The dam was built in 1967, a piece of post-war infrastructure for a part of Northern Ireland that had grown out of small mills and small farms. The engineers chose to face it with stone quarried from Corkey, a working pit opened almost three miles away specifically to supply the project. That decision shows in the wall today. Where many reservoirs of the period present a flat concrete face to the moor, Altnahinch wears the local geology on its skin, blocks of dark Antrim basalt set into the contour of the hillside as if the dam had grown there rather than been built. The water it holds is fed from Altnahinch Burn and from the upper reaches of the River Bush itself, the two main streams joined by a fan of tributaries draining the greater Glenbush valley.

From the dam, the Bush has roughly twenty-five miles still to travel before it reaches the Atlantic at Portballintrae. Along that route it passes under fifteen road bridges, a small number when you say it quickly, but a generous one for a river of this size. Each bridge marks a crossroads of rural life: a school run, a creamery lorry, a farmer moving cattle between fields. The river slides under them all, broadening as tributaries join, until it tips over the last fall at Bushmills, the village whose distillery has drawn its water from these same currents since 1608. The whiskey is famous. The river that makes it possible is not.

Locals know Altnahinch differently. To them it is one of the better fishing waters in this corner of County Antrim. The reservoir is stocked with native brown trout, the small wild fish that have always lived in the upland Bush, and with rainbow trout for anglers who want the harder fight. On a still summer evening the lake holds the colour of the moor above it, a soft tea-brown with the sky doubled on its surface, and the only sound is the rise of a fish or the low murmur of wind through the firs of Slieveanorra. The setting carries a kind of working-day beauty that does not announce itself. You find it, or you do not.

Slieveanorra Forest climbs the hill above the reservoir, a state plantation of Sitka spruce planted in the middle of the last century to put working trees on land that had been bare blanket bog. The forest gives the dam its dark backdrop, and the moor above the trees stretches to Trostan, the highest of the Antrim Hills. Walking the track around the reservoir, you can see all three layers at once: water below, conifer wall in the middle distance, and beyond that the open heather of the Antrim plateau. The whole catchment is upland Ulster compressed into a single view.

Altnahinch is the kind of place that does the work other places get the credit for. The Bushmills brand, the Giant's Causeway tour buses, the famous run of the river through Bushmills village itself, all of it depends on this small upland reservoir holding steady through dry summers and wet winters. The dam keeps the supply even, the trout keep the locals interested, and the River Bush keeps doing what it has done since the last ice age pushed the moraine at Armoy and shoved its course westward. Stand on the dam wall, look downstream into the trees, and you are looking at the first quiet step of a long Antrim story.