Sounds strange, but conside using terrains for the lake surface as well, can give more motion to the water, depends on the lake, I've seen some where you get a strong centre flow through the lake and others that are flat as a pancake. I also tend to go with Muddygrubs style of multiple terrains to make the surrounding banks rather then a single terrain.

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Toolset: Blender, GIMP, Indigo Render, LuxRender, TopMod, Knotplot, Ivy Gen, Plant Studio.