The Baldeneysee is the largest of the six Ruhr reservoirs. It is located in the south of the city of Essen between the districts of Werden, Bredeney, Heisingen, Kupferdreh and Fischlaken. The first plans were made in 1927 by the first Ruhrverband managing director, Karl Imhoff, who was supposed to make the Ruhr cleaner again. He planned the Baldeneysee along with other Ruhr reservoirs in order to strengthen the self-cleaning power of the river. The flow rate is reduced by damming, so that suspended matter settles. This enabled the Ruhr to clean itself naturally through sedimentation and the breakdown of impurities from microorganisms and algae. Today, this task is primarily performed by numerous sewage treatment plants. At that time, attempts were made to ensure the supply of drinking water despite increasing consumption and deterioration in quality due to industry and population growth. The already successfully built lakes Harkortsee and Hengsteysee confirmed the project. So between spring 1931 and March 1933 a Ruhr weir was built in Werden. The name Baldeneysee comes from the fact that initial plans included the weir at the level of Baldeney Castle in the Baldeney district.