The story normally told of the Thames is of dramatic improvement as a result of cleaning up pollution. In the tidal Thames – from the tidal limit in west London down to Gravesend and beyond – there is much truth to this. But for the rest of the catchment above London the story is more one of efforts to stop the long-term decline.
There’s a long history of cleaning up pollution in the Thames in London, and of recording the effects of that clean up. For example, at the end of the 19th century, Charles Cornish in ‘A naturalist on the Thames’ noted that:
“Since the middle of June, 1890, large shoals of dace, bleak, roach, and small fry have appeared in all the reaches, from Putney upwards. A few years ago hardly any fish were to be seen below Kew during the summer, and these were sickly and diseased. Last year they were in fine condition, and dace eagerly took the fly even on the lower reaches. Every flood-tide hundreds of “rises” of dace, bleak, and roach were seen as the tide began to flow, or rather as the sea-water below pushed the land-water before it up the river”
But after the World War II things again got worse and by the 1950s long stretches of the Thames in London were severely impacted by sewage pollution with no fish populations over a 69 km stretch from Kew to Gravesend. Since then upgrades to sewage treatment works have substantially improved the situation and the river is now classified as being at ‘Moderate’ ecological status downstream of Richmond on the five point Water Framework Directive scale of High, Good, Moderate, Poor, Bad. But cleaning up the London Thames is a bit like painting the Forth Bridge as population and storm water runoff has increased. Now to keep on top of sewage pollution the £4 billion ‘super-sewer’, the Thames Tideway Tunnel, is being constructed carry wet weather sewage overflows to Beckton sewage works rather than straight into the river. This will reduce sewage discharges into the river to less than a tenth of their current level from 40 million down to about 2 million cubic metres per year.
Pollution in the rest of the Thames catchment
Away from central London, we are still in the phase of managing and trying to stop the decline in freshwaters. Despite improvements at sewage works and efforts to control farm and urban pollution, rivers, ponds, lakes and streams are still very widely polluted, and sensitive plants and animals at best hanging on, at worst declining and disappearing. Important freshwater wetlands – like those of the wet heaths of Surrey and Hampshire, and floodplains of the Upper Thames in Oxfordshire – need constant vigilance if we’re to keep the clean water they still support, clean. Even with the fairly modest target of the Water Framework Directive’s ‘Good’ status, just 8% of the waterbodies, mainly rivers and streams, that are monitored currently achieve this in the Thames catchment. For the vast majority of waters, which are not monitored at all, the picture is probably even worse than this. Nationally, we know that the number of high quality ponds has been declining and, although the Thames catchment still has many wildlife rich waterbodies, the situation is probably no different here. In thousands of kilometres of smaller streams and headwaters we do not know what their condition is at all – but we know it is not good.
 Cornish CJ (1902). The naturalist on the Thames. Seeley, London.
 Attrill M (2006). How clean is the Thames? Lecture to the Gresham Society.