily find itself stranded
but it has a curious ability that might just save it.
If it can't swim, it'll crawl.
Specially adapted fins allow the catfish to walk on its elbows.
Like the lungfish, it can also breathe air.
It may have to perform this trick many times
as its habitable world gets smaller by the day.
Meanwhile, the kob are on the move again.
Their world is expanding rapidly.
As the floodwaters recede, a huge green carpet of new grass rolls out before them.
This explains how the Sudd can support such massive herds.
But is this the same flood and fertility that sustained Ancient Egypt?
There's no doubt that the White Nile has enough water
and nutrients to completely transform the Sudd.
But the great marsh holds onto its riches.
It soaks up the water like a massive sponge
and the labyrinth of papyrus traps any sediment.
Despite a great influx of water,
the flood goes no further North -
the flow of the White Nile out of the Sudd changes little through the year.
Ancient Egypt's desert world
relied on the annual flood brought by the Nile.
With it arrived millions of tonnes of fertile soil.
So where did it come from?
The search upstream from Egypt
and then up the White Nile stalled at the Sudd.
This branch of the Nile is not the source of the great flood...
But there is an alternative...
to return to Khartoum -
and take a different journey
along the Blue Nile.
Upstream from Khartoum,
the Blue Nile flows through dry savannah.
Then, on the fringe of the Ethiopian Highlands
the river emerges from deep inside a gorge so steep and hostile
that it has claimed the lives of many
who have tried to explore the river further.
But the bottom of this gorge shows little promise of a great flood.
It's barren.
This is a tough place to make a living.
They're crossing the dried out river bed of the Blue Nile.
Right now it's no more than a trickle -
just enough to warrant a trek of many hours
to bring livestock to drink.
But the huge boulders that cover the river bed suggest
that at some time a powerful force of water must flow through here.
The lammergeyer,
Africa's mountain vulture,
seeks out those who succumb to the heat and the drought -
in this brutal season
the lammergeyer will have plenty to choose from.
Ethiopia hosts eighty percent of Africa's highest mountains -
a legacy of the region's volcanic past.
Thirty million years ago,
a huge plug of molten volcanic rock welled up through cracks in the Earth's crust,
then cooled and solidified to form the Ethiopian Highlands.
Since then, erosion has carved the edges of this rocky dome to spectacular effect.
And there is a surprise up here - in this arid land...
a lake!
...at nearly two thousand metres,
Tana is Africa's highest.
Tana's hippos live at a higher elevation than any other.
Could Lake Tana be the elusive source of the Niles great flood?
The mere trickle flowing out of it doesn't look promising.
But the rocks here tell a different story...
only a raging torrent could have carved these shapes.
Further up on the highland dome above Lake Tana,
the land is similarly dry and barren.
Yet if the Blue Nile is responsible for the great flood,
it must surely start somewhere up here.
These mountains might appear lifeless,
but look closely and they are alive with rodents.
These moor lands support one of the highest densities of rodents on Earth -
up to three tonnes of them in every square kilometre -
something that has not gone
un-noticed by other local residents.
The Ethiopian Wolf,
also known as the Red Jackal, is a master rat-catcher...
...normally...
The parched vegetation gives very little cover to the rodents -
they have to risk it out in the open...
...but it's not much good at concealing a wolf either.
The rodents aren't necessarily safe underground.
The wolf has acute hearing, amazingly sensitive smell,
and a narrow, stabbing snout full of sharp teeth.
Danger's past, but they must always be on the alert...
Gelada Baboons
no monkey lives at higher altitude -
they are found only in these mountains.
They use the steep cliffs of the Blue Nile's gorge
to provide overnight protection from leopards and humans.
In fact, the rodents have nothing to fear
the baboons feed exclusively on grass
the only monkey that does so.
Males use those teeth to intimidate rivals.
They live in a complex ordered
society of many hundred animals...
It can be bitterly cold up here.
Geladas shuffle while feeding,
to save energy -
crucial with such a poor diet...
It probably stops their bare bottoms getting cold too.
This is a tough time of year
they are forced to dig for roots.
How can such a place provide the water for the Nile's great flood?
From June, moisture-laden clouds
from the Congo basin collide with these steep mountains and the heavens open.
A metre of rain and hail fall in just a few weeks.
The Ethiopian highlands are known as "Africa's Water Tower".
The roof of Africa can be a miserable place
during its three month rainy season.
But the rain brings the mountains back to life.
The rodents' world is transformed.
The lush vegetation provides them with as much food as they need,
and cover from predators...
...like this Augur Buzzard.
Provided, of course, they see it before it sees them.
The soil here is volcanic.
That means it contains a rich cocktail of nutrients.
