From Croatia to Bhutan, Charles Rangeley-Wilson, star of tonight's new BBC2 series, 'The Accidental Angler', will travel far to fish a spring-fed river bursting with trout

The best fishing book ever written opens with the modest explanation that this is "just a book about some fishing rods and the places they take you to". Negley Farson was a foreign correspondent for the Chicago Daily News. His bookGoing Fishing - Travel and Adventure with a Fishing Rod is as much a travel book as a book about fishing. His rods took him deeper into the places he was posted to than he otherwise would have gone, giving him a reason to describe his journeys beyond the news copy he had to file. And the book makes better travel writing for all that.

That provided the inspiration for me to try writing about fishing when I gave up teaching 10 years ago. I was lucky enough to get commissions that took me, like Farson, to some far-flung corners. Those 10 years have taken me to every continent - except the polar ones - on the trail of all sorts of weird and wonderful fish, in awe-inspiring landscapes. But from among the swamps, tundra, jungles and highlands that I've visited, the places that excite me, that stick in my mind most profoundly, are always the spring-fed rivers and where they come out of the ground.

A path in Dorset runs along the foot of a chalk cliff, sandwiched between a wood and the river Stour. In winter, the Stour runs heavy and brown, sliding past like lava flow. But after heavy rain dozens of springs burst at the foot of this slope and run across the path to the river. In one place the springs break underwater in the river itself, clearing a hole in the opaque water like a view through to blue sky on a cloudy day. Further west, in another valley, a stand of trees grows around a hollow where springs break through their roots and collect in a pool. The pool spills over a derelict sluice and forms a stream that runs across a farm track. At dusk in midwinter the stark trees are like black bars against the glowing western skyline. The sounds of rooks and tumbling water highlight the isolation and quietness of the place. I've spent hours in both of these spots and others like them, captivated by the sight of clear water bursting from the ground. The fascination comes from somewhere deep inside me. The Romantic poets, Coleridge especially, also had a thing for spring-fed rivers. He wrote:

Unperishing youth! Thou leapest from forth

The cell of thy hidden nativity;

Never mortal saw

The cradle of the strong one;

Never mortal heard

The gathering of his voices;

The deep-murmured charm of the son of the rock,

That is lisp'd evermore at his slumberless fountain.

There's immortality in there, life force, fecundity, sexual imagery. Everything the Romantics got excited about - a freshwater spring is as bewitching as a view out to sea, or as a panorama of sky. But it is also more life-affirming than either. In a freshwater spring the romance meets the utilitarian; the history of London's houses can be traced in relation to where the springs were.

For me I can't say which came first but my fascination also has a more prosaic root. It starts with a fish. Spring-fed rivers, wherever they occur, always make for the best trout fishing. Rain falls on permeable ground, seeps through fissures in the rocks, coalescing into trickles and rivulets, even underground streams. This invisible migration of water gets complicated. Weird things happen. In Slovenia, one river has seven names as it emerges and disappears again and again. In Provence, one of my favourite rivers comes out of a natural well so deep that even Jacques Cousteau gave up looking for the end of it. But whenever and wherever springs gather to form a river, the water will be cool and fertile, immune from raging torrents or extremes of drought, the perfect environment for millions of insects and therefore for the fat, fussy trout that eat them.

Nowhere are they fatter or fussier than in the River Gacka in Croatia. A legendary river before 1939, for half a century the Gacka fell off the map, locked inside a Communist regime and then a war-torn region. This inaccessibility made it all the more appealing to me. As soon as I could I went out there to fish it. I stayed at a pizza bar, was ferried around by a football-crazy lad called Milan in his dad's pizza delivery van and at the source of the river discovered a freshwater spring to make Coleridge's opiate visions humdrum.

With hefty Croatian disco funk rattling the panels of Milan's Renault we scrambled round hairpin bends from a bluff overlooking the valley to the base of a steep limestone escarpment. Milan dropped me by the pond at the end of the road. "This is where the river starts," he explained as he left. I watched the pond for a few minutes and noticed that it shimmered with the pressure of up-welling water. A leather-skinned farmer in overalls paddled a punt across it scything out weed. Big trout darted out of his way. At the far edge water spilled over a sill through a line of four derelict flour mills, cascading over rocks and the beams and boards of broken buildings. The farmer came over. He had no English, so he gestured at the sky, at the clouds, put his thumb in the air, and spread his arms wide. By which I understood, the weather is perfect today, you will catch a big fish.

The river flowed from the mill through a meadow of wild flowers bordered by an orchard, and a farmhouse. A heavy-set lady in a black scarf leant on her fence, and gestured me across her garden to the river. Under the apple trees cutting hay with a scythe, her husband stopped to wave. Music was playing on a radio. Coloured rubber boots were lined neatly under the stairs, and a black cat watched me from the shade of the wood store.

The river was perfect. Full of fat, grey trout that shimmied nervously over the chalk white bed and very occasionally took my fly. I could have fished it for ever.

But spring-fed streams are not always as exotic, or even as Arcadian. In south London the River Wandle rises in springs by Beddington Sports Ground and Ruskin Road and flows north to the Thames at Wandsworth. It might once have been one of the finest spring-fed rivers of the lot. Isaac Walton loved its "trout spotted like tortoises". John Ruskin, the Victorian art critic and traveller, reckoned of all the places he had been to, none compared with the springs of the Wandle at the back door to his aunt's bakery near Croydon. Nowadays the river that once flowed through open countryside is completely urban, and the springs that fed it have been replaced by sewage outfalls. Even so, I like to walk and fish it, to notice how it is holding on, defying the city. Outside the Saver Centre in Merton the river looks bizarrely perfect. Blind yourself to the odd plastic bag, ignore the noise of the traffic on Merton High Street behind you and the river could be flowing through the open downland of Dorset. The gravel bed is clean; bright green water crowfoot vibrates in the fast current and small fish sip midges off the surface.

