Building Kielder Water meant flooding the upper valley of the North Tyne, a remote stretch of Northumberland upland that had been farmed in a thin, hard way for centuries. The reservoir absorbed numerous farms, a school, and most of the valley's tiny population - about ninety-five people in all - who were bought out and relocated. The old line of the Border Counties Railway, which had run along the valley before Beeching closed it in 1956, was submerged. So were stone bridges, farm tracks, and field walls that had taken generations to build. The hamlet of Yarrow, where the dam was sited, ceased to exist. What replaced them is now the longest shoreline of any reservoir in Northern Europe - twenty-seven and a half miles, looped around drowned valleys and submerged hills. On a still day, when the lake is glass, the geometry of vanished fields can sometimes still be sensed just under the surface.

Kielder Water sits inside Kielder Forest, one of the largest man-made woodlands in Europe. The forest was planted by the Forestry Commission from the 1920s onward, mostly with non-native conifers - Sitka spruce, Norway spruce, Scots pine - that would grow on poor upland soils that nothing else profitable would touch. By the time the reservoir filled in 1984, the forest had reached maturity, and the lake came into being already surrounded by 250 square miles of dense, dark woodland. The combination created something unusual in England: a genuinely large, genuinely empty landscape. Red squirrels still hold out here against the grey invaders that have driven them out almost everywhere else in England. Ospreys have returned to breed. The dark sky observatory at Kielder is one of the best places to see stars in the country, because there are almost no lights for thirty miles in any direction.

Kielder has been called a white elephant since before it was finished. The scheme was justified by projected industrial water demand that never materialised, and the reservoir's primary water-supply function - transferring water through pipelines and rivers to Teesside, Tyneside, and Wearside in times of drought - has rarely had to operate at the scale originally imagined. Whether the scheme was foolish or merely unlucky depends on how charitably you regard 1970s economic forecasting. The reservoir does have other jobs. It generates electricity through England's largest hydroelectric plant, a six-megawatt installation opened by the Queen in 1982 using a 5.5 MW Kaplan turbine and a smaller 500 kW Francis turbine that runs continuously on the compensation flow into the North Tyne. It saves around 8,500 tonnes of carbon dioxide a year. It supports trout fishing, sailing, kayaking, cycling, and over 250,000 visitors a year. It may not solve the problem it was built for. It solves several other problems instead.

The name Kielder was first recorded in 1309 as Keldre, and it began life as a river name. Linguists trace it to Brittonic caleto-, the same root that gives Welsh caled meaning hard, suggesting either a hard rocky river bed or a tough piece of country to live in - probably both, given the upland geography. The Brittonic Celts named these northern uplands long before the Anglo-Saxon and Norse settlement that followed. Their language has almost vanished, but it survives in dozens of river names across northern England: Kielder, Tyne, Wear, Derwent. The hard country has stayed hard. The drowned valley simply became hard underwater instead. Standing on Tower Knowe in the cold spring rain, watching the surface of an entirely man-made lake ripple over what used to be somebody's pasture, the antiquity of the place name feels appropriate. It is the kind of land that has always been called hard.