My underground river is the only water on the map, and it happens to be a vertical river.
There are maybe 12 tiles of moving water total before it plunges into the depths. It'll be a heck of a challenge tapping it for a water supply.
It's not really a vertical river. Underground rivers stretch more than 1 map square, like chasms. Using the finder and specifying "underground river" amongst other features (HFS, chasm, etc) means it's very likely the finder will pick out a region containing only 1 square of underground river. Which also makes the underground river very hard to find when you embark.
This one really is just plain vertical. It starts out mysteriously at one point, and the water travels a horizontal 10 or 12 tile distance, then disappears into the depths.
I used reveal to see it. Want to see the map? (warning, hfs on the map, don't poke around too much or you get spoilers)
http://www.mkv25.net/dfma/poi-11321-undergroundriver
Yeah, you got a one tile river. A decent find since it allows one to use a smaller map and still get both ends of the river.
You all aren't thinking big enough. Trade depot in the shaft the river falls into. Needs to be VERY carefully planned out, and you'll need lots of hatches, grates, doors, floodgates, mechanisms, floor tiles, bridges, and dwarves, but the end result is an easy to use trader drowner.
Last time I tried it, I had the very bottom z level covered in grates to pass the water, next level up was the depot with hatches on 3 sides to pass the water normally and start filling once the traders arrive, then another layer of grates one level below the river. If it weren't for the loss of the primary work material, dwarves, I would have had a nice super drowner that quickly filled a massive tube above the traders, and they would then drown. I was actually thinking of designating the top grates as a meeting zone so the dwarves would get to watch, but then I was reminded of dwarven stupidity regarding water.