Why London's rivers are teeming full of life once again

The rain is coming down in sheets, and there is a lone figure striding against a shimmering river's current. Thigh-deep, he has been playing with his prey for 15 minutes. "It's a tricky customer," he shouts, pressing his tongue against his lower lip. His rod bends against the strain of a Merton-based Moby Dick, a resilient denizen of south London's murky river Wandle. Soon, it is writhing around, hooked on to the end of the angler's line after chomping into a suspiciously supine parachute Adams nymph. A bus driver swings past on an adjacent road. "What are you doing, you prat?"

But Charles Rangeley-Wilson, fishing author and broadcaster, is lost in what he's doing. He flaps his limbs and fibreglass rod and tugs its line towards his body. After several minutes' horseplay, during which the assembled observers retreat from London's grey summer sky to huddle beneath a nearby pub canopy, he lands his hands on the slippery prize. It is the proverbial whopper he had hoped for: a 2lb brown trout. The moment is thrilling, because, with the same obsession that dominates many of the sport's aficionados – and separates them from those who simply don't get it – Rangeley-Wilson has been trying to catch a trout here for a decade. This, he says, is "unicorn stuff".

Rangeley-Wilson's forthcoming book, The Accidental Angler, spun off from a recent BBC series and published later this month, charts how some of London's rivers are in the rudest health they have been in years. It is a little-known fact that two centuries ago London had innumerable rivers teeming with fish. Over time, they became clogged with whatever the steaming growth of the metropolis threw at them. The fish died, and many of the rivers were concreted over. Recently, however, thanks in part to the ecological awareness and active interest of the capital's fishermen, London's waterways are being restocked. "It's quite easy for someone to see a couple of swans on a river and think all is well," Rangeley-Wilson says. "But fishermen come to notice things that others might not see."

A lucid depiction of how the city has changed can be seen on a 1746 map by John Rocque; the map is one of Rangeley-Wilson's favourite pictures. Rocque's skilful inking – hedgerows, lanes, trees and meadows – shows a city suddenly ending halfway up Farringdon Street. Battle Bridge (now King's Cross) is a rural hamlet. The map shows how at one point the Thames was fed by a string of rivers radiating out from the city centre like spokes from a hub. Battle Bridge was in the valley of the river Fleet, which began (and still begins) up on Hampstead Heath. The Tyburn can also been traced: it flowed south from Hampstead under Buckingham House (as the palace was called in those days) to Westminster. The Westbourne flowed through Chelsea. Their names, you might say, can be read off like a roll call of the capital's war dead: like the Fleet, none of them can be seen in central London today.

The Fleet is a sad case study for the city's deceased natural waterways. The river once formed the westernmost boundary of the Roman city, and in its prime it bubbled and sloshed all the way across London town. There was even a harbour at its mouth where it met the Thames. But centuries of people discarding into it what Swift described as "drown'd puppies, stinking sprats, all drench'd in mud" turned it into an open sewer. Rangeley-Wilson says several attempts were made to clean it, but it always reverted back to the stinking norm. After the Great Fire, Sir Christopher Wren even designed an elegant canalisation of the lower river, but the funk became too acrid, and it was paved over.

Now, only the odd clue hints at the river's path from the Thames up to its source. One tell-tale sign is the striking dip in the street between Ludgate Hill and Ludgate Circus on either side of Farringdon Road – the remnants of the old river valley.

As one drives north, the names of the streets are a giveaway: Water Lane, Spring Place, Angler's Lane. In Kentish Town, a trickle can still be heard beneath manhole covers dotting the trendy north London roads. At Hampstead Heath, on a rainy August afternoon, the only exposed part of the river can be seen in the bathing pools, purposely created by bottlenecks placed in the river's course. The ponds skip down the hillside from a tiny spring, which in summer does not flow. Here, on a wet summer's afternoon, is a hardy family, semi-drenched, searching for fish that they are certain dwell here, suspicious-looking men who conceal themselves in hides under the pretext of ensnaring dace, roach, chub and barbel.

