and why Egypt and Ethiopia have been fighting over it.

The problem is this dam being built in Ethiopia.

Once it's finished it'll be the most powerful hydroelectric dam in Africa.

But Egypt says its supply of water is in danger of drying up.

After years of talks they're now saying they're close

to signing a deal on how to share the water.

But there are lots of details that need to be worked out.

So why has it been so hard to end the battle for the Nile?

Its official name is the Grand Ethiopian Renaissance Dam.

And it's huge.

When they're done building it

the reservoir will be bigger than Greater London

and cover almost 1,700 sq km.

It's pretty much the largest project

the government has ever embarked upon.

The original cost of it was around $5 billion.

It will probably be significantly more than that.

But before we get into the dispute about the dam itself

it helps to understand the Nile basin and what's at stake.

Eleven African countries are in it and 280 million people live there.

The Nile is also, fun fact

the only major river that runs south to north.

One of the river's tributaries, the Blue Nile

starts off in Lake Tana in Ethiopia

and converges the White Nile in Sudan's capital Khartoum.

From there the Nile flows downstream to Egypt.

There are several dams along the way, including Egypt's Aswan dam.

But Ethiopia is building its power plant upstream

right here on the border with Sudan.

Now as the name Renaissance suggests

this dam is supposed to revive Ethiopia's economy.

It'll provide electricity to millions of Ethiopians.

And that's big because right now more than two-thirds of the population

aren't connected to the grid. That's about 75 million people.

Over several years it will produce 6,000 megawatts.

Ethiopia's current generating capacity is only 4,000 megawatts.

So this dam is also going to generate

more power for Ethiopia than it needs

meaning it can export electricity.  

That's all very good news for Ethiopia.

But if you ask Egypt this dam will be catastrophic.

The Nile has been at the centre of Egyptian life for millenia.

An entire ancient civilisation was built around it

and relied on the river for transport and irrigation.

And that's still the case today.

The country relies on the Nile for almost all of its water.

It's a rain-starved, largely desert country.

So obviously any potential interruptions to the flow of the Nile

is justifiably of huge concern for Egypt.

Egypt controls most of the flow of the River Nile

into its country, at least for now.

Egypt's biggest worry is how quickly Ethiopia will fill the reservoir.

And our colleagues at Al Jazeera Labs have studied a few scenarios.

Say Ethiopia agrees to fill the reservoir over the next 10 years.

AJ Labs says that would cut Egypt's water supply

by 14% and destroy 18% of its farmland.

If Ethiopia fills the dam more quickly, say over seven years

one-third of Egypt's farmland would be lost.

And a five-year window would destroy half of that land.

According to their simulations this is how the Egyptian delta

would change over those 10 years.

So Ethiopia's best-case scenario

filling the dam as soon as possible

is Egypt's worst nightmare.

It's no wonder these negotiations have been tough.

It was during the Arab Spring in 2011

while Egypt had a revolution on its hands

that Ethiopia started building the dam.

Ever since then Egyptian leaders have tried to stop the project

and military conflict never seemed far.

“Last year, Ethiopia accused Egypt of sending rebels to neighbouring Eritrea

to sabotage the dam, prompting another Nile-sharing country

Sudan, to send its troops to its border.”

In 2018 the operating manager of the Ethiopian dam

Simegnew Bekele, was found dead.

“Bekele's sudden death drew crowds and raised suspicions.”

It was reported he'd been shot in broad daylight which led to protests.

But in the end the authorities said Bekele took his own life.

In 2019 Ethiopia's prime minister, Abiy Ahmed, even said:

"...no force could stop Ethiopia from building a dam.

If there is a need to go to war we could get millions readied.

If some could fire a missile, others could use bombs.”

This is ultimately an age-old debate about who owns the Nile.

In 1929 Egypt signed a treaty with Britain, a colonial power in Africa

at the time, that basically guarantees its access to the Nile and gave

Egypt a veto over construction projects on the river.

But Ethiopia didn't sign that treaty.

Then in 1999 all the Nile basin countries, including Egypt and Ethiopia

signed an agreement on managing water.

But that deal was compromised in 2010

when six of those countries decided to draw up another treaty.

This time Egypt rejected it because it threatened its historic rights to water.

It's Egypt's concern that if it moves into

It's Egypt's concern that if it moves into

then it will be left short of water resources

and also its control over the situation will be massively reduced.

And as far as Sudan goes it's sort of caught in the middle.

Egypt is Sudan's political ally

but the government supports the Renaissance dam

because it'll be a source of affordable energy

and it will help regulate flooding.

The good news is that Egypt, Ethiopia and Sudan

are in talks with one another.

The bad news is that they still haven't answered the tough questions.

Questions like, when there's a drought

will Ethiopia consider Egypt's needs?

And when is a drought a drought?

Those conversations will only get tougher

because more dams are being planned

populations are growing and the climate is changing.

All of which will make the Nile and the supply of water

increasingly worth fighting over.