About Oker Dam

The Okertalsperre in the western Harz mountains of Lower Saxony holds 46.6 million cubic metres and forms the centrepiece of the Harzwasserwerke reservoir system. Completed in 1956, the 63-metre concrete double-arch gravity dam impounds the Oker river — the main drainage artery of the western Harz — just above the town of Schulenberg im Oberharz. The Okertalsperre is the primary drinking water source for the Braunschweig, Wolfenbüttel, and Salzgitter regions, supplying treated water to roughly 700,000 people in the Lower Saxony industrial heartland. The reservoir works in an interconnected system with the Innerste, Grane, Söse, and Oder reservoirs across the Harz, enabling water transfers between catchments during drought or maintenance periods. The steep Okertal gorge below the dam is a designated nature reserve, and the stretch of the Oker river immediately downstream of the dam supports important populations of brown trout and rare aquatic invertebrates dependent on cold, well-oxygenated release flows. The Harzwasserwerke manages the ecological minimum flow regime to protect downstream habitat. The reservoir valley is surrounded by the western Harz national park boundary and the forested hillsides above the waterline support Germany's largest breeding population of black stork.