If it's taking so long to render, it's also because water is rendered under the terrain, where you don't see it. So if you take that part out (by mask) it won't be rendered and speeds up the render time. A mask can be made by painting over the creek from above, or even this way, by using the render painted over (better of course), where the rest is black, water area is white; this image map can then be projected through render camera, and used as input in a default shader's opacity tab, where the default color is turned to black. Input this into a colorless surface layer where your water shader resides as a child, and your done.
