The Hengsteysee is a long, narrow reservoir on the Ruhr between Hagen, Herdecke, and Dortmund, formed in 1929 when the river was dammed for hydroelectric power. The lake is now a recreation area with a paved 10-km cycle and walking loop, rowing clubs, a sailing school, and a popular lakeside beer garden.
The hills on either side rise steeply, giving the lake an alpine feel that you would not expect on the Ruhr. The Hohensyburg ruins and the Vincke Tower on the northern shore both make worthwhile stops — the tower offers a long view over the lake and the city. Easy half-day combination by car: start at the tower, walk a section of the lake, end at the beer garden.