Quote -
Thanks Eonite, I didn't know that but it is good to know 16 bit is that much better. I am achieving my river using the Function Editor. I used an image node. Is this what you mean or is there another method?

You can have rivers in a procedural terrain without having to use greyscale maps.
There are several methods.
One method is to take a Simple Fractal Noise node followed by an Absolute filter node. An example using this technique on a large scale terrain can be downloaded here

http://www.eonmusic.ch http://www.artmatica.ch