A journey in search of the source of the Clyde reveals much about the river's place in the nation's heart

BLAME Kenneth McKellar. The late singer is the reason why I'm standing in Leadhills in the rain on a Friday morning. As a Glaswegian, I have long wondered where, precisely, the river that runs through my city begins.

• Peter Ross fords the meeting of Daer Water and Thick Cleuch burn, said by Tom Weir to be the start of the Clyde Photograph: Robert Perry

In his famous rendition of The Song Of The Clyde, McKellar is unequivocal. "Of all Scottish rivers it's dearest to me," he sang in that unmistakable tenor full of whisky and shortbread. "It flows from Leadhills all the way to the sea."

Leadhills. The South Lanarkshire village high in the Southern Uplands. So that's where the Clyde starts. Simple. Except that the locals don't seem to know much about it. "Couldn't help you love, sorry!" says a bemused lady behind the counter in the shop. "I've never heard of the Clyde starting round here."

In a whitewashed cottage on the main street, Lee Gilmore, a petite middle-aged woman, runs Bear-ly Reminiscent, a shop selling teddies dressed in the tweeds and bunnets of miners from yesteryear. Can she confirm Leadhills is the source of the Clyde, as McKellar said? "No, that's a bit of a lie," she says. "The source is actually Elvanfoot, the next village along, but that wouldn't have fitted so well in the song."

It's a great place for disputed claims, Leadhills. In the graveyard there's a man said to have lived until he was 137, though some argue that's totally ridiculous and he actually died at 133. Leadhills is also locked in a perpetual cold war with nearby Wanlockhead over which of the two is the highest village in Scotland. And now there's this business with the Clyde.

What is undisputable is that Leadhills is home to the world's oldest lending library, founded in 1741. The Miners Library was established for the elevation of the minds of men who worked underground. One wall is lined with brown leather-bound volumes, mostly religious and scientific titles, the air smells pleasantly foosty, and against one wall stands a wooden pulpit with "Learning Makes The Genius Bright" painted in gold along the top.

Andy Foley, 40, one of the volunteers who look after the library, hands me a newspaper clipping from 1956 concerning the opening of the Daer Reservoir. "I think people round here generally accept that the Daer is close to the source of the Clyde," he says. "If you find it, you should plant a flag."

But where, exactly, to go next? Elvanfoot perhaps. It is a short drive along the B7040, through a landscape of grouse butts, cairns, disused mines and a dismantled railway. This is burns country, with many small streams flowing from the heathery hills: the Shortcleuch; Scapcleuch; Over Cleuch; Nether Cleuch. For anyone trying to find the Clyde, it's enormously confusing.

Happily, the OS map seems pretty clear. The words "River Clyde" - in bright, confident blue - appear for the first time a little north of Elvanfoot, just beyond the point where the Elvan Water flows into the Daer. The river is making its first official appearance. It is lexicographically endorsed. But you couldn't, in truth, call it the source. Already, it is 25 yards wide. And while the people at Ordnance Survey are pretty sure that this is where the Clyde begins, the people of Elvanfoot seem uncertain and a little underwhelmed.

"No," says Bill Goodfellow, a white-haired farmer, standing at his door and pointing south-east, "the source is the Little Clyde Burn, up on the other side of the motorway. What's the old saying? ‘Annan, Tweed and Clyde, rise oot o' ane hillside.'"

There is a long tradition that the Little Clyde Burn is the true source of the Clyde. Neil Munro, author of the Para Handy tales, popularised the idea. In 1907, he wrote The Clyde: River and Firth, an account of travelling the length of the river. The book, long out of print, is newly republished by The Grimsay Press. Munro gives an account of visiting Little Clyde Farm and following the burn to its origin in a spring. Celebrating the discovery with a bottle of hock, the author and his giddy companions used the cork to plug the spring, then travelled back to Glasgow where, they were relieved to discover, the Clyde had not dried up.

Little Clyde Farm is still there, close these days to the M74, and looks much as it did in Munro's day. Smoke is rising from a chimney. Jim Humble, 62, comes to the door. He has a tremendous face. White hair, white beard and a Roman nose. Caesar in green wellies.

