1. DiemelseeHeringhausen The Diemelsee or Diemel Reservoir is a reservoir with a surface area of 1.65 km² and about capacity of 19.9 million m³ on the River Diemel in the counties of Waldeck-Frankenberg in North Hesse, and Hochsauerlandkreis, Westphalia, Germany. It is part of the Diemeltalsperre hydropower system comprising the Diemel Dam, the equalizing basin, the power plant and the reservoir itself, owned by the Federal Waterway and Navigation Authority and managed by its Hann. Münden office. The Diemel Reservoir, along with the Edersee is part of the water regulation structure in the catchment area of the River Weser. From Wikipedia, the free encyclopedia.