assuming that you're using Max, but these work with any package assuming you're using Mental Ray.
personally, you can't go wrong with RealFlow. I used it for my thesis and it's super quick to pick up (especially the fluid simulator), and exporting it to Max is a breeze.
if not, i'd recommend one of two modes:
HEAVY DETAIL: model and texture the riverbed, then boolean the water so it's a solid sitting on the water. if you look on jeff patton's site (
http://jeffpatton.cgsociety.org/blog/archive/2007/1/), he has a great caustics water tutorial which when using max2008/2009, is super quick to get fantastic results. You can have caustics, refraction, the whole shabang.
LIGHT DETAIL: model the river bed, and then put in a plane for the water surface. You can then just use the arch&design water material, and set it so it's more river-like (default is ocean).it's fully animate-able and with a little tweaking you can go from silt still water to a gushing rapid. and if you really want to get nutty with it, you can add noise modifiers to tweak the plane before hand.
hope this helps