A dispute over the Nile, the world’s longest river, is coming to a head. At stake are the lives and livelihoods of millions of people who depend on its water.

Egypt is objecting to efforts by Ethiopia to start operating a $4.8 billion dam on a major tributary of the Nile, a hydroelectric project that it hopes will power a social and economic transformation of the country, without a binding agreement that preserves Cairo’s rights to the waters.

The project is regarded by Egypt as a threat that could cut off nearly all of a key source of water for the country’s rapidly growing population. The country has exerted outsize control over access to the Nile since colonial times.

Water scarcity has become a geopolitical, economic and business concern around the globe, with populations competing with farming and industry.

The dispute over the Nile is one of the world’s major flashpoints over water rights. Disagreements have intensified in recent years over how much water each country along the Nile and its tributaries can tap, with the U.S. and other African countries seeking to help mediate disagreements.