If you're on an older machine and concerned about "FPS death", then you might consider Niccolo's tip on going with an aquifer instead of a river. Rivers tend to be more machine intensive.
If you're worried about safely digging your way through an aquifer to build your fort, the
DF2012:Aquifer page gives a lot of detailed solutions. At least two of these methods have step-by-step illustrations.
Alternatively, you may find an underground source of water. I've read that if the water happens to touch a map border/edge, then it should refill indefinitely.
On FPS and system requirements:
According to the wiki (under
System Requirements) and comments on the
FPS issue on slow PCs thread,
RAM Latency is the most significant hardware factor:
RAM doesn't really have much effect on DF - it rarely takes up more than 1 GB at a time, and that's during worldgen.
From the wiki:
The most important thing to the performance of the game, however, is undoubtedly RAM latency—the amount of lag the RAM has when working. Dwarf Fortress works the RAM every single frame for every single creature, every single item, every single piece of liquid, the temperature of every tile—you get the picture. The gigantic amount of operations working at the same time—which any current processor could handle much faster than what you see—is primarily bottlenecked by RAM latency and RAM speed.
CPU and FPS are mildly correlated, but this correlation has been attributed as a non-causative one. Rather, newer CPUs seem to come with faster RAM, but for the purpose of a Dwarf Fortress computer, RAM is more important.
RAM speed is definitely more important than multiple processor cores as DF can't take much advantage of that.
For more in-game tips on maximizing your FPS, see the
DF2012:Maximizing framerate wiki page.