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."