Any reveller who happens to fall in the river at this weekend's Thames festival will not need to get an immediate stomach pump as in Victorian times, but none of the crowds celebrating the revival of community and ecological life on Britain's greatest waterway should assume it's clean. Nor should they assume that just because people have seen dolphins in the river that the Thames flows smoothly. London's antiquated sewage system, its inhabitants' extraordinary litter habit and the cult of the £500,000 riverside penthouse are all posing new perils.
First the bad news. The biggest ecological threat to the Thames comes from London's decaying Victorian sewage system, completed in 1865 by civil engineer Joseph Bazalgette. After the “great stink” of 1858, when the 
Houses of Parliament became so smelly that the members demanded action, Bazalgette built 83 miles of drains that stopped raw sewage from running into the Thames and took it directly to the east of London. Less ecologically aware, Bazalgette was responsible also for the great Thames embankments which effectively canalised the river betwen high walls.
Following heavy storms, Bazalgette's drains these days regularly overflow and the river becomes flooded with land water contaminated by sewage. The result is an oxygen deficit known as a “sag”. One occurred on July 8: in the morning a dolphin was spotted in the river - suggesting that the water was remarkably clean. Within hours, storm waters had flooded over and the lethal sag stretched roughly 15km from Putney to Wapping.
It killed by suffocation an estimated 430,000 flounder fry and hundreds of thousands of young smelt between Chiswick and the centre of London. In Chelsea Harbour 100 kilos of larger roach, dace and perch died. The dolphin died also, but its death is thought to have been unconnected to the pollution.
Dealing with a Thames sag is tricky. The July 8 event was quickly brought under control, thanks largely to two boats, The Thames Bubbler and Vitality, which patrol the river and can pump up to 30 tons of oxygen into the water in a day. There are also two riverside plants which can inject hydrogen peroxide into the water.
“When London's drainage system was built there were no fish in the river so it was not perceived as a problem. When there is sewage in the river, bacteria will use it as food and take up oxygen in the process,” says Environment Agency scientist Richard Oatley.
The river is most vulnerable in summer when flows are slower, there is less fresh water coming from upstream, less retention of oxygen and higher temperatures. In winter it might take three weeks for the river to flush out a sag; in summer it could take up to four months.
Sewage overflow has become such a large problem that the Environment Agency, Thames Water, the Office of Water Services (Ofwat) and the Department of the Environment (Defra) are now launching a five-year study into a £1bn overhaul of the system. But the engineering work required could take decades.
Litter is also a huge problem. Each year the Port of London Authority (PLA) fishes up to 1,000 tonnes of it from the river at a cost of almost £500,000. “The Thames is a sump for London's litter,” says Mark Lloyd of Thames 21, which co-ordinates the PLA debris clearance.
And the river is not what it once was. “If you stand on Tower Bridge
at low tide,” says Steve Colclough, of the Environment Agency, “the river is a quarter of the width it was in Roman times.”
The latest threats from housing developments which, despite the Environment Agency's and many campaigning groups' efforts, have eroded the foreshore, destroying vegetation, depriving marine life of food and effectively squeezing the banks together. A narrower river causes a faster tidal flow. Small fish that hitch a lift on the Thames, clinging to weeds along the foreshore as they go, are swept along at ever increasing speed. The young migrating fish find it difficult to attach themselves to plants and frequently die. And as the banks of the Thames contract, the rising tide exacerbates the risk of floods. According to the Environment Agency, by 2030 many of the existing wall and embankment defences will be at the end of their useful life.
Now the GLA is drafting its own document, the Blue Ribbon strategy, for the Thames. Lady Dido Berkeley, who sits on the committee and runs the pressure group Thamesbank - one of 400 organisations involved in maintaining life in, on and by the river - is fighting for more legislation to protect the Thames from what she calls the “horrendous, accumulative damage of building encroachment” and the restrictions on public access to river facilities.
“The river is the reason we all are here. The Royal Parks are highly protected. Why hasn't the Thames got the same protection? Many people think the Thames should become a national park,” she says.
The Environment Agency recognises the problem but although it is a statutory consultee in planning applications along the Thames it lacks legal teeth. “Our encroachment policy is an educational programme rather than one that can be enforced,” says Steve Colclough.
Nevertheless, old father Thames has known far worse times. Thanks to previous efforts, pollution is now a tenth of what it was in the 1950s when the river was biologically dead. Over one hundred species of fish have returned. And that murky brown colour? In fact, it is one thing that cannot be blamed on litter and pollution. The natural silt and clay sediment of its bed mean the Thames is unlikely ever to be mistaken for the allegedly blue Danube.
The good news is that the river is now one of the most important fish nurseries in Britain and the southern North Sea. The brackish waters as far up as Chiswick are the main nursery in the lower North Sea region for sea bass. Grey mullet reproduce in Chelsea Creek in the warm water outfall of Lots Road power station, where up to a dozen herons sometimes feed. The rare sea lamprey, a popular food for Londoners during the 18th century, is reported to be breeding again near Barnes. Flounder and smelt spawn at Wandsworth. Meanwhile, farther downriver, Dover sole and sprats have been caught at Greenwich and Essex fishermen regularly land anchovies. Salmon, which spawn in the river Kennet, have been observed in the Thames since 1974, after an absence of 150 years. Salmon ladders have been installed upstream to help their return but although juveniles and adult fish are seen, there are not as many as hoped for. No dead salmon were found after the July sag. However, the smelt, a related species with a similar life cycle, were doing well until that incident. In a recent survey a sea trout was found at Putney.
The fish which floated to the surface were just some of 118 species the Environment Agency has recorded in the tidal section between Teddington and Tilbury, about 50 of which are residents. More than 350 invertebrate species including a variety of crustaceans live in the river, which also supports a host of bird life and occasional marine mammals.
