Diemelsee

Diemelsee, Diemel Reservoir Diemelstausee, Diemeltalsperre
The Diemel Dam: dam and reservoir, compensating basin (Ausgleichsweiher) and power station. Foreground: the village of Helminghausen. Left: the Eisenberg
Location	Waldeck-Frankenberg (HE), Hochsauerlandkreis (NW)
Coordinates	51°22′20″N 8°43′26″E / 51.37222°N 8.72389°E / 51.37222; 8.72389Coordinates: 51°22′20″N 8°43′26″E / 51.37222°N 8.72389°E / 51.37222; 8.72389
Construction began	1912 to 1914 and 1919 to 1923
Dam and spillways
Impounds	Diemel, Itter
Height (foundation)	42 m
Height (thalweg)	36.2 m
Length	194 m
Width (crest)	7 m
Width (base)	31 m
Dam volume	72,000 m³
Spillway capacity	113 m³/s
Reservoir
Total capacity	21.75 M m³
Active capacity	19.9 M m³
Catchment area	103 km²
Surface area	1.65 km²
Maximum length	4 km (Diemel arm), 3.1 km (Itter arm)
Maximum width	300 m
Normal elevation	376.2 m above sea level (NHN)
Power station
Installed capacity	1.04 MW

The Diemelsee or Diemel Reservoir (German: Diemelstausee) 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 (DiT) 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.

The Diemelsee is located a few kilometres northeast of the Upland in the northeastern foothills of the Rothaar Mountains that lie in the northeast of the Rhenish Massif. It lies mainly within the county of Waldeck-Frankenberg, its smaller northern section and its dam belonog to the county of Hochsauerlandkreis. In is also within the Diemelsee Nature Park between Willingen and Marsberg on the territory of Diemelsee and the borough of Marsberg on the River Diemel and its tributary, the Itter. Its dam stands about 500 metres south of the village of Helminghausen (southwest of Marsberg). The only shoreline village is Heringhausen.