As you know, Healey, I use GHQ Terrain Maker for my terrain layouts. Many of the things I do and techniques I use can be applied to other terrain systems as well.
Terrain Maker uses the 1/4" hexes for streams and rivers. The bottom of the hex is the riverbed and 2 pieces of cut 1/4" hexes form the banks on either side if a stream or on one side if part of a larger river.
Here are a couple pictures showing the basic riverbed with paint applied.


These pictures show the water part after all the other stuff that I do to the hex. You can see the flat base coat on the edges and then the gloss.
Here are the basic steps.
1. Paint the basic color. I use Behr's Tannenbaum house paint for this and paint 1 coat and let dry.
2. Put on a second coat. I use the paint, unthinned, and brush it on the hex. While the paint is still wet, I stipple it with a somewhat stiff brush to get some ripples so it is not completely smooth. Let this dry overnight.
3. Spray on the colors. For the final top colors, I used 2 shades of Luftwaffe Greens by the old Polyscale sprayed from my airbrush. I used a lighter, more muddy olive green near the banks and ford areas while using the darker greens in the middle. This gives a bit of "depth" without having to resort to pouring resin.
4. Apply a clear gloss over the paint to make it look wet. For me, this seems to work pretty well.
That is basically how I do it. I usually do a bunch of river/stream hexes at once.
More coming.