Travel: Britain - A piece of Powys that passeth all understanding

Overcrowding in some of our most beautiful landscapes is reaching crisis point. As the temperatures rise, so columns of townsfolk wind into Dartmoor, the Peaks and the Dales, all intent on experiencing "unspoilt wilderness". The irony, of course, is that in summer the "wastes" that thrilled Boswell, Wordsworth and Hardy are likely to be gridlocked.

For all that, though, there are still hidden gems, and the Elan valley is one of them. Extend a straight line from London through Oxford, Cheltenham and Hereford, and shortly before reaching Aberystwyth you find yourself in the joyful emptiness of eastern Powys.

Given the valley's beauty and its comparative proximity to well populated areas (it is only two hours from Birmingham or Bristol), it may be surprising that the area is so under-visited. It certainly wasn't always so.

Shelley was among the first visitors: he fled here to his uncle's house after being sent down from Oxford, and was so taken with the wild beauty that he tried to set up a writers' commune nearby. The Victorians agreed with him, founding a string of spa towns along the eastern edge of the mountains that form the backbone of Wales (Llanwrtyd, Llangammarch, Builth and Llandrindod all boast the epithet "Wells"). Yet the area's popularity has waned, and left it with one of the lowest per capita incomes in Britain.

For the handful of visitors who do stumble across it, the rewards are significant. The backdrop of the Cambrian mountains are formed from some of the oldest rocks in Britain and come with a desolate beauty of their own, different from that of the better known crags of north Wales and the Lake District.

A century ago the entire catchment of the valley was bought by the Corporation of Birmingham, anxious for a reliable source of drinking water. In an ambitious engineering project, four huge dams and a 73-mile pipe were built to supply a mushrooming Midlands population. By modern standards these massive stone edifices are beautiful, if austere, and heavy rain makes them truly spectacular, cloaking them white with millions of gallons of cascading water. Thanks to this and deserted mountain roads, the area features heavily in car and mobile phone advertisements (not to mention the recent TV series Mortimer's Law).

In contrast to other reservoir projects, the dams have been an environmental godsend. In the cause of water purity, the authorities have always kept a tight leash on human activity, beginning by evicting some 400 tenants (most of whom, in fact, lived above the waterline). As a result the hillsides are dotted with ruins - not to mention two abandoned mines. The Elan Valley Trust, which manages the catchment, has continued to exert strict (if less brutal) control. Farmers face restrictions on stocking and chemical use, but when it comes to visitors the picture is ambiguous. On the one hand there is an excellent visitor centre and an effective "freedom to roam" policy; but water sports, camping and off-road driving are banned. In addition, the Trust seems reluctant to advertise the valley's charms widely, which explains why it is so empty, even in mid-summer.

As a result it remains a stronghold for many rare species which thrive in its hanging oak forests, heather and scrub. Birdwatchers are drawn by red kites, peregrines, redstarts and golden plovers; others hope for a glimpse of polecats, otters and badgers - and the rare upland orchids found in the unimproved hay meadows. Another great attraction is the "friendly" nature of the hills, quite unlike the precipitous peaks of Scotland and the Lakes. And the views from the tops are still breathtaking: on a clear day the panorama stretches from the Black Mountains to Snowdonia.

Outdoor activities are not confined to hikers - this is one of the few hilly areas in the country where you can plan a relatively gentle cycle tour through spectacular scenery. And if even this seems too energetic, flyfishing for the monster brown trout that lurk in the lakes is cheap and easily arranged.

Finally, it's worth noting that the area is pervaded by a genuinely friendly atmosphere. Be prepared for gentle gossip with the newsagent and pub locals, all curious as to how on earth you heard about the place.

Getting there: by public transport, the Elan valley is difficult to reach. The railway station at Llandrindod Wells, on the Heart of Wales line, is about 12 miles east. There are sporadic buses.

Being there: the Elan Valley Hotel (run by former actors, 01597 810448) is two miles south west of Rhayader. B&B costs pounds 24-pounds 30 per person. Bikes, ponies and fishing can be arranged from here. Kitewatchers Wildlife Breaks (01597-811169) has a variety of guided tours.