OK. So here's a picture of my fortress' demise.
Yay devastation...
So the cause of this cataclysm appears to be the big diamond room I made upper right. It was really late last night and I completely forgot about the 7x7 rule because I wasn't making a normal square room. Lesson learned.
I got the message "You hear a distant rumbling" or whatever, and before I could react, there was water in every room west of the underground river, despite several doors in the way. No water east of the river, and no obvious room collapse, despite of what I figured caused this whole thing anyway.
The huge blue ripples of water coming out the front doors are amusing, also.
So it seems a little buggy that a simple room collapse deep in the fortress would cause the entire area west of the river to be completely flooded. The well in the room wasn't completed, btw. And the flood didn't go away, either. It drowned everything and lagged down my computer (3.4 GHz) in what seemed to be an attempt to calculate the various directions of the water bouncing in and out of rooms.
I'm posting it here because I assume it isn't normal. And unfortunately there's no save game for you, since it bogged down the game so much after a few minutes that I had to crash the game myself.