CAIRO — Every day now, seasonal rain pounds the lush highlands of northern Ethiopia, sending cascades of water into the Blue Nile, the twisting tributary of perhaps Africa’s most fabled river.
Farther downstream, the water inches up the concrete wall of a towering, $4.5 billion hydroelectric dam across the Nile, the largest in Africa, now moving closer to completion. A moment that Ethiopians have anticipated eagerly for a decade — and which Egyptians have come to dread — has finally arrived.
Satellite images released this week showed water pouring into the reservoir behind the Grand Ethiopian Renaissance Dam — which will be nearly twice as tall as the Statue of Liberty. Ethiopia hopes the project will double its electricity production, bolster its economy, and help unify its people at a time of often-violent divisions.
#FillTheDam read one popular hashtag on Ethiopian social media this week.
Seleshi Bekele, the Ethiopian water minister, rushed to assuage Egyptian anxieties by insisting that the engorging reservoir was the product of natural, entirely predictable seasonal flooding.
He said the formal start of filling, when engineers close the dam gates, has not yet occurred. Effectively, that will be the moment when Ethiopia launches its huge project and gains tremendous control over the flow of Nile waters into Egypt.
Despite those assurances, downstream in Egypt, which relies on the Nile for 90 percent of its water, the images prompted consternation.
For nearly a decade, Egypt has been negotiating with Ethiopia over how the dam should be filled and operated. The latest, last-ditch effort ended inconclusively on Monday, and the satellite photos, combined with news reports from Ethiopia, fueled speculation that the dam’s reservoir had, in fact, begun to fill up.
“The question is: What will we do?” the television host Nashat el-Dihi said on Egypt’s privately-owned Ten TV station on Wednesday. “The people are worried, and that worry must have an influence.”
Egyptian fears are amplified by the repeated insistence of the prime minister of Ethiopia, Abiy Ahmed, that his country will close the dam gates by the end of this month, come what may.
“If Ethiopia doesn’t fill the dam, it means Ethiopia has agreed to demolish the dam,” Mr. Abiy told lawmakers on July 7.
Although the dam’s reservoir will take at least seven years to fill, the start of the process has acquired an intense significance for both countries — a hydrological Rubicon that, if crossed without agreement, could push their dispute in a new and unpredictable direction.
At the United Nations last month, Egypt’s foreign minister, Sameh Shoukry, reiterated warnings that his country viewed it as an issue of “existential” significance.
“Survival is not a question of choice, but an imperative of nature,” he said.
Ethiopia’s ambassador to the United Nations, Taye Atske Selassie, retorted that the dispute over the Nile, which flows through at least six countries, was equally important for Ethiopia.
In reality, Egypt faces no immediate threat to its water security.
Even if Ethiopia proceeds with the fill as planned this month, less than one-tenth of the reservoir will be filled. With ample reserves of water behind Egypt’s own dam on the Nile, at Aswan, there is little risk of parched Egyptian fields or taps in Cairo running dry.
But the two countries remain bitterly divided on key issues, rooted in history, pride and money, that in essence amount to a dispute over control of the Nile itself.
“This is an important moment,” said William Davison, an analyst with the International Crisis Group. “It raises the prospect of the two downstream countries pulling out of talks, which may increase tensions.”
Egypt wants legally binding assurances that, in the event of prolonged drought, Ethiopia will slow or halt the filling of the dam. Cairo also wants a say in Ethiopia’s development of any other dams on the Nile in the future.
Ethiopia rejects those demands, which it views as a violation of sovereignty. Egypt must accept that its centuries-old dominance of the Nile has come to an end, they say.
Trust is low on all sides. One Egyptian official, who requested anonymity to discuss sensitive international talks, accused Ethiopia of seeking a deal with such vague commitments and with so many loopholes that he likened it to a block of Swiss cheese.
Ethiopian officials shoot back with accusations that Egypt is behaving in a typically highhanded manner, and they point to strong backing from ordinary Ethiopians — many of whom have a financial stake in the project.
Like tens of thousands of Ethiopians, Yacob Arsano Atito, a political-science professor at Addis Ababa University in the Ethiopian capital, bought government bonds in the dam years ago. He hopes his investment of $225 will soon be repaid.
“I’m very happy the project is on its way to completion,” he said.
In an interview, Mr. Bekele, the Ethiopian water minister, said that the dam was not fully constructed. It currently rises to more than 1,800 feet, about 260 feet short of its final height. Not all of its 13 turbines have been installed.
Although the dam’s reservoir has a maximum capacity of 19.5 trillion gallons, much larger than the reservoir behind the Hoover Dam, its optimal operating size will be about 13 trillion gallons, the equivalent of one year’s flow of the Nile.
In many ways, though, the dispute is as much about politics as hydrology.
Mr. Abiy, the Ethiopian premier, swept to power in 2018 with a reputation as a reformer, and last year, he won a Nobel Peace Prize for his success in forging peace with Eritrea. But recently, his country has again become mired in violent upheaval over the status of the Oromo, its largest ethnic group.
In the east of the country, the insurgent Oromo Liberation Army carries out attacks on the security forces, and at least 166 people were killed in late June during protests over the killing of Hachalu Hundessa, a popular Oromo musician and activist.
For Mr. Ahmed, who has repeatedly vowed to fill the dam this month, the project provides “something that Ethiopians can unite around,” said Mr. Atito, the political scientist.
There are also perils for Egypt’s authoritarian leader, President Abdel Fattah el-Sisi. His security forces do not anticipate major demonstrations if he fails to strike a deal with Ethiopia, officials said.
But after building up the dispute for so many years, Mr. el-Sisi’s legitimacy could take a knock if he fails to act, particularly at a time when he is also facing down threats from other regional rivals.
In a meeting with Libyan tribal leaders on Thursday, Mr. el-Sisi reiterated a threat to send Egyptian troops into Libya to fend off forces backed by his rival, Turkey.
Stuck in the middle is Sudan, which lies between Ethiopia and Egypt. Sudan stands to benefit from cheaper electricity produced by the dam, but worries that any sudden release of water could damage its own, smaller Roseires Dam.
Western diplomats observing the crisis say that veiled Egyptian threats of military action against Ethiopia are unlikely to be carried through, although Egyptian officials refuse to rule it out. But for now, the focus is on a political deal.
The African Union, headed by South Africa, is expected to call an emergency meeting to discuss the crisis next week. Mediators hope that, with one final push, they can bridge the differences between Ethiopia and Egypt, before seasonal rains fill the gap for them.
Simon Marks contributed reporting from Palau, Sardinia; and Nada Rashwan from Cairo.