It was soil as fertile as this which
nurtured the crops of Ancient Egypt.
With the plentiful rain and intense mountain sunshine,
the grass here grows fast.
This explains why these highlands can support such huge numbers of animals.
The rains are transforming the highlands in another way.
Night temperatures can be low enough to freeze any water in the ground.
As the ice expands, it breaks up the topsoil.
When the ice melts the next morning,
the soil is left crumbled and fine
Millions of nest-making rodents loosen the soil further.
The highlands are full of people, too.
They have exploited the rich soil here
since the time of Ancient Egypt.
The rainy season turns the highlands into a busy patchwork of farmland.
This is one of the largest and oldest areas of cultivated land in Africa.
Inevitably, this great bounty attracts unwelcome visitors.
Ethiopian children play their part in protecting the family crops.
The flute is played to scare away animals
at least that's one theory.
Sometimes more direct action is necessary.
The rains continue for up to three months.
It may appear bleak,
but in fact it's a time of celebration and thanksgiving for the highland farmers.
They're accustomed to the yearly rhythm,
and they see the rains as a blessing.
Beneath their feet, the land is undergoing one final dramatic change.
The rain-sodden earth can hold no more water.
It spills out across the mountainsides, creating a host of tiny streams.
As they merge, the cargo of fertile soil driven downstream increases.
Streams become rivers.
Finally these rivers flow into the mountain bowl that is Lake Tana.
The lake is filling up all the while,
creating marshland on its shores.
Here, Weaver birds turn the flower heads of papyrus into building material
and they set about constructing
a woven city over the growing lake.
Papyrus is one of the Niles great gifts growing along much of the rivers length
it provides a strong waterproof
building material for expert craftsmen such as these.
The people of the Nile have also found it useful for building.
The Ancient Egyptians even used it to make the paper
on which they recorded the story of their great civilisation.
People here still put papyrus to good use.
The design of these boats has changed little since ancient times.
Tana has become a vast mountain reservoir for the Blue Nile.
The dissolved volcanic soil has enriched the lake.
It's good fishing for people and animals.
It's thought that the Ancient Egyptians heard rumours of this lake,
but they never realised the critical
role it played in their lives.
Every day pelicans gather near the lakeshore...
...in response to especially dense concentrations of fish.
But sometimes the fishing is easy.
The pelicans aren't fussy about where their food comes from.
Fisherman's off cuts dojust fine
as Lake Tana's pelicans have learned.
Like the Ancient Egyptians,
the people of the Ethiopian highlands
know they must live within the Nile's rhythm.
It can be good to them and give them all they need.
But it can also turn on them
and even threaten their lives.
Some years the flood is so great that crops,
livestock and even homes are swept away
Even though most highlanders are devout Christians, older, pagan beliefs,
with a deep connection to nature, still survive here.
Discreet offerings are made to Gihon
a river spirit.
Usually, Gihon is asked for help.
But if the flood is too great, the spirit is begged for mercy.
More than sixty mountain rivers have flowed into Lake Tana,
but only one river flows out - the Blue Nile.
Virtually dry just a few months ago,
it has now become a very different river...
The highlanders call these falls Tis Isat.
It means "smoking fire".
This, surely, is a force powerful enough to carry
the great flood thousands of kilometers to the deserts of Egypt.
Gathering speed and power,
the floodwaters sweep on into the Blue Nile Gorge.
Well over a thousand metres deep,
nearly twenty kilometres wide and over six hundred kilometres long,
this is Africa's Grand Canyon.
From all over the highlands,
other huge rivers pour into the Blue Nile Gorge.
By the time it leaves Ethiopia,
the Blue Nile will be fifty times the size it was in the dry season.
Now the Blue Nile is carrying a hundred and forty million tonnes
of fertile soil.
For a million years, the Blue Nile
has been carving this huge gash through the Ethiopian highlands -
grinding down the volcanic rock, grain by grain, to form the enormous gorge.
Finally, the Nile's great flood spills out of Ethiopia
and across the flat lands of Sudan
It sweeps past Khartoum,
elbowing the steady flow of the White Nile aside.
And on through the desert to revive
and enrich the parched lands of Egypt.
Thousands of years ago this was the great flood
that allowed the Ancient Egyptians to build a great civilization.
But they passed into history without ever finding out
where the riches they depended on really came from.
The Nile has never given up its secrets easily.
Next week...
for centuries,
mankind struggled to find the source of the White Nile.
It was this great prize that drew an army of explorers to Africa -
men who were ambitious, dedicated, obsessed.
Their endeavours made them legends.
Doctor Livingstone I presume.
And revealed why a river that flows through a desert never runs dry.