There is only one way to appreciate the scale of Egypt's monumental Aswan High Dam, and that is by standing directly on top of it. Beneath your feet lie 43m cubic metres of granite, an edifice that took 10 years and a billion dollars to assemble.

To your south 2,000 or more square miles of water stretch out towards the Sudanese border, forming Lake Nasser, one of the largest reservoirs on Earth. To the north, gurgling out quietly from deep within the barrage, there is the Nile, now tamed, steady, and ready for use by 80 million people downstream.

In terms of sheer technical ambition, not to mention its impact on Egypt's economic fortunes, political might and cultural identity, nothing has rivalled the High Dam since the pyramids.

Judging from the nationalist symbols plastered all over a nearby celebratory monument, and from the soldiers patrolling the dam's walkways, this vast structure means as much to Egypt today as it did when it was completed 40 years ago

Symbolising the country's historical mastery over the world's longest river, it also marks the spot where, should upstream African countries have their way, surrender of that control will first become visible.

The consequences of any reduction in Egypt's share of the Nile's flow will be felt across the country, not least on the brackish fields of the Nile Delta, about 500 miles away, where farmers are already struggling to find fresh water.

Near Aswan it becomes clear how important the Nile is to Egypt, which relies on the river for 90% of its water supply. On the west bank of the city, on a 600-metre wide strip of verdant land that quickly gives way to rocky desert, Omar and his fellow farmers produce grapes, figs, watermelons and other crops for export to food markets in Cairo. Temperatures here can reach up to 45C in summer but irrigation canals and oxen-powered pumps keep Nile water streaming in all year round.

"The Nile is everything to us, it's liquid gold," explained the 25-year-old as he oversaw the day's mango harvest. "We're like fish here: take us from the water and we'll perish."

In the fishing and agricultural districts of upper Egypt there is little sympathy for the countries upstream that are threatening to boost their share of the Nile's resources. The stance is echoed by technical experts in Cairo, who claim that Egypt's share of the water in the region is already dangerously small.

"Nile-basin countries as a whole receive 7,000bn cubic metres [bcm] a year of rainfall," said Khaled Abu Zeid, a regional water resources programme manager at CEDARE, the Centre for Environment and Development for the Arab Region and Europe. "In the Nile basin itself, you're looking at 1,660bcm of annual rainfall. And then from all this, you have Egypt taking 55.5bcm a year from the Nile, our only source of fresh water. So we have to ask ourselves exactly what we're talking about when terms like 'water sharing' are used. Egypt is a desert environment, whereas some of the upstream countries could not get any greener."

Given the precarious state of Egypt's water security, it is not surprising that successive political leaders have described any possible alterations to the distribution of the Nile as an existential threat to the nation. Anwar Sadat, Egypt's president, famously declared himself ready to go to war against any attempt to limit Egypt's dominance of the river; recently, Egyptian columnists have characterised the actions of upstream states as a "genocidal war" against Egyptians. Some writers have suggested that Egypt's strident rhetoric has hampered the spirit of co-operation between Nile states.

But Abu Zeid said: "Egypt sincerely wants to work with upstream countries, and I hope that those countries don't look negatively upon these statements about the Nile being a 'red line' for Egypt. But regardless of what language you choose to employ, the fact is the Nile is a national security issue for Egypt."

As an indication of how seriously the Egyptian government is taking the present crisis, responsibility for the Nile basin dispute was removed from the water and foreign affairs ministries last month and put into the hands of Egypt's intelligence and security chief, Omar Suleiman.

Suleiman was in Uganda this week holding talks with the country's president about the Nile issue, as Egypt stepped up efforts to persuade other countries, such as Burundi, not to sign the rival River Nile basin co-operative framework agreement threatening Egypt's hegemony.

Egypt makes much of its water recycling and desalination programmes, arguing that the country's barren environs have forced it to use meagre water resources efficiently. But critics dispute these claims, pointing to the outskirts of big cities like Cairo where luxury residential developments are accompanied by a rash of water-intensive landscaped gardens and golf courses.

There are many who believe that Egypt's Nile predicament also reveals a long-term political malaise, which has seen the country's status as the pre-eminent regional power slowly drain away.

"President Nasser cultivated a sense of post-colonial solidarity with upstream states based around the non-aligned movement, yet under the regimes of his successors Africa has been neglected," said Nabil Abdel Fattah, a research director at the Al-Ahram Centre. We have seen a marginalisation of the African affairs institutes at universities, a marginalisation of African news on our TV screens. The problem here is … the perception we have of Egyptian identity. Our politicians see Africa as a backwater, its countries as underdeveloped."

Omar could not agree more. "This would never have happened under Nasser; if he were still with us nobody would dare try to take our water."