I've been preparing a lame-ish lava flood defense for a couple of in-game years (interrupted every other season by seiges). I have a channel that wends its way to the front of my fort where several channels outside would keep it fenced in once I prepare a floodgate and unleash FIREY DOOM. That's the plan, anyway.
There are 3 floodgates on this channel:
- one halfway from the lava fields to the chasm to keep pesky magma men from coming down and destroying my carefully planned defense.
- one a little past the chasm aquaduct for redundancy
- one connected to the cave river via channels
Now, I had put the last one down in order to initially flood that channel with water. It would allow me to build stone bridges instead of having to reinforce with steel (of course, logic rears its head now to tell me I could've done the same without it and closed the 2nd floodgate instead, go me!). However, I had forgotten to cover all sides with channels so that no rampant flow would stick around and keep my channel flooded. Meaning that once I closed that floodgate, it didn't drain. :/
So I steam-cleaned it by opening the 2nd floodgate (after preparing my bridges). This purged all the water. Except that now those same channels would not transmit the water from the cave river to the 3rd floodgate. I have had to dig a new channel (and drown another stupid dwarf) to get water to run down the lava channel.
That's not all. At one point, during a seige, I had inadvertantly trapped some lava in between the 3rd floodgate and the cave river. Because lava flow still existed on both sides (from the floodgate one-tile flood), the empty channel code it generates when a floodgate closes had nothing to work on, so nothing happened.
Here's the crazy part. I re-closed the floodgate and it sent the "empty channel message" down each individual channel it was connected to INCLUDING the main lava channel. My lava was draining away... all the way to the lava fields, leaving me with a useless channel in the midst of a goblin siege.