In 1878 the pleasure steamship Princess Alice sunk in a river collision. Most of the 600 or so passengers who died did so because they were overpowered by a noxious cocktail of human and industrial filth before they could reach safety.
"By the 1950s the Thames was in an even worse state," said Martin Attrill, a marine biologist at the University of Plymouth, England. "A 20-kilometer [12-mile] stretch of river was completely devoid of oxygen."
After London's Natural History Museum declared the Thames biologically dead in 1957, work began to try to rehabilitate the river. Government measures improved sewage waste treatment and banned industry from discharging pollutants into the river.
Today more than half of London's sewage sludge is sold in pellet form as fertilizer for agricultural use.
Attrill says water quality has continued to improve since the 1970s. "There's been a clear and very dramatic decrease in levels of heavy metals and pesticides," he added.
And yet the Thames's ecological renaissance remains a well-kept secret, according to a survey commissioned by the Zoological Society of London (ZSL). Published last week, the survey found that 83 percent of Londoners thought dumped shopping carts were the objects most likely to be found in the river. Just 6 percent of respondents knew about the return of Dover sole. Only 7 percent thought seals could be seen.
"It seems that Londoners know more about tropical rain forests than the river on their doorstep," said Alison Shaw, aquatic conservation manager for the ZSL. "People can't believe there's anything in [the Thames], because it looks so brown and dirty. But that's just the nature of estuaries; they carry a large amount of sediments in the water column."
Shaw hopes the ZSL can help the river shed its dirty old reputation through a project that will investigate how wildlife uses the Thames estuary.
For instance, researchers are currently tracking juvenile European eels on their spring migration upriver. The eels travel thousands of miles from the Atlantic Ocean's Sargasso Sea, where they are born. "The Thames estuary is also an internationally important area for migratory birds," Shaw added. "It's their first landing stage when they arrive in Britain. Large flocks feed on intertidal mudflats and grazing marshes."
She says it's important that Londoners are aware of the river's burgeoning biodiversity, not least because pollution threats remain.
During violent storms last summer, London's antiquated drainage system was inundated. Some 600,000 tons of raw sewage was released into the Thames to prevent the waste from flooding people's homes. As a result, many thousands of fish were killed.
The U.K. Government has so far stalled on plans for a 3.8-billion-dollar (U.S.), 22-mile (35-kilometer) tunnel under the riverbed to dispose of storm water and displaced sewage.
Perhaps the sight of dolphins surfacing opposite the Houses of Parliament will help swing the debate.
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