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. Facilities belonging to the Oker Dam Pre-Dam 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. Romkerhalle Hydro-electric Power Station The hydro-electric power station in Romkerhalle is fed via a pipe from the Oker Reservoir and has a capacity when completed of 4.14 MW. It was opened in 1956 and is equipped with a single Francis turbine. It has a descent of 80 m, the nominal discharge is 6.5 m³ per second and it generates 12.5 million kWh per year. The power station is operated as a peaking power plant working for about 8 hours per day. It discharges directly into the river bed of the Oker. At this point the whitewater section of the Oker begins and is especially popular with canoeists. Large and sometimes abrupt changes in water level on the Oker result from the unregulated flow of the power station from here as far as the stilling basin of the Oker Reservoir. The Romkerhalle power station is also called the Oker Hydro-Electric Power Station (Wasserkraftwerk Oker). Oker-Grane Tunnel The 7.4 km long Oker-Grane Tunnel, with its intake near Romkerhalle and outlet on the eastern shore of the Grane Reservoir, conducts water from the Oker Reservoir (power station or bottom outlet) into the Grane Reservoir where it can be purified for use as drinking water. The Grane Reservoir itself only has a relatively small catchment area and