IN 1970, the Tees estuary was virtually dead, so horribly polluted and starved of oxygen that life was impossible in its poisoned waters. Many heavily-industrialised rivers were the same, turned into lifeless waterways by a deadly cocktail of industrial toxins and untreated sewage.

Thirty one years later, the estuary has undergone a remarkable transformation. Salmon and trout head upstream in growing numbers, cormorants skim low over its waters and the seal colony of Seal Sands is growing. Although the problems have not been fully resolved - water quality is nowhere near perfect and industrial chemicals continue to be discharged - the Tees can rightly claim to be the river that came back from the dead.

The patient had become sick because, for many decades, the river had been regarded as a dumping ground by the many companies, a large number from the chemical sector, which established plants in the area. As the level of poisons rose over the years, the tiny invertebrates which supported the food chain perished and the fish died out. Birdlife dwindled and otters were consigned to the history books as the stinking river sustained horrendous damage from which some feared it may never recover.

Naturalists like Jeremy Garside remember it well. Now Chief Executive of Tees Valley Wildlife Trust, he says: "When I started working for the trust in 1989 the estuary was very poor and it stank. I remember when the National Rivers Agency carried out a fish trawl - and came up with three dead fish."

The Environment Agency, which replaced the National Rivers Agency, puts it even more graphically. Its official description of the estuary in 1970 was "virtually dead".

There were those, however, who were convinced that the estuary could revive and groups like the wildlife trust and the Billingham-based Industry Nature Conservation Agency, established 11 years ago to help Teesside industry protect the local environment, started lobbying for a new approach.

The movement was aided by tough new British Government and European Union environmental legislation and the emergence of enlighted industrial managers prepared to embrace the green challenge. Another factor was one which, ironically, brought heartache for the area but gave the environment the break it needed.

Jeremy says: "There were a lot of industrial closures. It was not good economically but it was good for the environment because we lost some of the most polluting industries and the river is now a lot cleaner."

Environment Agency statistics tell their own story. In 1970, industry discharged more than 500 tonnes of oxygen-reducing chemicals into the estuary each day. By last year, it was about 20 tonnes. In 1994, 25 tonnes of ammonia per day went into the estuary. Within three years emissions had been reduced to about five. And Northumbrian Water has invested some £240m on the massive new Bran Sands complex near Teesport and the Seal Sands chemical complex, which purifies sewage and industrial waste before it is discharged into the estuary.

The effect has been a drastic improvement in water quality reflected in the return of the wildlife. The increase in fish numbers has been spectacular; in 1995 12,500 salmon and sea trout migrated along the estuary, four years later it was 26,000. Salmon were even spotted leaping the Tees Barrage in 1998. And sea lamprey have returned, spotted recently at the barrage, having last been recorded in the estuary in 1913.

The birds are back, too: wading numbers are increasing, cormorants are thriving and sand martins are nesting on the estuary banks once more. But, perhaps the most remarkable recovery has been the seals. They were driven away by industrial development and human disturbance towards the end of the 19th Century but returned about 100 years later - the first example in Europe of seals re-establishing themselves in an industrial area. Since their return in the 1960s, numbers have grown at Seal Sands, and now there are more than 80 harbour and grey seals compared with just 14 in 1970.

Even otters are making a tentative return to the estuary and further upstream are becoming an increasingly common sight, attracted by the return of the fish.

Amid all the excitement, it would be easy to conclude that the renaissance is complete but there are plenty of reminders that the Tees is a river which still needs regular check-ups.

Environment Agency figures for a year and a half ago did indeed show an improvement. In 1970, more than half the estuary was marked red for "bad" quality on the map. Today, there is no red but plenty of yellow, denoting that it remains "poor". However, about half the map now shows blue for "good". There was none in 1970.

Naturalists would ideally like to see no industrial waste discharged into rivers but are realistic enough to realise that a balance needs to be struck between industry and job creation and the necessity to control the level of pollutants - a balance that was missing for much of the 20th Century. Jeremy says: "The job is not done. Some time ago a diver went down at the barrage and came up saying he had found his first mussel. I thought that was good but when I thought about it I realised that he should have found hundreds down there and the big task should have been cleaning them off because there were so many.

"A river can cope with pollution as long as it has the oxygen and the organisms which break down pollution.

"It is a case of setting the right pace of change and there are also socio-economic factors to be considered if the river is the be sustainable. However, it has been cleaned up and the wildlife is on the increase."

The barrage remains an uncertain quantity. When it was planned and opened in the early 1990s to regulate flow from upstream, there were fears that pollutants would be filtered out to create a highly toxic lower end of the estuary, something known locally as the Middlesbrough Soup.

That never materialised, but it has certainly resulted in sections of the river upstream becoming totally freshwater, whereas previously, the tide would push the salt up as far as Preston Hall near Stockton. It has also altered tide patterns which means reedbeds are growing along some of the riverbanks.

The long-term effects are yet to be become clear. Indeed, they may never become clear. According to Jeremy Garside: "There is so much happening on the river and other factors to consider, such as climate change. We will probably never know exactly what effect the barrage has had."

But whatever happens, it has got to be better than the dark days back in 1970.

*In the final part of the series, John Dean examines a picturesque dales river, which most people wrongly assume is free of problems.