The Itchen: Travails of the riverbank

All seems idyllic on the Itchen – but it's one of Britain's threatened rivers, under pressure from water companies and overcrowding. Simon Usborne looks beneath the surface

All should be quiet in Roger Harrison's dining room. Kept cool in the early summer heat by thick walls, some of which were built in Tudor times, it lies at the heart of a house that hugs a kink in a nameless country lane. We're only a 15-minute drive from Winchester in Hampshire but few people save Harrison find a reason to steer their cars past his door. And yet a low rumble fills the room, almost causing the candlestick holders to tremble. "You can really hear it, can't you?" Harrison says.

The noise comes not from the undisturbed ribbon of asphalt outside but from the river that runs through – or under – Harrison's house, a converted mill. From its source outside the village of Cheriton in the shadow of the South Downs, the Itchen, which is one of Britain's prettiest and most precious waterways, meanders south for nearly 30 miles, passing through Winchester before it eventually spills into Southampton Water.

At Itchen Stoke Mill, where Harrison, 77, lives with his wife, Victoria, and their black Labrador Amy, the river once turned great grinding stones. Now it fuels Harrison's passions. He takes us out into the garden, where the sun blazes and the low burble becomes a dull roar. Pristine lawns run to the banks of the stream, which is as clear as gin. "Our bedroom is at the top of the house and we always look out, thinking, God, this really is magical," Harrison says.

The mile-long stretch of the river Harrison calls his own (although he sees himself as more of a steward) is more than merely stunning. The Itchen is the country's best surviving example of a chalk stream, one of just 200 in the world that flow only in southern England and parts of northern France. Rain water soaks through chalk, becoming filtered yet infused with nutrients as it springs at a steady, naturally cooled temperature to create trickles that become streams.

The water here is so rich and pure that, allowed to flourish, chalk streams support a dizzying array of wildlife. I've come to see for myself, guided by Harrison and Rose Timlett, who heads up freshwater policy at the World Wildlife Fund (WWF). Shod in gumboots and led by Amy, we cross the little wooden bridge in Harrison's garden and go for a stroll.

"Look, do you see him?" Harrison asks, stopping on the path beside the river. Sheltering from the stream behind a clump of water starwort, which has is swept back into a subaquatic smudge of green weed, a wild brown trout forages for nymphs as they hatch from the gravelly river bed. Given the chance, they will then rise up to spread and dry their wings, reborn as flies. While we watch, a grayling, part of the salmon family, darts through the water as a gadwall, a type of small, dabbling duck, drifts on the glassy surface.

There's more going on in the sky, which, even by lunchtime, is still filled with the trills and tweets of chiffchaffs and sedge warblers, among other birds that nest here. A hundred years ago this month, US president Theodore Roosevelt, who was seeking new way to occupy his time having left office in 1909, travelled to Europe. He stopped first for a day in the valley of the Itchen, where he joined Edward Grey, a fellow ornithologist and the British Foreign Secretary. "Altogether I passed no pleasanter 24 hours during my entire European trip," Roosevelt later wrote of his visit, during which he lost count of the animals he observed.

More recently, Chris Packham, the BBC nature presenter, came here to retrace some of Roosevelt's steps with Harrison. Some of the species the president and Grey had spotted – such as the redshank – proved elusive, but so keenly has the ecosystem been preserved by Harrison and the people who lived here before him, Packham recorded 42 distinct bird songs during one short walk.

If you're lucky you might also spot an otter or a white-clawed crayfish here. There are mink, badgers, voles, hare, shrews, bats and deer, too. Two years before Roosevelt's visit, Kenneth Grahame published Wind in the Willows, which was set on the banks of a fictional chalk stream not unlike the Itchen. But what makes these exceptional waterways such havens for flora and fauna also make them popular with people – and not only twitchers or writers. "Water companies love chalk streams," says Timlett. "The water is cheap, it's there and nature has already done an amazing job of cleaning it."

The Itchen has been used to supply public water for centuries but never has the demand been greater. In 2008, 130m litres of water were diverted or pumped – abstracted, to use the proper term – from the Itchen for public use. An estimated 300,000 households get water from the river, and more still is used by fish and industrial-scale watercress farms. Meanwhile, local authorities plan to build as many as 80,000 new homes near here in the next 15 years.

The threat to the Itchen is as clear as its waters but worse, Timlett says, is the attitude of consumers, some of whom use as many as 180 litres of water per person per day. "People think water comes from a reservoir but there aren't any round here," Timlett says. "It all comes from the river but even when they walk along a stretch like this they don't realise it's the same water coming out of their taps."

Thanks to Harrison, conservationists and groups of local volunteers who share his love of the river, the Itchen is being held up as a model for the preservation and management of chalk streams. Earlier this year, the WWF published Riverside Tales, a report that used three rivers to offer lessons they hope local authorities and water companies all over the country can learn. Southern Water has committed to reducing abstraction from the Itchen and plans to introduce water meters to all the homes it serves in a bid to raise awareness and reduce consumption.

Get it wrong, and chalk streams can quickly go bad. The Mimram and the Beane in Hertfordshire, the other rivers in WWF's report, miss the glamour or protection of the Itchen, but should be no less attractive to wildlife. But over-abstraction and neglect by all but a devoted yet frustrated minority of local wildlife and conservation groups means that, in the summer, both rivers run dry in places.

"We've got such a unique ecosystem in chalk streams and for thousands of years we've managed to live in harmony but it's almost like we're on the edge now of pushing rivers too far – of taking too much," says Timlett, who believes water companies and local authorities too often pointing fingers at each other and commissioning studies rather than taking action.

"We think of chalk streams as Britain's rainforests. It's all very well to care about things happening overseas but if you really want to take sustainability seriously you have to start at home, where, on our watch, we're already seeing a decline. What's so important about the Itchen is that it's one of the last remaining places you where you take people and inspire them – to say this is what it's all about. This river isn't just water running between banks, it's sustaining a whole ecosystem. Wildlife comes here to survive."

Such is the fragility of a habitat like the Itchen, no amount of love or investment can safeguard everything. At various intervals along Harrison's stretch, he and volunteers have erected cages to protect river water-crowfoot, an aquatic species of buttercup that produces beautiful white flowers but which is food for the booming swan population.

Towards the end of the tour, when Harrison's house comes back into view, we step into one of Hampshire's last surviving water meadows. By flooding flat areas of land next to rivers, via a carefully carved and managed network of channels that were controlled by wooden gates, early farmers encouraged grass to grow early in spring. This meant sheep could graze sooner than normal and hay was abundant.

Less plentiful in the water that flows back towards the mill, however, are shrimps. Harrison stoops with knees that seem to belong to a much younger man to pull up a clump of duckweed, inspecting it like a chimp checking an infant for nits. "What we are most worried about is their absence," Harrison says. "Look, there's nothing, nothing at all," he gestures. "There ought to be more shrimp."

Another lesser-spotted species that used to flourish here is the bogbean, a wild flower that likes wet ground. As we near Harrison's garden, he spots a flower about 10 yards off. "It's the most beautiful thing," he says, bounding over. "Look, there's another. Aren't they lovely."

The bogbean has feathery white and pink flowers that grow on thick stalks and are pollinated by bees and butterflies. "When you get a whole area of them in white they really are absolutely spectacular," Harrison says. "This is just the sort of place they should grow well but they never seem to flower in the same way. Even here, their habitat is shrinking."