The Great Pumpkin's aid, aka Daddy's Little Ninja, got the the 6th World almanac for Halloween and looking at it i noticed in western Germany near benelux there's a large body of water. When did that appear. I don't remember anything like that in RL Germany.

Hm... i don't have that map here. But maybe is it part of the Eiffel? I vaguely remember slight volcanic activity there. Maybe some existing body of water got deepened/broadened by that. Maybe the Ruhrstausee?

That's not a lake, If you take a look at the legend section of the the full map, you see that color is used to mark noteworthy areas.
Bodies of water are marked with much darker blue.

ah, ok. I thought there'd been some other sort of disaster. we've done runs in France and the low countries and Switzerland but I haven't known too much about Germany. kind of west of the Rhine

It's probably the Rhine-Ruhr Metroplex.

It's Rhine-Ruhr alright (it's even labeled as such).