The waterfalls needed to conform to the underlying rock, so I decided to rely on vertex painting to "stream" the water flow, and also to push it outward around the rocks as it cascaded down.
The mesh for the waterfall was therefore fairly dense, and the vertex colours allowed me to paint in opacity and position offset.
Although this worked okay for rolling down rocks, getting a natural looking curve where the water was seeming to bounce off the rock and then arc downward was a bit tricky using vertex painting, so I made a second, thinner mesh which was already arced.
The overall effect ended up much less dense-flowing water than I had planned, but I actually think this is a little more practical, and I've warmed to the way it looks now.
In the case of the enourmous waterfalls in the vista, I felt I needed them to block light to the rocks behind them, so I made a material which allowed me to paint blackness, which looked like moderately convincing soft shadows.
At the bottom and top of the waterfall, a cloudy mist of water is created using a particle emitter. This was particularly hard to tone-adjust for night because the particle system was already adjusting the vertex colour in order to fade each particle over time, and UDK's material instance control through matinee (which is what I've been using to adjust the amount of light on materials so far) doesn't seem to play nice with emitters.
I was able to find a workaround by allowing matinee to control a variable on an actor (i happened to use the brightness of a tiny out-of-level light, but I could have used anything). This is read ten times per second using kismet, and applied to all emitters. This was a pain to set up and is messy and probably highly non-optimal, but I really couldn't stand such bright water mist at night!