But... look at the ground and you'll just see a flat greenish noise texture. They try to cover it with clumps of grass and vegetation, which works to a degree, but not completely.
Look at the water flowing - particularly on any kind of whitewater rapids/waterfall - compared to the character textures and the buildings, it's really quite disappointing.
You need a fairly complete simulation to really fool the human eye and we're a long way from that.
As for a "VR" river, there's nothing wrong with filming a real river with a 360 degree camera, and then augment that scene, rather than "building a river". A real river on a virtual Mars, might look better than a virtual river on a virtual world.
Of course you can dream up a game that hard to bring to life exactly as you envision it using current technology. That doesn't mean new technology doesn't open up a few new possibilities. If you run into problems thinking big and complicated, try thinking smaller and simpler. I'm sure a Populus style game would be a lot of fun with the DK2 (isometric sandbox god simulator game with slightly cartoony graphics). Why does it have to be realistic?
I'll bet dollars to doughnuts that even twenty years from now, by far the cheapest way of creating a completely photorealistic VR river scene is to build the actual river and record video of it. Then you "only" run into the problem of storing and distributing the petabytes of data you record.