A question of textures

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Maybe you could also simply create a nice effect with the uv animation in the material editor? Lava isn't exactly translucent anyway and it doesn't really have waves and ripples like water either as it's much thicker and viscuous. So just make a cool glow, diffuse and normal map and have it scroll so it looks like its slowly flowing in the direction of the river. Of course the uv coordinates of your river mesh need to be laid out correctly in the direction of the scrolling or it will look weird. You could also make things like lavafalls with this kind of material....

Not sure, maybe it would look crappy but I'd at least try it out.

Edit: Damn, ninja'd by Traggey. -.-