I thought I understood how water works in this game, but... maybe not.
I have a brook on Z=147. I dug a tunnel to it, with a diagonal jaunt (which is supposed to remove pressure) into a 90 degree turn, then a three-level drop (through a staircase) down to the center of a reservoir on Z=144. I put a floodgate at the end of the tunnel (right by the last square of soil that I'd have to remove to breach the brook), linked it to a lever, then opened it. Then I channeled through that last square. When the reservoir was full, I pulled the lever again, closing the floodgate. So, there should be TWO things stopping the water from having pressure: the floodgate and the diagonal path in the tunnel. Right?
On Z=145, right above the edge of the reservoir, I channeled three holes for wells. This is my first year in the fortress, so I didn't have the supplies to build all three yet; so I built one well in the center hole and left the other two open.
Then I dug out a space for underground farms, and a matching space above that, with channel in the upper space for bucket irrigation. The upper level is very near the well(s).
Once that was dug, I designated 4 pond zones for the first farm plot. Dwarves grabbed buckets and headed to the wells. Next thing I know, two of them are in the reservoir (with their buckets) directly underneath the open holes (not the well in the middle), and there's a flood of water all over the well "room" on Z=145 above the reservoir. The tunnel on Z=147 (brook level) is missing some of its water. As near as I can tell, some of the water was sucked upward from the reservoir into the well room, and is now being replaced by water from the upper tunnel. And two dwarves are in the reservoir.
Did the dwarves jump into the reservoir because the single well was in use, and their bodies displace a whole bunch of water upward into the well room? Or did the use of the center well somehow cause the reservoir to gush upward into the room and knock the other two dwarves in the open holes?
Will I ever understand how to avoid premature aquatic fun?