An extract from "A Walk along the Thames Path", by Gareth Huw Davies, published by Michael Joseph, 1990.

I decided to walk the banks of the river Thames, the 180 miles form its source to the Thames Barrier at Woolwich, with no firmer purpose than to watch an astonishing national institution unfold, and to write an entirely personal account of what I found upon its banks. My plan was to steal upon an unexpecting river and seek some of its personality as it is reflected in the people who live and work beside it, or upon it. And, where it allows me, to tease out some of its secrets.

There are many grander and more testing walks than this in Britain. I shall need neither heroism nor map reading skills. All previous long distance trails have linked remote, cast-out places. The Thames Path is a masterstroke, a peoples path accessible to the merest Sunday afternoon whim or, over its full length, the mightiest Summer holiday ambition outside Britain's coasts and hills. For many miles it will run exactly where it ought to, within inches of the water. Only when it reaches London does its riverside grip slip, like a badly-printed overlay. My route will be set with regular refreshment for body and spirit, in pub and church and preserved building.

There will be conversation and diversion. There are many books to read. Yet, too, there will be unexpected moments of bewildering isolation. And I shall learn that to be on the north bank when you require the south, or the other way round, is a rare frustration in an age that has declared the ferryman obsolete.

Victorian writers on my path could amble for hours through river meadows, where brown-armed hay makers would look up from their backbreaking toil. Many of these meadows are now under a spiritless form of mechanised agriculture which extends to the very water's edge, evicting Ratty from his "bijou riverside residence, above flood level and remote from noise and dust". Yet I will restore my spirits by recounting many a conservationist's bankside victory.

R. L. Stephenson found it astonishing what a river could do, "all by following gravity in the innocence of its heart." The Thames falls by only 600 feet from the source to London Bridge - 300 feet in the first nine miles; 100 in the next 11 miles; 100 in the 72 to Great Marlow; 100 in the last 48 miles. Veterans of uplands treks will find the river's incline unexceptional. Walkers will not encounter an easier long distance route. So I set off, downhill all the way, to discover the world's most illustrious river.

It was a slight beginning. A wide, low shouldered valley and the faint memory of water, long ago flowed away. A thousand dandelion clocks marking time past or time to come. At the stile, a scrum of Fresian cows loiter indifferently. "Obstruct you? We always stand here." An ash sprawls a branch out overhead, like a giant athlete straining for the tape. But this is the beginning of the course. In the shelter of a high spinney of hawthorn and sycamore stands a plain, marble obelisk. "The Conservators of the River Thames, 1857-1974. This stone was placed here to mark the source of the river Thames."

It is marble, the material of high monuments, celebrating heroes and great moments in history. Formal enough for those places of official ceremonial, many miles downstream, but it seems too grand for this pure, small, corner. But it serves the purpose of those who put it here. Its unyielding surface is unmarked by vandal. There is no water anywhere. No signs of a great river about to be born. I find signs of recent excavation.

People come to find water, see none, so instinctively dig for it. Water diviners hang there sticks here and concentrate, but in vain. Nothing gurlges from this shallow mark under its high protecting ash. A rook probes for insects in the meadow. A chiff-chaff chimes noon in the spinney. A wren squeaks a sweet concerto for tiny voice, so rapid and complex it would defeat any human notation. How did the Thames ever escape this quiet place in rural Gloucestershire? I have no doubt it was once here. Simple clues confirm it. An ancient line of tottering oak trees head east on its original line; their roots may still find some refreshment in deep vestiges of the old river.

And so I follow a memory, on a morning in late spring gently tipping into summer, along the fold in a field, the cradle of the Thames smoothed and softened by ploughing. I strike east to find water and the 20th Century.