If you're trying to render a relatively tight shot of the riverbed; I'd recommend using the surface replicator. In a nutshell; model the rocks and boulders first; then create a terrain in the basic shape you're after. Use the surface replicator to disperse the rocks on the terrain, all nice and random.

Carrara's terrain editor won't give you that much detail up close. If you're new to Carrara; Mark Bremmer's tutorials cover the process and are well worth picking up :)

If you have some pictures of what you're after; you could also convert them to grey-scale and make displacement maps as an alternative. (You may have to actually model the terrain in that case)

JT