I don't believe (going off my memory because his posts were edited) he was doing anything with any special/unique feature, he was just using the river object in a clever way.
If you have the river's path follow the flow of the waterfall, and then add particle effects to the top, along the body(where the water stream diffuses somewhat) and at the swell at the bottom, it can provide a fairly convicing waterfall effect. That's for shorter waterfalls, though. The taller the waterfall, the more the water diffuses into a mist in the air(extremely tall waterfalls just turn into an actual mist before it really hits the bottom, for example) so part of it is just deciding the artistry/effect you're wanting to go for in your scene, but there's no special trick or voodoo with this.
Other ways to do waterfals are special meshes with animated textures that scroll across the water surface. You can see some examples of that in play if you grab the Pacific art pack: http://ghc-games.com/SC/Pacific_Package.rar
and look at the waterfall meshes that are there. You can see it in action in the level(if you drop the pacific package's files on a Full template directory, you can then load up the pacific level and fly around it) as well, which also uses some rivers in conjunction to the waterfall meshes to create a fairly convincing look, so that's probably worth checking out as well.