The dam grew out of a particular German partnership: a university professor with a theory, and an industrialist with a problem. Otto Intze taught hydraulic engineering at the technical college in Aachen, and his 'Intze Principle' described how a curved gravity dam could lean into the rock at its abutments, transferring some of the water's enormous push into the valley walls themselves. Robert Boker, a Remscheid industrialist, needed clean water for a city whose tool factories and tenements were outgrowing their wells. The two found each other, and between 1889 and 1891 they raised a granite-sheathed wall across the Eschbach valley. The first water flowed into Remscheid's pipes that same year.

Word travelled. A dam for drinking water was novel enough in Germany that visitors arrived to see it almost before the mortar had finished curing. Prince Friedrich Leopold of Prussia made the journey on 15 July 1897. Two years later Emperor Wilhelm II himself came, and pronounced the structure a great work of Bauteknik und Wassergewerbe - construction technology and water commerce. In the writings of the era the dam was usually called the Remscheid Dam, because German custom of the time named impoundments after the city they served rather than the stream they crossed. The Intze Principle would soon be copied at dozens of sites - in the Eifel, the Harz, and eventually in reservoirs as far away as China and the Americas - but it began here, in this narrow valley.

By the late twentieth century the original masonry had been holding back water for nearly a hundred years, and it was tired. Between 1991 and 1994 engineers refortified the upstream face with a thirty-five-centimetre concrete liner, threaded an inspection walkway behind it, and installed new drainage and monitoring systems. The dam still looks like its 1891 self when you stand below it - a curved stone wall in a German forest - but its interior is now stitched with the diagnostic equipment of a modern water utility. It belongs today to the Wupperverband, the river-basin association that manages drinking water for this corner of the Bergisches Land, and is linked through pipes and tunnels to the larger Neye Dam upstream.

What you see from the air is small by modern standards - a slim, twisting blue ribbon nestled in beech and spruce woodland southeast of Remscheid, with the curved masonry plug at its downstream end. A road circles the reservoir, threaded with a nature trail that has drawn day-trippers since 1977. From the terrace of the A 1 motorway service area at Remscheid, drivers heading toward Cologne can pause and look down on the same water that Wilhelm II once admired. The Eschbach Dam was never the largest dam in Germany. It was only the first of its kind, which is enough.