A few hundred yards away, cranes and diggers are building earthworks for a 10.5-mile bridge, costing pounds 550m, that will link the Portuguese capital to the sleepy south-eastern shore and change its face forever. Environmental pressure groups, including the World Wide Fund for Nature (WWF), say it is being built in the wrong place and flouts European Union environment rules. They want the project, the biggest of its kind in Europe, halted.
But the first piles have already been driven and executives of the Lusoponte consortium, led by the British company Trafalgar House, moved into their site offices on the north side of the estuary this week to supervise the construction. Completion in March 1998 will coincide with the opening of Expo 98 whose vast site adjoins the bridge. Martin Edwards, a senior Lusoponte spokesman, was sorting out his files and still staggered by the view.
"The bridge will take traffic out of Lisbon, which is far too congested, will sweep away unsightly shanty towns and open up areas of prime real estate for development," he said. "We are aware of the environmental concerns and are confident the measures we have adopted will keep the environmental impact to a minimum."
The bridge will cut across one of Europe's most important wetlands, in an EU protected area, Jorge Palmeirim, president of Portugal's Nature Protection League, said. Two weeks ago the WWF called on the European Commission to halt the work and hold a public inquiry into the bridge, which is partly-funded by the EU and the European Investment Bank.
Mr Edwards conceded there would be some disturbance during the building phase, but said that landscaped sound barriers, a viaduct over the salt pans and special shaded lighting will minimise long-term damage to aquatic birds. Some salt pans are to be recovered as a nature reserve, he added.
Across the estuary in the Alcochete headquarters of the Tagus Estuary Natural Reserve, opposition is fierce. Jose Manuel Marcos, a biologist working at the reserve, said: "The problem is not the bridge, but the new motorways and uncontrolled urbanisation that it will bring. It is not a problem for today or tomorrow, but in 20 years when new urban areas will cut the birds' migration routes. The benefits are doubtful, but the damage to wintering aquatic birds and breeding birds is certain."
The Communist mayor of Alcochete, Miguel Boieiro, is less pessimistic. "Almost all the local people wanted the bridge in this area because we thought it would be a factor for development. We'll be nearer to Lisbon, it will bring greater dynamism and more jobs. But there will be disadvantages too and we must fight for environmental protection measures to be taken and respected," he said.
In Mr Palmeirim's view, instead of plunging into empty land the bridge should have linked the capital at its busiest parts, the suburbs of Chelas on the north side of the estuary and Barreiro on the south bank, "bringing life back to the dying parts of the city, instead of destroying open land for speculative reasons".
That route, which the government turned down, would have served the transport needs of more people and taken pressure off the existing bridge, Mr Palmeirim said. He points to a government-sponsored report predicting that even when the new Vasco da Gama bridge is open, traffic on the old bridge will continue to rise, and at a faster rate than that using the new bridge.
Part of the new bridge's funds come from tolls levied on the old 25 April bridge, and when tolls were increased by 50 per cent in June last year, users threw up barricades in protest. A two-month "won't-pay" campaign forced the government to back down and give special concessions for frequent users. The row "disrupted the financing of the new bridge", Mr Edwards said.
He believes the success of the new bridge depends on the completion of ring roads round the city and motorway links to Spain and south to the Algarve. But Mr Palmeirim says that the thinking behind the project is based on grandiose and outdated concepts of private road transport that do not meet public transport needs.
Inside the church in Alcochete's tranquil central square are 14th-century green and ochre tiles whose formal symmetrical patterns indicate that the building was once an Arab mosque. It symbolises a fusion of African and European culture typical of the Iberian south.
The area is also, because of special climatic conditions that combine Mediterranean and Atlantic influences, a rare crossing point for African and northern European wildlife, Jose Manuel Marcos said.
One of the flamingos feeding in the salt flats unfurled its vast wings to reveal a flash of deep coral pink. Jorge Palmeirim lowered his binoculars. "These birds are doomed," he sighed.