Theres a shot in the behind the scenes stuff on the Star Wars Episode ONE DVD of one of the ILM compositors doing this very thing.
Check it out!
He basicly explains how he maps waterfall footage to curved planes positioned over the source footage combined with particles and displacements.
Took me in directions I never would have thought of.
Also, if you are deadset on using ALL particles for your shot, you might wanna try adding a turbulence field, AND a drag field to the lip of your waterfall.
(the part where it curves downward)
Set the fields volume influence to a cube, and scale it to cover the horizontal length of your crest.
That way, you'll get better accumulation at the crest of the waterfall where waterfalls normally, (in SOME cases) get foamy, and misty and held-up before they begin to plummet to thier doom
Just another idea, but for reference, check out that DVD!
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS