Er, sorry if I'm being bothersome, but I think my questions were skipped... they're a few posts back. Basically:
1. I don't have any rivers or magma pits. How can I use my bridge to kill people (read: friggin' migrants) more easily? (Crushing is difficult - it's so small that I only have one tile long crush-space, so it's hard to lure people in)
2. Why did my migrant die of thirst while drowning in 4/7 to 5/7 water?
Also, I have another question:
3. How can I make a decent waterfall without access to rivers? (I do have murky pools though)
1: Make several bridges 1 tile long and however many tiles you need it to be to span the width of your entrance hall, and build several of them in strips along your hall. Like this:
##|===|## <--bridge
##|===|## <--another bridge
##|===|##
##|===|## <-more bridges
##|===|##
Link them all up to one lever. When they close, the bridges will all become solid tiles and crush anything on them. No chance they'll survive it, unless you let some slip past. You can also make a retractable bridge over a deep pit.
2: You probably wouldn't think about drinking if you were drowning, would you? Or are you asking why he didn't drown? Dwarves will
prefer to swim in 4-depth water, but it's not drowning-depth. Dwarves will only drown in 5-depth water or higher, and they only have to be in shallower water for a moment to get a breath of air. Your dwarf might have gained some swimming skill by thrashing around in there too, which will allow him to stay afloat in drowning-depth water. Dwarves only need novice skill in swimming to do this so it's not entirely unlikely. I suggest you add more water and build a floor over the pool - IIRC dwarves can't breathe in drowning-depth water if there isn't open air above them, even if the water isn't completely full below it.
3: If your map has plenty of rain, water should be a renewable resource. Evaporation is the enemy here though, never let the pools get below 2 units deep or water can start to evaporate in the pools. Once that happens, rain might not be able to replenish them fast enough before it evaporates. I suggest you channel around a pool until the water spreads out a little and reaches 2-level depth, wait for it to fill up, and then repeat until you have a large pool. This will be your buffer zone. The bigger it is, the better, and if you can find a way to make it deeper as well as wider, all the better. Again, never drain it below 2 units. Pump the water into a cistern whatever size you need it, if the water in your buffer gets low you'll have to wait for it to rain and pump some more. The cistern will most likely have to be slightly smaller than the buffer to compensate for the water left behind in the channel, otherwise you won't have enough to fill it to 2 units. Keep the cistern as full as possible at all times It's kind of a pain but it should work. Once you've done that dump it out of the cistern whenever you need to.