"It's quite a source of pride being the first home on the Clyde," he says, leading the way behind his cottage to the burn. It's a brown trickle with midges hovering close to the surface. "The Lanark mills were built because of this. Strange to think of all that coming from such a small source. If you follow it up the hill, which is quite a steep climb, you get into the woods and it's just a different world. After a hard week at work, there's nothing better than going up to the Clyde."

For three weeks during the hard winter, snowed in and with their pipes frozen, Humble and his wife used the burn as a water source. They drank it and bathed in it, not even bothering to boil it first. They love the river and - when it mattered - it was there for them. It's great to think that the people living closest to the source of the Clyde appreciate its significance.

Is it really the source, though? Neil Munro believed so, but since his time opinion has swung against Little Clyde. Some even mutter darkly that railway workings in the early 20th century diverted the burn so that now it actually flows into the Annan and out to the Solway Firth.

A better bet seems to be to return to Elvanfoot and follow the Daer Water south. According to no less an authority than Wikipedia, and verified by a trusted source - two men shouting to me from a fishing boat - the Clyde is formed when the Daer merges with a tributary, the Portrail. This happens near a small settlement called Watermeetings. The farmer there, a Lancashire man called Len, explains that Watermeetings gets its name not from the confluence, but from the prevailing climate. "It rains that hard round here, it's where the water meets the ground." Only yesterday, he says, a man was asking about the source of the Clyde. Len told him to head for the Daer Reservoir.

The dam is huge. It contains 5,300 million gallons of water, which flows out through thick moss-green pipes on its way to 250,000 homes and businesses in Lanarkshire, including the Coca-Cola plant in East Kilbride. Coke, in other words, has the Clyde in it, which is a pleasing thought.

It's lonely round the reservoir. A row of modern houses, once inhabited by workers, have fallen into disrepair. Broken venetian blinds hang like rib-cages behind smashed windows. Swallows have built mud-nests in the kitchens. All very spooky. What a relief to get away and seek the river once more.

This is the most difficult leg of the journey. About half a mile past the dam, the road becomes little more than a wet and rocky track. Worst of all is an ersatz bridge made of rotting wood and scavenged iron. The Clyde does not give up its secret without testing the courage of those who seek it.

Eventually, the track turns down to the left and then runs out. You have to walk from here, though not far. It's boggy ground, sharp with reeds. Here, in a natural amphitheatre formed by several cloud-shrouded hills, the Thick Cleuch burn meets the Daer. The land narrows to a V and the two shallow brown streams become one. According to Tom Weir, the late walker and broadcaster, who filmed a visit to this spot in the late 1970s, this newly formed river is the Clyde. Bending down, I cup it in my hands and drink. It tastes cold and clean. By the time this water gets to the Broomielaw, I'd be less inclined to swallow it, but here it feels like the natural thing to do - a small communion.

Why, I wonder, should it feel so moving just to be here? Well, the Clyde is not Scotland's longest river, and it is not the most beautiful, but it is, I would say, the river which lives most vividly within the minds and imagination of the Scottish people. It carries a freight of meaning. Through its association with shipbuilding in particular, the Clyde has become is emblematic of what we like to believe are national characteristics - strength, rough humour, ingenuity, comradeship and a love of adventure.

The song of the Clyde is, in some ways, the song of Scotland. Here, though, high in the hills, it's hard to catch even an echo of that. This shushing, lullabying river will make its way north then west. It will take a deep bow at the Cora Linn falls, marking the end of its first act. From the waterfall, it will pass through the industrial heartland, bisect the city, then flow out to the coast. The end of that 100 mile journey - the towns and islands of the Firth - has long been associated with pleasure. But to witness the beginning of the river's journey is, above all, a privilege.

Even Kenneth McKellar, had he made his way here from Leadhills, would surely have been quick to agree.

This is the first in a three-part series of Peter Ross At Large on Scotland's most iconic river. Don't miss his next instalment next Sunday.