I have no idea what the slider does either. It's something to do with noise, but I can't imagine what. It was one of only 3 things I've ever used the manual for, and, like the other 2, it wasn't really mentioned. My advice is forget about downloading textures and make your own. It takes time (I can change a texture 100 times before I'm happy with it) but at the end of the day you'll get a result tailored for exactly what you want and... it'll be all your own work. Try using an altitude dependant mat on the trees, so there's green, slimey moss at the bottom, graduating to more bumpy, darker bark further up. I've just used this method on a wall in a render that's working away ATM and it works well. You need to reduce the reflection on the water too, it looks like mercury. Try a bit of transparency. If you've got CPU to spare, try using 4 or 5 identical water planes with 100% transparency and 110-ish diffusion, stacked one just underneath the other, so the water seems to get murkier the deeper it gets, whilst at the top it's still transparent. It's hard to do, but you can get amazing results (and render times too!)