Ethiopia is building the Grand Renaissance Dam, which when completed next year, will be Africa’s biggest hydroelectric power plant.
Its construction began in 2011 on the Blue Nile tributary in the northern Ethiopia highlands from where 85% of the Nile’s waters flow.
However, the mega dam has caused a row between Egypt and Ethiopia, with Sudan caught in between, which the US is now helping to mediate.
Why is it contentious?
At the centre of the dispute are plans to fill up the mega dam as Egypt fears the project will allow Ethiopia to control the flow of Africa’s longest river.
Hydroelectric power stations do not consume water, but the speed with which Ethiopia fills up the dam’s reservoir will affect the flow downstream.
The longer it takes to fill the reservoir, which is going to be bigger than Greater London, the less impact there will be on the level of the river.
Ethiopia wants to do it in six years.
“We have a plan to start filling on the next rainy season, and we will start generating power with two turbines on December 2020,” Ethiopia’s Water Minister Seleshi Bekele said in September.
But Egypt has proposed a 10-year period – this will mean that the level of the river does not dramatically drop, especially in the initial phase of filling the reservoir.
Three-way talks between Egypt, Sudan and Ethiopia over operating the dam and filling its reservoir have made no progress in four years – and now the US is trying to mediate.
Explore the Nile with 360 video
Media playback is unsupported on your device
Join BBC reporter Alastair Leithead and his team, travelling in 2018 from the Blue Nile’s source to the sea – through Ethiopia and Sudan into Egypt.
This 360° video is a version of the first VR documentary series from BBC News. To view the full films, click here.
Why is Egypt so upset?
Egypt relies on the Nile for 85% of its water. It has historically asserted that having a stable flow of the Nile waters is a matter of survival in a country where water is scarce.
A 1929 treaty (and a subsequent one in 1959) gave Egypt and Sudan rights to nearly all of the Nile waters. The colonial-era document also gave Egypt veto powers over any projects by upstream countries that would affect its share of the waters.
Ethiopia says it should no longer be bound by the decades-old treaty and went ahead and started building its dam at the start of the Arab Spring in March 2011 without consulting Egypt.
Egyptian President Abdul Fattah al-Sisi was quoted as saying in September that it would never have got under way had Egypt not been distracted by the political turmoil.
The North African country’s main concern is that if the water flow drops it could affect Lake Nasser, the reservoir further downriver, behind Egypt’s Aswan Dam, which produces most of Egypt’s electricity.
It could also affect transport on the Nile in Egypt if the water level is too low and affect the livelihood of farmers who depend on the water for irrigation.
Why does Ethiopia want such a big dam?
The $4bn (£3bn) dam is at the heart of Ethiopia’s manufacturing and industrial dreams. When completed it is expected to be able to generate a massive 6,000 megawatts of electricity.
Ethiopia has an acute shortage of electricity, with 65% of its population not connected to the grid.
The energy generated will be enough to have its citizens connected and sell the surplus power to neighbouring countries.
Ethiopia also sees the dam as a matter of sovereignty.
The dam project does not rely on external funding and relies on government bonds and private funds to pay for the project.
The country has been critical of what it considers foreign interference in the matter.
Does anyone else benefit?
Yes, neighbouring countries including Sudan, South Sudan, Kenya, Djibouti and Eritrea are likely to benefit from the power generated by the dam.
Many of these countries have huge power deficits.
For Sudan there is the added advantage that the flow of the river would be regulated by the dam – meaning it would be the same all-year round.
Usually the country suffers from serious flooding in August and September.
Could the dispute lead to a war?
There have been fears that the countries could be drawn into a conflict should the dispute not be resolved.
In 2013, there were reports of a secret recording showing Egyptian politicians proposing a range of hostile acts against Ethiopia over the building of the dam.
President Sisi has also been quoted as saying that Egypt would take all the necessary measures to protect their rights to the Nile waters.
Last month, Ethiopian Prime Minister Abiy Ahmed told MPs that “no force” could stop Ethiopia from building the dam.
The fact that the US has intervened shows the seriousness of the situation – and the need to break the deadlock.
So what happened at the talks?
The three sides agreed to hold further talks in the US, and to reach a final settlement of the dispute by 15 January 2020.
If they still can’t agree, they have said they can jointly request further mediation.
The meeting also agreed that the US and the World Bank would attend future negotiations as observers.