On October 6, 2003, I wrote that the little River Wandle, a tributary of the Thames and for 100 years a rainbow-coloured sewer running through South London, had been given new life.

Thanks to changes in local industry, improved legislation and years of effort by angling working parties, the Wandle was fishable again. Its waters were running swift and clear and pure. The prams and rusting washing machines and old bikes were coming out of it. Coarse fish of many species had grown large in it. Huge carp idled under Wandsworth Bridge. Trout had been stocked at Croydon. Since then, even a fly-fishing club has been formed.

Two weeks ago, anglers fishing the Wandle saw the river change colour and smelt fumes. Within minutes, thousands of fish were tumbling dead along the bottom or drifting at the surface, white bellies up. Countless millions of aquatic larvae, nymphs and shrimps were swirling dead in the backwaters like so much silt.

For miles, much of this historic chalk stream - the very stream from which Frederic Halford, the father of dry flyfishing as a sport, caught his first trout in 1868 – was all but dead. Theo Pike, the director of the Wandle Trust, a conservation body founded by anglers, says that it will take years to recover.

It is a catastrophe of the most visible kind and a public-relations disaster for Thames Water, which allowed it to happen. The Wandle was a flagship project for everyone associated with it, not least for the Environment Agency, which regulates rivers and lakes.

Conservationists not only in Britain but overseas were following it as an example of what can be achieved in inner-city streams, given investment and community support. Local children were rearing fish for the trout-stocking programme. The Wild Trout Trust, a charity of which I am president, was in part inspired by developments there when it drew up its “Trout in the Town” project, a programme for improving neglected urban rivers nationwide.

Now this. Several things need to be said. The first is that the disaster was not malicious but a result of human error; chemicals being used to clean screens at Thames Water’s sewage treatment works in Beddington, were inadvertently discharged into the river instead of being neutralised elsewhere in the system.

The second thing to say is that human error should not have been possible. On any river, but on this river above all, fail-safe systems should have been in place that would have overridden or alleviated carelessness or chance.

The fact that Thames Water admitted guilt promptly is helpful in reducing delays and costs, not least in the legal proceedings that will follow. But the Wandle is no one-off. Last year, not for the first time, Thames Water topped the league of the worst polluters among the water companies in England and Wales. It has a trail of such disasters behind it. Meetings are being planned between the highest levels of Thames Water management and representatives of the Wandle Trust, of the Anglers’ Conservation Association (ACA) – the sport’s formidable legal and pollution-fighting arm – and of the Environment Agency, which, its regulatory involvement aside, must be biting its corporate knuckles after the effort that it also has put into the river.

Locally, several actions need to result. The first is that those essential fail-safe systems and buffers are installed and properly maintained, regardless of cost. A second is that the river needs to be speedily restocked along such lines as the angling groups and the Environment Agency recommend. A third is that Thames Water’s funding of the Wandle Trust, drastically cut back this year, needs not only to be restored but also enhanced.

The Wandle Trust, a community charity, works to improve the river and its wildlife and the pollution has, Pike says, “come as a terrible blow”. Ensuring that this group’s work is healthily funded is one small way in which Thames Water can show repentance and maybe reap some eventual benefit.

Elsewhere, other actions are needed. One is that lessons learnt from the Wandle are acted upon by the other water companies. A second is that the system of so-called “deemed consents” – a provision that, at times of heavy rain, prevents the Environment Agency from acting against water companies regardless of what they channel into rivers – needs to be ended.

Deemed consents – there are 3,476 of them at present – are effectively licences to pollute. They were seen as a temporary expedient to ease water privatisation, but, 18 years on, they are being exploited. They are, Mark Lloyd, the ACA’s director, says, a practice that allows rivers “to die the death of a thousand cuts” and, in the 21st century, they are an anachronism.

Finally, policing and punishment need to be more effective. Year-on-year cuts imposed on the Environment Agency by government ministers have drastically reduced the agency’s ability to pursue polluters and only the most serious cases are taken to court.

When offenders are brought to court, penalties, too often paltry, need to reflect society’s – and allegedly the Government’s – mounting concern for the environment at large.

It will all cost money and little of it will come about overnight. But come about it must. Rivers are the veins and arteries of our landscape. Their wellbeing is in the hands of those who regulate and exploit them. It is not only fish and fishing we are talking here, but also our children’s environmental heritage.

— Brian Clarke will be speaking on fish, fishing and the environment at the Guildford Book Festival at 7pm on October 17 – in Waterstones’ high street branch – to mark the launch of his new book, On Fishing. His angling column appears here on the first Monday of each month.