The River Wandle flows through London from Croydon to Wandsworth, though 200 years ago it was among the finest trout streams in England. Izaac Walton, Horatio Nelson and Frederic Halford all fished it before urban sprawl engulfed the river and it became an open sewer.

Ten years ago I walked the river looking for a trout. Back then it seemed a Quixotic quest, but I’d heard rumours they were there – an amazing thought because the last properly recorded one had been caught in 1936. The idea of trout finding their way back into a once-dead urban river held such promise of the power of nature to regenerate. I went back again and again. I saw the river improving, but until 22 August last year I hadn’t found what I was looking for.

That day I was with a photographer on the river on Merton High Street. We walked upstream to Merton Abbey Mills and stood by an A road. I noticed a good fish under the bridge. It was the wrong shape for a chub, the wrong colour, too, and while I half-wondered I never really dared believe. Not until I hooked and landed that fish did my decade of hoping come to a conclusion. I had seen and touched my SW19 trout, my reason to believe that the rivers of England have been through the worst, that they are getting better. And if that can happen in London, it can happen anywhere.

Four weeks later, on 17 September, anglers were the first to notice hundreds of dead fish rolling down the Wandle. They called the police, who notified the Environment Agency (EA), which had people on the scene within 30 minutes. Text messages and emails spread the news. By late afternoon a huge number of people – fishermen, locals, friends of the river – had heard that a part of the Wandle had been destroyed. It seemed that all the hours, the years of back-breaking work spent regenerating the river had been brought to nothing.

Andy Perks, a volunteer for the Wandle Trust, went to the river as soon as he heard. “In Ravensbury Park the EA was pulling fish out. Everywhere you could get to on the bank there were dead fish in bin liners or piles of them on the bank.” It transpired that about five kilometres of river had been affected. Theo Pike, a director of the Wandle Trust, described the channel across Mill Green: “It was squeaky clean, bleached like your bath.” No weed was left, nor any fish. The EA recovered one-and-a-half tons of dead chub, barbel, dace and roach – an estimated 2,000 in number. An underestimate in all likelihood, according to Tom Cousins, the EA fisheries specialist who had the job of weighing and listing them. Even for Tom, who has seen fish kills in his time, it was a deeply upsetting task: “We know these fish. We’d stocked them as juveniles and seen them year after year in our surveys.”

I asked whether there were any trout. “A few small ones,” confirmed Tanya Houston, the area fisheries officer. These are likely to have been fish from the Wandle Trust’s “Trout in the Classroom” project, reared from eggs by local school children and released by them into the river.

The pollutant became more dilute as it flowed downstream and beyond five kilometres it seems that the Wandle managed to survive. Even so, the scale of the pollution is a huge setback for what had been an astonishing
success story. Mark Lloyd of the Anglers Conservation Association (ACA), which became involved immediately, working on the basis that it would be prosecuting the culprits, said as much: “It has reversed years of work by the local community, the Environment Agency and Thames Water themselves.”

On 19 September Thames Water (TW) announced that it had been responsible for the disaster. It had been cleaning part of the Beddington sewage-screening mechanism – an operation aimed at improving water quality – when the cleaning agent sodium hydrochloride was flushed into the river. Richard Aylard, TW’s external affairs and sustainability director, explained that two years previously 18 of TW’s sewage works had been failing their consents. Cleaning Beddington, which hadn’t failed but was in danger of doing so, was part of its amelioration programme.

But when I asked Cousins what had driven the recovery of the Wandle – notwithstanding the efforts of all the Wandle Trust volunteers – I discovered that it had been gradually improving water quality from the same sewage works. This twist of events asks a big question about the future of urban rivers. While thousands of people would delight in seeing a vibrant, wild river in the heart of London, and hundreds work tirelessly to realise it, can that ideal be reached, and how safe would it be if all can be undone in an instant?

There is a feeling among the EA, ACA and the Wandle Trust that this disastrous turn of events can be used to the river’s advantage, that it will provide a chance for everyone to focus on what the river needs. As Pike says, “This time we’ve really got to grasp the nettle and solve the Wandle’s problems once and for all, while it’s still up there in the news.”

Thames Water has pledged to support the work of the Wandle Trust with a considerable (unspecified) sum of money: enough to fund a development officer and ambitious plans for the river. Aylard’s intentions are twofold: “Firstly there’s the clean-up and compensation and getting the Wandle back to where it was and the other part is to invest in continuing improvements,” to create “a flagship project of urban regeneration”. This is an exciting idea, but there will be trade-off between the ideals of this ambition and the reality of funding. I had to press him for the magic number. He was reluctant to pin it down but admitted TW was thinking of about £500,0000, with the second half of that figure to be used to unlock additional funding from other sources.