Until recently, it has been rare to snare trout within the M25. But a few years ago the city's fishermen started noticing different breeds on their lines. That trout could live in Wandle was once considered an ecological impossibility; now, they seem to be flourishing, though they remain difficult to catch. In 2003, a fly fisherman caught a 2 1/2lb trout next to Wandsworth council's bin lorry depot. It was thought to be the first trout caught in London for 70 years, and the first caught on a fly for more than a century. Trout are sure-fire evidence of a river's health: considered by many to be the king of catches, trout need clear, oxygenated water to survive, something that's been available only recently.

The river's rebirth has been possible because it is the most suitable sort for trout: a chalk stream, fed by pure water from springs in the North Downs. After a 1995 sewage spill nuked thousands of fish, crucial fish food such as shrimp and water snail returned to the river. Since then, the Environment Agency has restocked it with several fish species. Rangeley-Wilson believes that the fish that he caught was a stock fish, because of its age and its tellingly damaged dorsal fin.

Along with the Environment Agency, anglers, too, have helped restore the ecological condition of rivers. Rangeley-Wilson founded the Wild Trout Trust, which promotes restoration and conservation work on degraded rivers. Its policies are followed religiously by the Wandle Trust, which runs monthly clean-ups of the river involving up to 50 people from the community, many of them fishermen, who get together to haul "unnatural" rubbish out of the water.

Because fishing is such a popular sport in this country, the impact of such endeavours can be profound. The Government says that four million people in the UK fish for fun, 750,000 of them specifically targeting elusive salmon and trout. Around 20,000 people are employed in the business of fresh-water fishing, and official statistics claim that the sport generates the quite ridiculous sum of £2.75bn for the British economy. No small fry.

Theo Pike, Wandle Trust trustee, can scarcely contain his envy of Rangeley-Wilson's big Wandle catch. "Catching a trout of any size, by any method, on the Wandle is still a rare event," Pike says. "So it's a major achievement to land such a good one on a fly. And with a camera to capture the moment, too! Needless to say, I'm very jealous."

At the bank of the Wandle, Rangeley-Wilson can scarcely contain his smile, sending texts to everyone he knows who has fished the river. This momentous occasion, and his passion for it, speaks volumes for his love of the sport. It pays comforting testimony for those putting effort into cleaning up London's rivers. His words, touching on a religious appreciation of nature, perhaps best sum up this zealous fervour. He writes of: "God at His best. The breath of a river. This is the edge." A day in his company is more than enough impetus to convert.

Fin city: metropolitan fish

Trout
Trout can only live in the very cleanest of rivers. They are still rare; the ones that do exist are elusive. Flies are often the most effective means of ensnaring them.

Barbel
A bottom-dwelling species. According to folklore, barbel used to be called pigfish, an unflattering reference to the way they forage. Following habitat improvement and targeted restocking programmes, their numbers have flourished.

Chub
Common across Britain, except northern Scotland, west Wales and Cornwall, chub like running water and overhanging trees and bushes. They feed on small fish and frogs, fruit, worms and shellfish. Chub are a free-biting fish and are easy to catch.

Carp
Another bottom-feeder, carp were introduced to Britain by monks in the 12th century. They feed on vegetable matter, worms, insect larvae and shellfish. When first introduced, carp had a reputation for being difficult to catch; now they are popular with anglers and found in the middle Thames between Eynsham and Hurley.

Dace
These slim fish are widespread in England. and favour fast, shallow water where they pursue insects and crustaceans. They are small and are often mistaken for chub, but can be identified by their cave anal fin.

The Accidental Angler, by Charles Rangeley-Wilson (£7.99) is published by Yellow Jersey. To order a copy with free P&P call Independent Books Direct on 0870 079 8897, or visit www.independentbooksdirect.co.uk