Elsewhere the river is more neglected. Under Armoury Way I once saw a teddy anchored to a trailing branch, drowned among all the other rubbish that careless Londoners had thrown in, that rose from the river bed like some kind of sarcastic coral reef.

But back when the Wandle was perfect and full of trout, there were no trout at all in the Gangtey, a spring-fed stream high in the Himalayas. This river is perhaps my fishing Shangri-La, but its trout have been there only for the past 40 years. They were carried there, on the instructions of the King of Bhutan, in clay urns from the rivers in Kashmir, and they had come to Kashmir as eggs packed in ice, by steamer all the way from Loch Leven in Scotland.

While we were busy destroying our wilderness - the Wandle eventually caught fire and its last trout was caught in 1934 - we were also exporting it. The film I made with the BBC this year had that idea as part of its theme. We went to Bhutan on the trail of these Leven trout (Loch Leven, like the Wandle is a shadow of what it was) and because I'd heard of this remote spring-fed stream. I never expected to find any place so perfect as the Gangtey valley. It was quite a drive, a full day heading east from the capital Thimpu, over a mountain pass so high our crisp packets swelled to look like footballs. We arrived in the dark and I got up at dawn to look around. Nathan followed me with the camera. It was ridiculously picturesque.

The sky was a high blue, while the sun, not yet over the hills, cut an angle of filtered light across the shadowed slopes, the beams drawing like a veil across the valley. Smoke came from each chimney. Cows wandered about the road and on to people's porches. Out of a house we were filming beside came a little girl in a traditional gho and pink wellies, with a toothbrush wedged in her mouth, followed by her mum and dad, also brushing their teeth. The little girl looked at us, mesmerised by the camera.

Bhutan is a land of precipitous slopes and cathedral peaks, but the Gangtey valley was like another country, softer, gentler. A landscape of permeable, porous rock. Spring-fed country. At the head of the valley the river was split by a spur; two streams met immediately before where the road crossed the valley. Somewhere in the forested slopes above, these streams had broken from underground.

The Gangtey is a sacred river. I had to get permission from the king to fish there and I did so for only a few hours. It was full of trout. Dark fish with vermillion spots that slashed at my dry fly until I had caught and released too many. It was strange to think that the Gangtey had flowed for so many years with no trout in it, that the enthusiasm of amateur Victorian naturalists and a fishing-crazy king had combined to create the perfect trout stream in a place so remote that it may just stay that way for thousands of years to come. As I wound my line in I thought about this and the River Wandle, a stream that flowed for thousands of years with trout in it and which didn't have them any more. I felt elated and privileged to be where I was - and pretty fed up about where I had come from.

Only the Wandle has a coda. There's a group of Londoners called the Jet Set Club that meets weekly to clean it. They're keeping pace with the fly-tippers; I spent a few hours with them recently removing enough junk to fill a lorry. And with the local schools the Jet Setters are stocking baby trout into the river.

In 1866, John Ruskin got as miserable about the rubbish in his beloved springs at Carshalton as I felt looking at that teddy under Armoury Way. He wrote about it in the Crown of Wild Olive and said: "Half a dozen men with one day's work could cleanse those pools ... but that day's work is never given, nor, I suppose, will be." One hundred and forty years later it has been. The Wandle is getting cleaner. A spring-fed river that had died is coming back to life.

Charles Rangeley-Wilson's new series, 'The Accidental Angler', will be shown on BBC2 at 8pm tonight. The book 'The Accidental Angler' is published by Yellow Jersey, priced £14.99

My top restaurant

A 20-minute drive east of La Sorgue is Roussillon, one of several hill-top towns in Luberon. You'll have to park below the ochre cliffs the town is named after and walk in - a steep climb up cobbled steps - and close to the top is a modest-looking restaurant called La Treille (00 33 4 90 05 64 47). It's run by the chef, who is shy, and his partner, who is chatty, and waits the tables; the cat-loving duo moved to Roussillon 30 years ago. They serve simple but superb food, stuff such as lamb stew with pine nuts, apricots, figs and couscous, and perfect with the Côtes du Ventoux red wines from the hills.

My favourite spring-fed rivers

La Sorgue in Provence is a short river that spills out of a deep, well-like cave on the edge of a towering plateau near Avignon. It tumbles quickly to Fontaine-de-Vaucluse, the small town where Plutarch lived, and more sedately on to the town of L'Isle-sur-la-Sorgue, through which the river runs in dozens of channels full of trout and grayling.

The River Itchen in Hampshire, pictured, probably isn't what it used to be 100 years ago, but even so it is as unspoilt as any spring-fed river in England. Three pure chalk streams, the Candover, Alre and Tichborne, meet just west of Alresford and for 10 or so miles to Winchester the Itchen slides with a limpid gracefulness that no other river can match.

The Chico Gallegos in Argentina is a tiny tributary of the much larger Rio Gallegos. It rises on a high plateau of wind-swept grassland and bare rock and hasn't a clue which way it is going, braiding and meandering in on itself over and over again. The valley floor is a marsh and the river can be hard to get to, but drop a fly on to the pools and it will last less than 10 seconds before an enormous trout smashes through the surface to eat it.