The Big Question: Why are our rivers so dirty, and what can be done to make them cleaner?

Why are we asking this now?

Three-quarters of the rivers in England and Wales have failed tough new ecological tests introduced by the European Union, according to a report by the Environment Agency. The pollution watchdog insists the news isn't all bad, with water quality having improved for the 19th year in a row and wildlife returning to many rivers where it has long been absent.

Nevertheless, only five of the 6,114 rivers in England and Wales are categorised as pristine. Conservationists and campaigners argue this proves that despite heavy investment in sustaining Britain's ecology, the Government isn't getting enough bang for its buck.

How bad are they?

The number of rivers classified as poor in quality is striking. One hundred and seventeen of the rivers in England and Wales – constituting 2 per cent of the total – were found to be the dirtiest and most inhospitable to wildlife, and are ranked on a par with the dirtiest rivers in eastern Europe. A further 742 are in "poor condition" and 3,654, or 60 per cent, are in "moderate condition".

But the picture is more complicated than that, because according to the Environment Agency there have been improvements in the quality of water in many rivers. Indeed, the watchdog says that seven out of 10 rivers in England, and nine out of 10 rivers in Wales, achieved either "good" or "very good" status in terms of chemical and biological quality in 2008.

This incongruity is explained by the fact that the EU's European Water Framework Directive (EWFD), which became law in the UK in 2003 and under which the picture for rivers in England and Wales is more bleak, sets higher standards. It does this by using a wider and more sophisticated range of more than 30 different measures of river quality.

Lakes, meanwhile, are faring little better. Only one out of 762 lakes in England and Wales is considered of high status, with seven considered "bad". Nearly 70 per cent of lakes are in line to miss the targets that have been set for them.

Which rivers are doing most well?

Four of the five doing most well are in Northumberland: Ridlees Burn, Barrow Burn catchment, River Till, and Linhope Burn. All tumble off the Cheviot hills, running to the rivers Tweed and Coquet. The only other pristine river is the Clettwr in Conwy, north Wales, which comes off the river Conwy.

Why are those rivers doing better than others?

There is no escaping the fact that those rivers that are cleanest – at least according to the stringent criteria laid down by the EWFD – are those that are furthest from large conurbations or industrial production. Those in Northumberland are young, fast-flowing, and full of waterfalls and pools. Cattle breeding nearby can reduce their cleanliness, but all remain conducive to a huge variety of wildlife, including fish such as salmon and wild trout. The Clettwr in Conwy is celebrated for the ancient oak trees that line its embankment.

Which rivers are doing badly, and why?

The dirtiest rivers are close to large conurbations. But there are other specific, local problems that exacerbate the deterioration in the river's health. Run-off from sewage works and pipes can be highly damaging, as can growing demand for drinking water and the heavy pollutants from farming practices. Parts of the Medway, Thames and Lee suffer from this affliction. The Stour estuary in Kent is part of a nature reserve run by the Royal Society for the Protection of Birds (RSPB), and has profited from private investment in recent years, together with more environmentally friendly farming techniques. But it too has been found to be among the dirtiest in England.

Isn't this embarrassing for the Government?

It is rather, because according to EU regulations, the Government is legally required to ensure 95 per cent of all British rivers are in "good" ecological condition by 2015. Just 26 per cent are currently in that category. According to the Environment Agency, with the current levels of steady, but unspectacular, improvement, only a further 5 per cent will meet this target – a huge shortfall. That could resurrect the court cases and fines of the early 1980s, when Britain was labelled "the dirty man of Europe".

What does this mean for Britain's wildlife?

Mark Avery, director of conservation at the RSPB, chides "[the] devastating impact on our waterways; pollution from agriculture, over-abstraction and poor town planning are all threatening an extremely important habitat for wildlife. Otters, water voles, kingfishers, and more than 30 species of fish rely on our rivers".

It's impossible to get a clear, overall picture of how the wildlife on British rivers is faring. It varies hugely. Whereas 50 years ago, no salmon were seen on the River Tyne, 10,000 have been seen this year already. Otters have been seen near Manchester and the lower Thames for the first time in 40 years. Salmon have returned to the River Mersey, once Europe's most polluted river. And a new fish pass on the River Hamble in Hampshire enables sea trout, lamprey and eels to flourish there for the first time in centuries.

Anglers are worried, however. "The new system of scoring rivers means that when you look at the ecology – the number of fish, plants, invertebrates and the tiniest bugs – it is not doing well", says Paul Knight, executive director of the Salmon and Trout Association. "The main problems are linked to over-abstraction and lack of river flow, and diffuse pollution from phosphorus from laundry detergents and nitrogen from pesticide run-off from farmland".

What else can be done to improve river quality?

Over the next five years, the Environment Agency will be working alongside farmers, water companies and conservationists to clean over 9,000 miles of river in England and Wales.

Plans to improve each river basin are being submitted to Hilary Benn, the Environment Secretary. And ministers have already banned the use of phosphorus in washing powders and industrial cleaning products, though the ban is yet to come into force. It could eventually cost £9bn to get 95 per cent of UK rivers to "good" status by 2015. If this isn't achieved, the EU allows interim targets to be set for 2015 and 2021.

Is it too late to restore the health of Britain's rivers?

Yes...

* Very few of them measure up to EU standards for rivers whose ecology is in rude health

* The demands of Britain's growing population mean that although there is still plenty of rural land, it is shrinking as conurbations grow

* Many of the species that thrived on our rivers are damaged irreparably

No...

* The pollution watchdog has announced bold plans to clean up over 9,000 miles of river

* Banning phosphorus from some industrial and domestic cleaning products will keep it out of rivers

* Better farming techniques and private investment could reduce the amount of pollution entering rivers