Oh my. After my first fortress worked out pretty well (it's going strong eight years in, doesn't seem particularly likely to fall to any sieges, and has no other real challenges besides building impressive things), I've moved on to a second--which, as you may have gathered, has the three things in the topic. I've got a few questions, though, for clarification on how exactly things work.
My location has a small brook waterfall (drop of 1 z-level). The muddy brook at the other end is underground. Would this little section be my underground river?
There's that issue with pressure for waterfalls. Look at the diagram below and tell me if I've got things right:
####### <--Level 1
# # <--Level 2
###~~~# <--Level 3
####### <--Level 4
#s are land, ~s are water. Level 1 is ground level, where the open space above the original brook is. Level 2 is the open space above the underground brook and 7/7 of the original brook. There's also a tunnel nearly into the open space of the underground brook. Level 3 is 7/7 of the underground brook. Level 4 is just rock.
If I open the tunnel on level 2 to the brook, will it flood? (I think the answer here is no).
If I channel from level 2 down to level 3 to access the brook, with the tunnel on level 2 flood? (I think the answer here is yes).
Can I tap into the above ground brook, drop the water down a level into my fortress (from level 1 to level 2) and then route it so that it dumps into the brook at level 2 (via tunnel?)
If I do this:
######## <--Level 1
xx # <--Level 2
##~~~# <--Level 3
######## <--Level 4
with the X's being a pump and the empty space in level 3 being a channel, will the tunnel at level 2 flood? (I think the answer is yes)
Would it be easier just to build a waterway and dump the water into my chasm? Will flying nasties be able to get into the fortress that way?
My magma question is a lot simpler. Fire snakes spawn in magma biomes. Does that mean the original magma pipe area only, or anywhere I route the magma?