This is very cool. Atmospheric and kind of peaceful.
The water has some very messed up culling for me though. Not sure what criteria it uses. When i look at the base of the waterfall or behind it, i can see the water, if i look further downstream it disappears. This kind of stuff happens with shader stuff in my experience.
The rock culling was also a bit messed up. They disappear when they are partly off-screen.
Otherwise really cool. Love the rain effect. I tried something like that back in DBC, but the framerate was appalling. Are you using plains, cylinders or what? I'd quite like to nick that if possible.
Framerate was pretty low on this, but seemed fairly smooth. 23fps mostly, but if i looked at a whole bunch of trees, it went as low as 13. I have a fairly old computer though.
Also, it keeps fixing the mouse position. I went to another window and would have been completely stuck without alt-tab or ctrl-tab, and i don't think your average joe knows how to do this.