For rivers that are deep enough that you cannot see the bottom, I paint the center of the river black. The edges I feather from black to whatever the prevailing ground color is. I just darken the parts in the "water" slightly from the dry land. Once that is done, I put in rocks and logs and weeds. Finally I put down some WS realistic water. A very thin layer. It's not great water but it's good enough. The WS water expands slightly as it dries and fills in around the weeks, rocks and twigs that are in close to the river bed. This makes an indistinct line for the water's edge. I've tried using plaster to model choppy water, but it never looked good.