London periodically goes through a drought every 10-15 years, thanks to a quirk of the Gulf Stream weather system. It brings warm air from the Caribbean to Western Europe all year, and why we don't get much snow in winter despite being the same latitude as northern Canada. The last dry spell was in the 00s, that lasted about 5 years. In that time the Thames was purported as the cleanest urban waterway in the world (it's still brown due to it being tidal/ estuarine), where it was said that if you dipped a cup, then let the silt sink to the bottom, it would have been safe enough to drink. Seals, salmon, even small whales started returning. The upper reaches one could swim in (though not advised, there are notorious undercurrents).
http://www.beforeiforget.co.uk/images/xanga/whale.jpg
Then the drought ended - and rowers started getting sick. In those 5 years over half a million extra people had moved in - the sewers, built in the 19th Century but future-proofed for the next century, had run out of its timeframe -they could no longer cope whenever a heavy rainfall happened and would discharge. And these rains are surprisingly rare in London, which is the driest part of the country and actually gets less annual precipitation than say places like Madrid, Melbourne, Tel Aviv (half the rainfall of NYC). It's overcast most of the time, hence the reputation but is not actually very wet, even outside drought years (where it technically falls into semi-arid territory).
Every summer the grass dies en masse, with or without drought -the city sits in a chalk valley and the parks become rather unpleasant dustbowls:
However whenever there is heavy rain the sewers now overflow -its estimated 20,000 tons of sewage goes into the water every year. Until the $5 billion Super Sewer gets completed in 2025 that's gonna remain the case. Wildlife has again abandoned the urban waterways.
In short the Thames used to be known as The Great Stink, an open sewer for centuries that in the summer months would force the monarchy and parliament to decamp as the smell was so overpowering. It then gradually got cleaned over a century. But now it's heading downhill again.
https://66.media.tumblr.com/cd3b8942...3l0o1_1280.png
the company Thames Water is periodically fined millions for discharging sewage, but they don't really give a shit