The only reason Egypt has even existed from ancient times until today is because of the Nile River, which provides a thin, richly fertile stretch of green through the desert.
Now, for the first time, the country fears a potential threat to that lifeline, and it seems to have no idea what to do about it.
Ethiopia is finalizing construction of the Grand Ethiopian Renaissance Dam, its first major dam on the Blue Nile, and then will eventually start filling the giant reservoir behind it to power the largest hydroelectric dam in Africa.
Egypt fears that will cut into its water supply, destroying parts of its precious farmland and squeezing its population of 93 million people, who already face water shortages.
Dam construction on international rivers often causes disputes over the downstream impact.
But the Nile is different: few nations rely so completely on a single river as much as Egypt does. The Nile provides over 90 per cent of Egypt's water supply.
Almost the entire population lives cramped in the sliver of the Nile Valley. Around 60 per cent of Egypt's Nile water originates in Ethiopia from the Blue Nile, one of two main tributaries.
Egypt hardly gets by with the water it does have. It has one of the lowest per capita shares of water in the world, some 660 cubic meters a person. The strain is worsened by inefficiency and waste. With the population expected to double in 50 years, shortages are predicted to become severe even sooner, by 2025.
Egypt already receives the lion's share of Nile waters: more than 55 billion of the around 88 billion cubic meters of water that flow down the river each year. It is promised that amount under agreements from 1929 and 1959 that other Nile nations say are unfair and ignore the needs of their own large populations.
Complicating the situation, no one has a clear idea what impact Ethiopia's dam will actually have. Addis Ababa insists it will not cause significant harm to Egypt or Sudan downstream.
Much depends on the management of the flow and how fast Ethiopia fills its reservoir, which can hold 74 billion cubic meters of water. A faster fill means blocking more water, while doing it slowly would mean less reduction downstream.
Once the fill is completed, the flow would in theory return to normal. Egypt, where agriculture employs a quarter of the work force, is worried that the damage could be long-lasting.
One study by a Cairo University agriculture professor estimated Egypt would lose a staggering 51 percent of its farmland if the fill is done in three years. A slower, six-year fill would cost Egypt 17 percent of its cultivated land, the study claimed.
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Internal government studies estimate that for every reduction of 1 billion cubic meters of water, 200,000 acres of farmland would be lost and livelihoods of 1 million people affected, since an average of five people live off each acre, a senior Irrigation Ministry official said. He spoke on condition of anonymity because he was not authorized to discuss the figures.
Other experts say the impact will be far smaller, even minimal. They say Egypt could suffer no damage at all if it and Ethiopia work together and exchange information, adjusting the rate of filling the reservoir to ensure that Egypt's own massive reservoir on the Nile, Lake Nasser, stays full enough to meet its needs during the fill.
Unfortunately, that isn't happening so far.
"To my knowledge, this situation is unique, particularly at this scale," said Kevin Wheeler at the Oxford University's Environmental Change Institute. "I just can't think of another case that has two large reservoirs in series without a plan on how to operate them together."
Originating in Ethiopia, the Blue Nile flows into Sudan, where it joins with the White Nile, whose source is Lake Victoria in east Africa. From there it flows through Egypt to the Mediterranean.
For Ethiopia, the $5 billion dam is the realization of a long-delayed dream. Ethiopia's infrastructure is among the least developed in the world, leaving most of its 95 million people without access to electricity. The hydroelectric dam is to have a capacity to generate over 6,400 Megawatts, a massive boost to the current production of 4,000 Megawatts.
The dam, around 60 percent complete, is likely to be finished this year or early next. Ethiopia has given little information on when it will start the fill or at what rate.
"We have taken into account (the dam's) probable effects on countries like Egypt and Sudan," Ethiopia's water, irrigation and electricity minister, Sileshi Bekele, told journalists. He added that plans for the fills could be adjusted.
In a 2015 Declaration of Principles agreement, Egypt, Ethiopia and Sudan agreed to contract an independent study of the dam's impact and abide by it as they agree on a plan for filling the reservoir and operating the dam. But the deadline to complete the study has passed, and it has hardly begun, held up by differences over information sharing and transparency despite multiple rounds of negotiations among the three.
Frustration among Egyptian officials is starting to show.
In June, Egyptian Foreign Minister Sameh Shukri spoke of "difficult talks" and complained of delays in the impact study.
A high-ranking government official acknowledged there's little Egypt can do. "We can't stop it and in all cases, it will be harmful to Egypt," he said, speaking on condition of anonymity because of the sensitivity of the talks.
Egyptian leaders in the past have rumbled about military action to stop any dam, but that option seems less likely after Egypt signed the Declaration of Principles.
Salman Salman, a Sudanese water expert, said Egypt has long had an attitude of "this is our river and no one can touch it."
Now, he said, "Egypt is no longer the dominant force along the Nile. Ethiopia is replacing it."