The Ennepe Dam is a masonry dam designed by Professor Otto Intze. It is approx. 51 metres high, with a crest length of about 320 metres and a crest width of 4.5 metres. The dam's height was increased shortly after construction, from 1909 to 1912, by about 10 metres.
The reservoir's main tributary is the River Ennepe, which joins the River Volme in Hagen and flows into the Ruhr four kilometres further downstream. The Ennepe Reservoir includes a pre-liminary reservoir in Osenberg and six secondary reservoirs. The preliminary reservoir and two secondary reservoirs in Altenfeld and Altena were dredged in 2008 and 2009, as they had shrunk to less than 60 per cent of their original size due to sedimentation over the years.
The dam was rehabilitated in 1997. In the process, a control passage was drilled through the dam's foundations with a jumbo drill. The lower half of its cross-section is located in the natural rock that forms the dam’s foundations, and the other half is located inside the wall. This required an enormous engineering effort, as the control passage had to follow both the dam’s curvature and the valley profile all along the interface of the wall and its natural foundations. The control passage serves to drain seepage from between the wall and the rock, as well as from the wall itself, thus reducing the uplift water pressure on the dam.
After the dam's rehabilitation, the crest was closed for traffic, which had previously been authorised.
The Ennepe Reservoir is used for drinking water production. The joint stock company AVU Gevelsberg, legal successor to the former public waterworks of the Schwelm District, currently abstracts nine million cubic metres of untreated water annually from the reservoir.
Moreover, in early 2006 a hydropower plant was installed at the Ennepe Dam and Reservoir. The cross-flow turbine has an absorptive capacity of 1.4 cubic metres per second. Average an-nual output amounts to 1.5 million kilowatt hours which are fed into the power grid. The hydro-power plant is operated by the the company Lister-Lenne-Kraftwerke GmbH in Olpe, a 100 per cent subsidiary of the Ruhrverband.