'We want to fight climate change. But we want to fight it with technology that doesn't destroy natural habitat.' — Reuters/File Photo

SLIMBRIDGE In this broad grey estuary, where thousands of migratory birds shelter and wildlife lovers walk, it's a battle of nature versus future.

Environmentalists want to build the world's largest renewable energy plant here to generate clean power, in what would be Britain's biggest public construction project since the Channel Tunnels were completed 15 years ago. But conservationists want to protect the mudflats and Atlantic salt marshes, which sustain ducks, swans and geese.

The plans for the dam on the border between England and Wales highlight the collision of two environmentally sensitive goals - protecting wildlife versus reducing dependence on fossil fuels. The gigantic scale of the $28 billion project has only increased the stakes.

 'Birds have been coming here for generations,' said James Lees, a gamekeeper at Slimbridge, a nature reserve the size of New York's Central Park on the banks of the Severn Estuary. 'They fly thousands of miles and know they can shelter here. It's irreplaceable.'

The proposed dam - one of a shortlist of five green energy proposals - would be made up of a 16-kilometer-long wall built across the Severn Estuary, a broad stretch of water that narrows into tributaries meandering through bucolic villages with names like Tintern, Dursley and Tenbury Wells. The wall will hold back these immense tides, then release the water through a series of turbines to generate electricity.

 Some environmental groups want the government to consider a smaller system of tidal 'reefs' - reversible turbines placed on the estuary floor to generate power while the tide moves in and out.

'Of course we want to fight climate change. But we want to fight it with technology that doesn't destroy natural habitat,' said Martin Harper at conservation group Royal Society for the Protection of Birds. 'A lot of people assume the birds will simply find somewhere else to go. They won't. It takes them years to find a feeding ground.'

 Though small tidal mills have been used to generate power in southern England since the Middle Ages, the potential for environmental degradation has hampered further construction and discouraged larger projects. A report by the World Energy Council said there are several suitable sites in the world, but large capital costs and environmental impact have made politicians hesitate.

The Cardiff-Weston project would be the biggest of its kind. Britain plans to derive 15 per cent of its energy from renewables by 2020, and a successful tidal power plan here could generate up to 8.6 gigawatts of power or the equivalent of eight coal-fired power stations - or 5 per cent of Britain's needs.

A similar plant at La Rance in northern France built in the 1960s, is comparable, but the dam there generates just 240 megawatts of electricity - 35 times less than what Severn could do.  Impact uncertain Since there's little to compare it to, no one is exactly sure what the impact of the Severn dam will be.

The government acknowledges the project could wipe out nearly 50,000 acres along the estuary but promises to set land aside elsewhere to compensate. Conservationists say much of the land is irreplaceable, especially at the Slimbridge reserve, founded 65 years ago by Sir Peter Scott, son of the famous Antarctic explorer.
 
This peaceful stretch of marshland about 193km west of London is one of the country's most popular reserves, attracting hundreds on sunny days. Local bird-spotters bring flasks of tea and sandwiches wrapped in paper and sit for hours in wooden hides or sheds, watching kingfishers and curlews through the mist. On a good day, they may see a peregrine falcon perched on a rock, looking out for wigeon and dunlin.

 Wardens worry that a dam downstream would disturb the tides that bring in food for the 60,000 migratory and wintering birds each year. The Royal Society for the Protection of Birds also says that the dam will harm the estimated 30,000 salmon and other fish that swim through the broad estuary to reach their spawning grounds in adjacent rivers. The dam could also affect local industry. Around 32km down river from Slimbridge, operators at the deep-water port in the industrial city of Bristol fear the project could reduce water levels and limit the number of ships coming in.

'The biggest ships will not be able to get through,' said Patrick Kearon at the Bristol Ports Company, which administers the port. 'We could look at solutions, like putting in locks to control water levels, but they will still be a deterrent to our customers as they slow down shipping and generate delays.'

 But others say the case against the Cardiff-Weston dam is overstated. Hydroelectric engineer Tom Shaw suggests the dam will in fact take some of the sediment out of the water and make it less muddy for birds and ships.
 
Unwilling to commit to either side, the government has agreed to fund further research into tidal reefs. In the meantime, it will continue consultations on other smaller projects, which include proposals for smaller dams at narrower points of the estuary and for tidal lagoons, or smaller-scale turbines that operate with less water. With an election likely next year, a decision on such a massive project is unlikely soon.

While the debate rages on, the moorhens, oyster catchers and white-fronted geese continue to waddle and swoop in the rich Slimbridge mud - while they still can.