Oker Dam

The Oker Dam (German: Okertalsperre) is a dam in the Harz mountains in Lower Saxony, Germany. It is fed by the River Oker.

The dam is in the shape of an arch (see arch-gravity dam). It is 75 m high, 260 m long, can impound up to 47 million m³ of water and provides hydropower, flood protection and water regulation at times of low water. It is also used indirectly for the production of drinking water. Its average annual discharge is 75 million m³. The hydro-electric power station at Romkerhalle is fed by the Oker Reservoir.

The construction of the Oker Dam was begun by Hr. Press in the years 1938 to 1942 and then completed between 1949 and 1956. Its completion became pressing following the devastating floods in Wolfenbüttel and Brunswick in the winter of 1946/47 when there were heavy falls of snow. Before work started the small hamlet of Schulenberg in the valley had to be abandoned; it was rebuilt above the reservoir.

The Oker Reservoir lies alongside the B 498 federal road, that follows the course of the River Oker between Goslar and Altenau. On the east bank is a forest road, that is out of bounds to private vehicles.

With its many side arms the Oker Reservoir resembles the much larger Bigge Reservoir in the Sauerland in west Germany. For the same reason it is also known as the "Vierwaldstättersee of the Harz".

From the beginning of March to the beginning of January a passenger boat plies the reservoir ferrying tourists (only at weekends during the off-peak seasons). The large tour on the MS Aquamarin takes approximately 1.5 hours and can be started from all landing stages around the reservoir. This boat service is the highest (above sea level in Lower Saxony. In addition, during the summer months, numerous, private, unmotorised water craft grace the waters of the reservoir. Swimming and diving are also permitted, apart from certain out-of-bounds areas such as the zone near the dam wall. In winter, when the ice is not thick enough to support weight, ice swimming takes place. There is a circular route, using roads and paths, around the Oker Reservoir which is accessible to unmotorised vehicles or walkers. Between the pre-dam and the primary dam there is a fully metalled circular path, which is used by cyclists, inline skaters and roller skiers. Only the path between the pre-dam and Altenau, which is barred to motor vehicles, is not metalled, but is surfaced with gravel and stone. On the B 498 in summer there is heavy motorcycle traffic (on the northern approach to Torfhaus). There are numerous restaurants and cafés along the road. In very calm conditions (no wind or precipitation and no man-made disturbances) a very flat surface forms on the reservoir which mirrors clearly the surrounding landscape.

The pre-dam (Vorsperre) of the Oker Reservoir is of the gravity dam type, with a height of 20.0 metres. The pre-dam can impound up to 520,000 m³ of water and its primary purpose is to control the water level and cleanliness of the Oker Reservoir. The dam site was designed as a retention basin (max. 15 m of water at the pre-dam) in order to create a wetland in the Kellwasser valley. Only when there is a very high level of water in the Oker Reservoir are both reservoirs at the same height. The water level in the pre-dam lake can even rise a few decimetres higher.

The bottom outlet is used only for floods and is only opened in case the normal outlet (located below the Windenhaus) is unable to discharge the water through it. It consists of a 1.2 nominal diameter pipe with a screen, an inspection seal, a shutoff valve on the water side, and a piston valve on the airside. It has a maximum output capacity of 15 m³/s.