I forgot to take into account the fact that water physics are very hateful. Tried building a well from a brook in my dining room, and, thinking that the water would simply stop at the end of the channel which had a hole leading up to the dining room where I wanted the well, didn't bother placing a floodgate.
I also happened to have my entire food and booze supply on that level. It all got washed into a corner, then the water moved along and began flooding the lower levels. (This is an early fortress, it seems I was trying to take on a luxury too early [Before I had doors sealing separate levels])
A dwarven child that came in with a few immigrants was in his room when the water began flooding the living quarters, which stupidly enough I had placed on the bottom level of my complex. Fortunately, I was able to forbid the door before the water flooded in, and though I had no plan for how to free him I supposed dying of hunger was less torturous a death than drowning.
At this point, I had started making a burrow on the highest level of my fortress, so that I could seal off the lower levels, to avoid having the dwarves drown. With a stroke of inspiration, I formulated a plan: Dig a stairway down below the living quarters, create an enormous basin, then open it up to the fortress to drain it for long enough a time to seal up the well. Out of sheer boredom (or want of fun) however, I built the stairway before the dwarves really finished the room.
At this point, several odd things happened. One, the miner who built the stairway was pulled up from where he built the stairway, putting him in the flooded living quarters. I had lost faith in the fortress at this point, and I decided to open up the door of that poor dwarf child who was trapped.
After that, I went off to go look around at how much water had drained from the upper levels. Unfortunately, the room I created was only large enough to drain one and a half levels (the second level being lowered to 5/7 throughout). At this point, the stupid miner had, for some reason, been unable to swim back down the stairway, despite the fact that he could swim directly over it. He subsequently drowned. Yet oddly, I noticed no dwarf child's corpse. Turns out he somehow managed to get down that stairway, and when I looked him up he was on the top of the plateau my fortress was in, going to drink from the same brook that had just imprisoned him.
All the housing rooms, except that dwarf child's, were intact and water-tight. I take this as a lesson in the importance of z-level isolation, and the power of rivers.