Thanks reason for the quick replies.
@hyndis
Just a bit confused on the channel bit though. Stairs? for a moat? Like, a ramp?
@reasonable
Well, i was digging a moat, and to get out of the incoming water, my dwarf had to swim its way out through the river. When it dug out the last portion, does it drop into the same level as the channel?
Oh, digging out the last tile to connect the trench to the liquid. You can do this simply by digging a channel from the Z layer above, or if you want you can do it with regular mining from inside the trench. This preserves the ground above if for some reason you want to preserve it.
Usually the easiest way is to dig out the entire moat/channel/trench/sewer and leave 1 square undug. Then dig out that final square from the layer above and you don't have to worry about losing dorfs to water or magma. You can build a floor over that tile to seal it off again. You can also build/carve a fortification inside the conduit to keep out swimming critters. I dont think it will work for vermin, but it definitely works for goblins and other large creatures. Fortifications also cannot be destroyed, unlike bars/gates/doors/floodgates.
For water flow control, its always a good idea to install a floodgate just behind the fortification, then connect it to a lever. You may never use it, but its always a good idea to have an off switch just in case.
Here's a diagram, viewed from the side:
XXXXXXXXXXXXXXXXXXXX....[channel]......[brook].....
[floodgate] [fortification] [water] [water]
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
Install the fortification first, either by simply carving it from native rock, or by constructing a wall and then carving the wall. Or, more directly, build a fortification outright from the same menu.
Behind that, build your door/floodgate. Connect it to a lever. Then once thats done, channel out the square just infront of the fortification. This should be the last undug square in your whole network. By opening the floodgate/door and channeling that out, you'll introduce the liquid into your system with complete safety.
Oh...and magma will melt most things. Water, however, is much safer.