oh wait on does your river have steep sides?....
the reason i ask is that at a certain angle on the terrain layer if it is like 60 degrees from neighbouring floor poly then sometimes if you go prone you can LooK through the walls, i wonder if somehow you can exit near a (almost) vertical terrain wall and slip through the terrain layer that way?..
Also crazy left field thought.. your terrain texture doesnt have alpha layer holes in it does it?... maybe retexture a test area and add water volume and see if that works?
anyway just the random thoughts of a mad mapper
You shouldn't even be thinking about such tinkering and instead improve your terrain so no one ever falls through.
If anything I wouldn't make the area dark but I would make the area damaging to players so if someone glitches through your terrain he gets killed and can respawn somewhere in the map again.
If you need to make it so players can't see turning off the lights won't do as they can still see what's going on where the lights are still on (ie. the playable area!). You could place a huge volume under the terrain which a) does damage to players and b) has black distance fog with a very low view-distance. Where it cuts off the fog you would still be able to see the skybox, which would look extremely weird but at least players can't see anything and if it looks weird it doesn't matter too much as it's a glitch anyway.
As I said, you should bother with such tinkering!
If you need steep slopes you are better of making StaticMeshes for them. Not only because this solves your problem of players getting too close to the edges of the slopes as the StaticMeshes are blocking them, but also because you can make it look a lot more realistic that way.