The Hengsteysee (Lake Hengstey) is a reservoir on the Ruhr river between the cities of Hagen, Dortmund and Herdecke, North Rhine-Westphalia, Germany. It was built in 1929 and is one of five reservoirs on the Ruhr.The reservoir is about 4.2 km (2.6 mi) long and has an average width of 296 meters (971 ft).