Google Maps are really quite funny - even if the images are sometimes unfortunately a bit thin in resolution. Here is the satellite image of Münster. The long blue blob is the Aasee - an artificial reservoir of the Münsteraner Aa created by Hermann Landois. The strange green ring in the city is the Promenade - a remnant of the old city fortifications and one of the main traffic routes for cyclists in Münster. The green outgrowth on the ring to the left, above the Aasee, is the Schlosspark. The blue strip from top to bottom in the right area of the image is the Dortmund-Ems Canal - the widening in the upper area is the lock system and the branch to the left into the city is the Stadthafen.
And as usual, I realize that a) I take too few photos of Münster and its surroundings and b) Wikipedia contains impressively many things.