Well, I just ran into my first cave river, and I am sorely disappointed. The long list of unseen dead critters, including cave crocodiles and olmmen, really set my expectations high. But when I finally found it, there was nothing in there but a couple of cave fish. I didn't even find any corpses. I guess they all fell into the mini-chasm that is the river's exit point.
Additionally, there is no dry land whatsoever in the river's cavern, so I guess I won't have an underground forest either. And my chasm (which was also hidden) is quite sparsely populated, too. The biggest threat my dwarves have right now is a single magma man that emerged from the magma pool I ran into. The only local threat left is hidden fun stuff, but I haven't started digging it out yet.
Anyway, is this the way cave rivers are supposed to be? Or did I just have bad luck and had one with a chasm that all it's inhabitants fell into and died?
I'm really thinking about abandoning this fort due to lack of local hostiles. I specifically searched with Regional Prospector for a place with plenty of baddies to keep me entertained, and now I'm sorely disappointed. The current fort doesn't have any iron, anyway (at least I haven't found any).
But before I embark again, I'd like some tips on finding features that are rich in bad guys. For example, do I need to select a cave river that has no ends in my fortress' local area, or will I lose frogmen even if there is no chasm for them to fall into?