The Blue Nile starts quietly. At Gish Abay in the Ethiopian highlands, where Amhara farmers live at 2,700 metres, a spring emerges into what Ethiopians have long considered a holy place - one of the rivers of Eden, the biblical Gihon, given to them by God. The water flows south, then loops east, then descends into a canyon 400 kilometres long that cuts the Ethiopian highlands to depths of 1,500 metres. A British team first descended the full length of that canyon in 1968, at the request of Emperor Haile Selassie, under the explorer John Blashford-Snell. They called it the Grand Canyon of the Nile. The river Ethiopians call the Abay had remained largely unmapped - marked, as British explorer Robert Cheesman remembered in the 1920s, 'on the map by dotted lines.'

The first European to see the source was the Spanish Jesuit Pedro Paez on 21 April 1618. The Portuguese Joao Bermudes had described the Tis Abay Falls in memoirs published in 1565. But the canyon defeated everyone who tried to pass through it until the 20th century. The American explorer W.W. Macmillan wrecked his boats trying in 1902. His Norwegian partner B.H. Jenssen was stopped by rapids at Famaka short of the Sudan border. Robert Cheesman gave up following the river banks and mapped the upper Blue Nile by travelling 5,000 miles on a mule through the adjacent country between 1925 and 1933. The 1968 Blashford-Snell expedition used specially built Avon inflatables and modified Royal Engineers assault boats. In 2004 the geologist Pasquale Scaturro and filmmaker Gordon Brown became the first people to navigate the whole river in one trip, which they filmed for an IMAX. In 2005 the Canadian Les Jickling and New Zealander Mark Tanner completed a fully human-powered transit - all 5,000 kilometres from Gish Abay to the Mediterranean - in five months.

Egypt has feared for a thousand years that Ethiopia might block its water. It is not an idle fear. Fifty-nine percent of the water that reaches Egypt originates from the Ethiopian highlands. In the medieval period the threat was religious - Ethiopian emperors reminded Egypt's Coptic Orthodox Patriarchate that they could obstruct the river, which provided leverage in negotiations over the appointment of Ethiopia's metropolitan bishop. The Solomonic emperors Dawit II, Yeshaq I, and Zara Yaqob all used the diplomatic threat. None of them actually tried to divert the Blue Nile, because at the time, no one had the engineering to do it. In November 2011 Ethiopia began construction of the Grand Ethiopian Renaissance Dam - GERD - a 5,150-megawatt hydroelectric project on the Blue Nile near the Sudanese border. Electricity generation began in February 2022. Egypt and Sudan, downstream, have voiced concerns about the potential reduction of water delivery. The thousand-year-old argument continues with turbines.

From June to September the summer monsoon saturates the Ethiopian highlands. The Blue Nile takes that rain and carries it down through the canyon as flood, stripping fertile topsoil from the plateaux and transporting it as silt. The water turns dark brown - almost black. For most of human history these were the floods that built Egypt. They laid silt on fields along 900 kilometres of the Nile every year, and in the rhythm of that flooding rose the pharaonic civilisation, the myth of Osiris and Isis, the granaries of Alexandria. When the Aswan High Dam closed in 1970, those floods ended, and with them a way of life older than writing. The Blue Nile still carries the silt - it just drops it now, uselessly, at the bottom of Lake Nasser.

In 2000 an American named Kenneth Frantz saw a photo in National Geographic of ten men on each side of a broken bridge over the Blue Nile, pulling each other across the gap with rope. The bridge had been built by Emperor Fasilides around 1660 using Roman bridge-building techniques brought to Ethiopia by Portuguese soldiers fighting alongside Christian Ethiopians against the invasion of Ahmad al-Ghazi in the 1500s. World War II bombing had broken its span. Frantz founded a charity called Bridges to Prosperity, and in 2001 and 2009 volunteers from the United States helped repair the old bridge and build a new flood-proof suspension bridge in its place. The Sebara Dildiy - the 'broken bridge' - is no longer broken. The story of how it was rebuilt is a small one, set against the thousand-year geopolitics of the river. Both stories are about water that runs through the Ethiopian highlands and will run through them long after the dams and the governments that built them are gone.