Aymenk: I like to use a plane object for water instead of a lake, for a couple of reasons; The plane can be scaled much easier than the lake, on either axis, it can also be rotated.
Long, thin rivers, imagine the example, would be better rendered as a long thin strip that covers only the area it needs to, not a massive disc of sub-terrain water that needs rendered but is never seen anyway. The lake wastes a lot of resources and time, in my opinion.
I do have a method for completely masking out sub-terrain water with a painted shader and the opacity channel of a default shader, you have used it here, I believe...
http://forums.planetside.co.uk/index.php?topic=6863.0. You'd have noticed that no water rendered beneath the surface of your terrain? This one actually uses an image map for opacity because it was made before the painted shader was released, simply replace the image with a painted shader and paint your own river in the 3D preview to keep the entire thing TG.
Most times, though, it's easier to just use a plane.(you can also mask this, so it makes no difference, really what you use, I just prefer a plane.
Also, I'd previously imagined the lake was responsible for the train-track/fanlines in water scenes. This has since happened to me with the plane aswell, though. It seems to me now, to be repeating fractals that makes it happen. This is helped by simply checking 'microvertex jittering' in the render node. As advised by Planetside.
My terrains are planes a lot of the time, aswell. I like planes.
