Lake Kettwig is the smallest in the chain of five Ruhr reservoirs. The weir blocks the Ruhr by six meters. The resulting lake has a content of 1.4 million cubic meters and a surface of 0.55 square kilometers. An important motive of the Ruhr Association for the construction of the lake in 1950 was the shooting of the Ruhr pump-back chain and the creation of safe fairway conditions on the river. However, like the other Ruhr reservoirs, the lake should also serve as a biological sewage treatment plant. Even today, microorganisms in the lake provide additional biological purification of the water.
Overflowing weirs and, since 1995, ventilated turbines have made a significant contribution to improving the Ruhr's oxygen balance. As a result of this measure, there has been no fish killing in the lower reaches of the Ruhr for many years due to a lack of oxygen. Eels, carp, tench, perch and pike are therefore in excellent health.