Red circle shows diagonal bend. Top leve has a lever to cut off the power supply. It is off in the image. Because more water is needed to keep flow in the upper level, the bottom is level is wider to hold more. When it is on, before the bend is usually between 4-6 units deep, after the bend between 1-3. water falls down intermittently, but there is a constant supply of mist. I'm not sure if the mist cleans dwarves or if they need to actually get hit by the 1 unit block of water to get clean.
My current setup was sort of shoe-horned in, so its far from optimal. Ideally I would put another pump on the other side, and then expand the main walkway to 3 tiles wide. The top level has a larger-than-necessary U bend in it because of where I had to place the pump, I think it could be done much more simply by placing the pump(s) one tile south from the actual waterfall zone. I'm considering building a new main dining hall now that my fortress is more mature, in which case I'd take what I've learned here and built wider and more efficient. Aside from traffic jams in the 2-wide hallway and the occasional need to toggle the pond filling job on, this appears to be working just fine.
If I get a nicer setup built I'll post that as well.