Just a waterfall model with an animated texture, and some particle effects at the bottom? The map Sawmill already has a waterfall, you might want to check that one out.
Reading about the "current" system, I just had in my mind that you were going to have a long river that would spawn from the waterfall and eventually drop off a cliff. Logs would roll down the river at slow speed and would change the gameplay because you can only cross at certain times. If anybody was still standing on the log after it reached the cliff, they would fall off and die.
You could create a water brush in Hammer for the river, and have a "push" brush inside the water brush to push players downstream.
Looking at your map, it doesn't seem like you're going to do that, but that's just an idea.
@Akrid: The mapping section of the site has some really helpful people who know what they're talking about. Since this is your first map, I would recommend you to go check it out even more. They've got a lot of threads that will help you understand Hammer better and, at least in the mapping sections, a lot of the people are pretty helpful.
Also, water is only a problem if it's moving. As in, the actual water brush is moving. You aren't going to get away with any sort of flooding effects, but a stream is going to be find, and a waterfall should be fine as long as it's just a model.