About 2,500 years ago, the Greek historian Herodotus called Egypt the “Gift of the Nile.”

Today, Egyptians say their ancient ancestors would have done anything to protect their indispensable Nile River, and so should they.

But overdevelopment and construction of a massive dam upstream in Ethiopia jeopardizes their vital water supply — and very existence.

As Hassan Hamid, 36, a boatman in Luxor who ferries passengers across the Nile, explained: The pharaohs “knew all the good in their lives came from the river. We only believe in one God now, but still the Nile is our life.”

The Grand Renaissance Dam, standing more than 500 feet tall, is slated to become the biggest in Africa when it begins operations later this year. The dam, about 450 miles from the Ethiopian capital of Addis Ababa, will generate 6,450 megawatts at full capacity — more than three times the energy produced by the Hoover Dam. Three-quarters of Ethiopians currently lack access to electricity, according to the World Bank.

“We’ve consistently been the fastest-growing economy in Africa, and this dam will help us keep up this level of growth,” Ethiopia’s top energy official, Motuma Mekassa Zeru, said in April when he announced the dam was 60% complete.

But Egypt and Sudan are worried that the dam will curtail their share of the Nile’s waters as global warming and less rainfall also threaten to lower the river’s level. The Nile provides nearly 100 million Egyptians with virtually all their water.

Ethiopia’s dam could drop the Nile’s levels by 25% for as long as seven years while the reservoir behind it fills up, according to a recent article in the Geological Society of America’s journal GSA Today.

That estimate was based on computer models, said Hany Hamroush, professor of geology and geochemistry at the American University in Cairo.

“It is alarming how much information is missing about the dam,” Hamroush said. “There has to be a complete transparency and honesty and full professional data to make sure that that dam will be safe.”

Egyptian President Abdel-Fattah el-Sissi has launched a diplomatic offensive to press Ethiopia to slow the timetable for filling the reservoir. He has visited the Nile basin countries of Sudan, Tanzania, Rwanda and Ethiopia five times so far this year.

“Egypt’s water security is non-negotiable,” said Ahmed Abu Zeid, spokesman for the Foreign Ministry in Cairo. “It is considered a red line that no one can approach.”

But even without the Grand Renaissance Dam, the United Nations estimates Egypt will face “absolute water scarcity” by 2025 for reasons largely of its own making.

Egypt’s population has almost tripled in the past 50 years to 97 million. Egyptians now have 15 times less water per person than the average American.

Pressures from the growing population also is resulting in 30,000 acres of land lost each year to illegal construction, most of it along the Nile, according to Egyptian government figures.

Such development is one reason that el-Sissi is pushing to build new cities in largely uninhabited desert areas, like the $45 billion New Administrative Capital 28 miles east of Cairo.

Authorities are taking drastic measures to protect the Nile’s banks from urban sewage and industrial waste.

In May, el-Sissi ordered the demolition of 50,000 illegally built homes on Warraq, a large island in the Nile in Cairo. The government claimed the homes were on state-owned property. In July, police clashed with the homeowners, killing one.

“Where does their sewage go?” el-Sissi asked at the time. “It goes into the Nile water, which we drink.”

Urban sprawl and changing agricultural practices — due in part to Egypt’s Aswan Dam that allows for year-long irrigation — have caused groundwater problems along the Nile.

“I used to drink directly from the Nile,” said Ahmed Sefelnasr, 43, a geologist at Assiut University. “I can’t do that now and would never recommend that my students do it.”

The U.S. government is helping address the issue at historical sites that attract tourists — tourism accounts for 13% of the Egyptian economy. The U.S. Agency for International Development is spending $14.8 million for Egyptian pumping projects at six key world heritage sites.

Those projects include operations at Luxor and the Giza Plateau, home to Egypt’s most magnificent pyramids and the iconic Sphinx, to prevent salt-saturated Nile groundwater from damaging the popular antiquities.

“We have to tell our children not to build on the banks of the Nile,” said Omar Badawi, 68, an engineer who helps manage miles of drains encircling the colossal monuments at anchient Karnak Temple.