Well, the river actually flows into a casm, my idea was to somehow block the flow causing whoever can't be on land to die, and I'm sure my hunter I got with migrants could take care of the rest with help of my miners making stair cases.
Edit: I see a spot were I could mine and cause the flow direction to change, allowing me to build a block in the main channel, then I could use a mechanism attached to a door in the alternate channel to stop flow completely.
Edit: Yay, my clerk was hauling rose quartz and was struck by a few mood :D
Edit again: Ok, I got hit with another group of migrants bringing my fort total up to 37, I'm having the hardest time actually getting my fort inside, currently all my stockpiles and workshops are outside. I guess my miners get to be just that for a good long time, and the bunch of dwarfs that I don't need at the moment (weavers, metalsmiths, bonecrafters and the like) get to be stone haulers making room for inside storage.