It had been transformed from a sewer into a pristine stream teeming with fish, but perhaps the most astonishing thing about the rejuvenation of the River Wandle was that it had happened in South London.

Suddenly the suburbs of Croydon, the industrial estates of Merton and the scruffier parts of Wandsworth had a waterway that resembled the most famous and exclusive chalk bed streams in Britain.

Angling clubs sprang up, a fishing tackle shop opened in Mitcham, carp, chub, roach and barbel idled beneath bridges as traffic rumbled in and out of London. It was a testament to what could be achieved with an inner-city river.

That was until the afternoon of September 17 when, in little more than 20 minutes, years of work was undone. Employees cleaning filters at Thames Water’s sewage works in Beddington accidentally allowed sodium hypochlorite, a bleach, to be released into the river. In the hours that followed, thousands of fish were killed, the luxuriant weed beds were destroyed and the gravels were bleached as clean of life as a bathroom floor.

This week the company, which accepted responsibility from the start, agreed a £500,000 payment to fund the restoration of the Wandle — the biggest pollution settlement in angling history. The Environment Agency may still prosecute Thames Water, but in the meantime the payment has been hailed by the Anglers’ Conservation Association as a welcome change after years of obdurate behaviour from many of Britain’s water utilities towards river pollution.

Theo Pike, 36, from the Wandle Trust, stands at the junction of the river where it is joined by a torrent of treated water from the sewage works. “Frederick Halford would have fished along here some time after 1868,” he says. “The waters were so clear that the trout could see you coming, so he devised a new method.” He invented dry fly angling to catch the wary fish and exported the code worldwide. He left the Wandle in 1868. “It was becoming too polluted,” Mr Pike says.

That was only the beginning. By the 1970s it was effectively a sewer, carrying the wastewater from the sewage works to the Thames.

But the tightening of environmental legislation in the following decades, and the efforts of bodies such as the Wandle Trust, helped to make the stream fishable once more. “We held regular cleaning days,” Mr Pike says. “We pulled out sofas, motorbikes, ironing boards, washing machines. In Earlsfield there was an entire restaurant kitchen and two staircases still in one piece.” Children from local schools reared trout in classrooms to help the growing stock.

Mr Pike points to one of the concrete banks. “Behind a clump of weed there was a barbel that was 2ft long,” he said.

He was fishing in Wales on the afternoon of September 17. He found dozens of messages on his mobile phone. Andy Quayle, a local builder, was fishing from a bridge on the Wandle when he smelt the bleach beneath him, making his eyes sting.

Ziggmunde Sinnette, a retired army officer and manager of Morden Hall Angling Club, was giving a tutorial to schoolchildren farther downstream when he had a call from the Environment Agency.

“That river is usually gin clear,” he said. “It was running opaque and there was a smell of chlorine. He marshalled a team and rushed downriver where, by closing a number of sluice gates, he was able to divert some of the pollutant.

“Then we got into the river and started pulling out floundering and dead fish,” he said. That evening the river turned silver; stretches were entirely choked with the bellies of dead fish. Dead water bugs piled up like banks of silt.

Despite the monumental damage, Mr Pike is now confident that the river can be restored. Thames Water has put in place security measures, including sensors developed in New York to protect from bioterrorism.

Mark Lloyd, executive director of the Anglers’ Conservation Association, said that with the funding received, the river could “literally . . . come back from Year Zero.”

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