So, after seeing Baboonanza's giga dam in his 3d Visual Fortress thread, I sought out a good site to build a dam. Since water won't back up past the fortress's tiles, and the rivers in DF don't seem to have a gradient, I figured the best solution was to find a suitable waterfall in with a long gorge.
The forum is surprisingly sparse on natural waterfall sites, so I slodged through and found a fun little area that suits my needs, after some modifications:
The area I am excavating needs to be removed to join the gorge and the depression. I'm going to dig the whole reservoir down to a tapered cross section so it can fill faster (less evaporation) and to expose bedrock to mine for the damn.
Other than a massive hydromechanical power plant, any suggestions on what I can do with the amount of water this will create? I DO have a magma pipe, so a mass-scale obsidian industry strikes me as a possibility, plus the absurd amount of fish this will create in the spring may spawn a sizable prepared meals industry.
This map freezes, BTW. With more work after the first dam is finished, I can enlarge the original waterfall's gorge and make a second reservoir and dam or expand the first with a second dam. The fortress area you see now is temporary; when I get some more laborers I'm going to start building a proper fort with the necessary equipment and form to work with the dam.