Well, so I was building a fortress the other day, next to a river/water fall. It was going along pretty nicely, I did have to seal the fortress to get away from the orcs, however. Just an underground bridge and an underground moat, nothing fancy there. Anyway, They left, and they aren't my problem. because I know all dwarves have an unusual love of mist, and I like the idea of a dwarven shower, I built some some grates and some floor near the top of the water fall, so my dwarves could clean themselves.
Now, I had TONS of carp in my river, so I thought, "Hmm, how should I get rid of these guys?" and I came up with a device that would allow my dwarves to drink fresh water as well as channel the river slowly off the side of the cliff, into the river below.
My carp were no longer a problem, as the closer they came to my fortress, the more likely they came to being air-drowned and eatten by my cats, under my fortress. So, carp, not a problem anymore.."Hmm," I thought to myself, "I think I should increase the water flow in my showers." So, I enclosed my 'cistern' with walls and stuff, so no more flow off the other side of the mountain. Before that, of course, I dammed the waterfall, so there was no water at all, flowing through my showers.
When I finished that..I checked out everything in my fortress, and everything looked OK. However, just as I was about to open my shower 'floodgates,' I was interrupted by real life stuff. How boring. I came back, to see my fortress..Almost completely covered. The only reason I had any rooms OK was because I often made 'balconies' for my dwarves, which water was flowing out of, down into the river below. As soon as I saw this, I didn't know what was going on. I couldn't find where the water was coming from, the river wasn't flooding, No doors were open.. I decided try to open my 'floodgates' to see what would happen, if not just for the sake of fun. When I opened this, water generously flowed down into the shower area, washing over the dwarves that had just died a second earlier, from thirst. After a while, and a few dwarf deaths, (I had 6 left, 5 if you don't count the crazy one.) The fortress seemed..OK. The water was slowly evaporating or pouring out the edge of the fortress, and I had plenty of food, as that room was barely flooded.
I still could find no reason how the water flooded my fortress, but I didn't care anymore. I just got a migrant wave! My fortress was saved. I channel the 'cistern' into the side of the mountain, as a celebration, and then.. VROOSSH. Water came speeding out of that cistern rocket speed. Lots, and lots of water. So I guess it was water pressure, that..somehow built up in my cistern somehow, with the use of no pumps whatsoever, and no river overflowing above my fortress. But I'm not sure..so..do you guys know?