Lake Wedau in Duisburg (Germany)

Wambachsee, Masurensee, Böllertsee, Wolfssee, Wildförstersee, and Haubachsee are the names of the bodies of water that make up the Sechs-Seen-Platte.

https://www.duisburg.de/wohnenleben/wasser/sechsseenplatte