I don't know if you've seen maps of Paris or Warsaw, but I can assure you, the Seine and the Vistula are not drainage ditches.
Why the hostile tone? You know as well as I do that, in the case of Warsaw for instance, the Germans would defend the western half.
Or in the case of "generic western european city", the Maus could be brought by rail in and just stay in its alloted districts. No need to be all condescending just for the sake of it.
EDIT: Again, this isn't meant as an argument in favor of building the Maus, but rather a "this is the best-case scenario for it", and it still sucks