As we hiked, from the car park below, left into the mountains to the end of the lake, it was a very nice tour. The way back on the lake was unfortunately not so tingling, since everything only asphalt road. Since you should look for another route.
The dam Oleftalsperre is located near the Eifel National Park near Hellenthal in the nature park High Venn-Eifel in the district of Euskirchen (North Rhine-Westphalia). The reservoir covers about 20 million cubic meters and is fed by the river Olef.