Damn, this looks good. Fast and effortless indeed, but clear and to the point. I really should try SAI. I have a trial version mouldering in my downloads folder.

One thing is, the river water doesn't look very, well, watery. With greenish water like this, you should decide - is it because of algae on the bottom of the river? (i.e. clear water, green bottom) or because of muddy water (hardly any change in toone from bank to center of current).