How clean is the Thames?
Concern about pollution dates back to Medieval times, when an Act of Parliament in 1383 ordered latrine owners to pay two shillings a year towards the cost of cleaning up their discharges.
Later Acts forbade the casting of rubbish and pollution into the Thames, but to little effect. In 1858, the smell from the river became so bad it disrupted Parliament in what became known as the "year of the great stink".
The event prompted the government to commission Joseph Bazalgette to build sewers on either side of the Thames. This greatly improved conditions for a while, but by the 1950s it was little more than an open sewer once more.
Only in 1974, when works to improve the sewage system were completed, did wildlife begin returning to its former haunts. Today, the Thames is considered the cleanest city river in the world and supports 118 species of fish, 350 species of invertebrates and 300 species of plants. Dolphin, seal and seahorse are among the less common species spotted in recent months.
Schoolchildren are helping to monitor the river's return to health through the River of Life Survey. Co-ordinated by the Zoological Society of London, regular wildlife counts are undertaken by children on field courses run by the Thames Explorer Trust. On the banks of the river in Chiswick, 20 pupils from Heathrow Junior School recently unearthed freshwater shrimps, mussels, snails and leeches.
Earlier this year, a freak storm dropped 40mm of rain on parts of London in three hours. With road drains feeding directly into the Victorian sewers, Thames Water was forced to release 600,000 tonnes of untreated sewage into the Thames or risk it flooding on to the streets. The Thames Bubbler and Thames Vitality, boats designed to pump oxygen back into the river in emergencies, were unable to operate and as a result more than 100,000 fish, including carp, chub, sea lamprey, bream, dace and tench, suffocated as bacteria in the sewage voraciously consumed the river's oxygen supply.
Plans to build a pound;1.5 billion tunnel under the Thames to help prevent future spills are now being drawn up.