But the Wandle is not an overgrazed rural trout stream whose problems can be solved with channel narrowing and gravel riffles. It is a river whose catchment has been paved over, that flows for the most part between harsh concrete banks, that runs through culverts and is divided by dozens of weirs and impoundments. I asked Cousins what sort of progress could be made with this kind of money. He was reluctant to limit thinking at such an early stage. “I’d much rather we went at this with a blue-sky approach, to look at what the river really needs, and then work back to a number.”

So what does the river need? In terms of habitat, it is those harsh banks and culverts that need to be modified. Houston explained that juvenile fish have few places to hide when the river is flowing hard. Pike pointed at the weirs: “relics from the industrial days. We need to bypass those so the river is no longer carved up into stair-steps of habitat.” But when pressed as to what was top of his wish list Pike was clear: “It’s still water quality. Sort that out and the river will heal itself. I’d like to secure the Wandle’s future against anything that could go wrong. We need reed-beds on the end of the sewage works, but also on the end of every other major input into the river.”
A reed-bed would act as a buffer below the sewage works. Aylard said that TW has agreed to look into the idea, but he seemed to doubt that it would conclude it was practical. He has never heard of a reed-bed serving a works the size of Beddington, which creates 90 per cent of the Wandle’s flow, processing the effluent of half-a-million people. But there must be some way of safeguarding against disaster.

“We’re fitting state-of-the-art monitoring technology at the inflows and outflows. We are looking at physical modifications to the plant to allow the flow to be diverted from the river, into either storm tanks or new balancing tanks. This would be an alternative to a reed-bed, or an addition to one.”

Sounds promising. But freak accidents notwithstanding – and Beddington has suffered only one other pollution like this one, back in 1995 and not the fault of TW – what else is it that consistently threatens water quality when for 99 per cent of the time the water flowing down the Wandle is, according to Tom Cousins, “easily good enough to support a good mixed coarse fishery”?

The answer is heavy rain – particularly in summer: oil, petrol, salt and other nasties accrete on roadways during dry spells and can wash off through storm drains. As perniciously but more deliberately, water companies invoke “deemed consents” to discharge untreated sewage when flows overwhelm their treatment works. “The first flush goes into storm tanks,” says Aylard. “But when they’re full we’ve got nowhere else to put it. It either backs up and goes into the streets and people’s homes or it goes into the river.” At any time this is unpleasant – between 12 and 15 September 2006 three million tons of untreated sewage was released into the Thames following heavy rain – but when a river is low and warm it can be a disaster: in August 2004 a big fish-kill in the Thames was the result of a downpour overwhelming a sewage works.

But building storage for a rainy day is expensive and the pension companies that own TW will look to profit before they look to trout in the Wandle. Aylard explained that TW has little room for manoeuvre anyway. “The regulator sets price limits between what we need to run the system and the profit we need to deliver to shareholders.”
In which case a healthy future for rivers like the Wandle must come through compulsion from the environmental regulators. Aylard says that the EA places stringent regulations on water treatment facilities. But as long as deemed consents remain there is room for the EA to get more stringent still. Compulsion comes down to political will. The answer is metering and a tariff taken off the price of water to compensate the environment. Right now the Wandle is undervalued.

With the Health Protection Agency warnings that: “Contact with tideway waters during and after stormy wet weather conditions represents a significant health risk to all users of the tideway,” and the Olympics fast approaching, the political will has been found to allow TW to create a £2 billion tideway storage tunnel. The government, aware of potentially huge fines for breach of the EU Urban Waste Water Treatment Directive, concluded the tunnel should be built in spite of attempts by Ofwat to find a cheaper solution.

But a significant storage facility for the Wandle? That pig will fly only if it becomes embarrassing for the government that our capital’s most iconic trout river has more turds in it than fish after a heavy summer shower.

Meanwhile the Wandle Trust will carry on pulling shopping trolleys out of the river. Erica Evans, who has been doing this work for years, admitted to a moment of despair at the news of the pollution, replaced quickly by a greater resolve: “It made me more determined actually. For me the more they chuck it in, the more I’ll take it out. While it was a big slump I think we are already on the upward curve.”

Find out more at the Wandle Trust website