Sorry to have taken so long, you guys!
Well, anyway, I might as well get around to the story now.
So, it started after a bit of wiki-fueled updating and base construction. Being better at the game than I was previously, I quickly began constructing a base in a great place I'd found. Shallow AND deep metals, flux stone, a brook, clay, no sand though. After a lot of digging out, I was creating an enormous main area to serve as the base. About five or ten Z-levels down, before the chalk ended, I carved out numerous locations. A hospital, a barracks, the mayor's and broker's offices, the office for the future captain of the guard, a dining hall, and an enormous set of storage rooms with their own enormous workshop. Gotta hold all that chalk somewhere! The last thing I needed to complete my base was a cistern, so I got to work making a well and hollowing out a reservoir beneath it. Floodgates were constructed, levers placed in the common room where the well was, and a backup emergency release valve was created so I could drain it all into the caverns if anything went horribly wrong.
A few issues I had with that were that the emergency levers were all located in the room that the well goes to, and the cistern is fed by the brook.
The funny thing is, for quite some time (I was watching actively) water pressure didn't kick in, and the cistern only filled up to the point which the channel leading into it was located, and above that there was little or no water.
I, thinking that pressure would be nothing to worry about, decided to hop onto the wiki for a while and read about some interesting topics.
Some amount of minutes later, my dwarves were drowning, and an enormous flood threatened to kill off my fortress.
!!Fun!! times!