Two good choices are (1) EnviroTex Lite clear resin and (2) Liquitex Gloss Medium.
(1) The resin is poured in 1/8" deep layers. Best effect is to pour the first layer clear then add any color tint -- be it blue, green, brown, or a combo, using acrylic artist's inks -- to additional pours.
(2) The gloss medium works well if you have put a lot of effort into texturing and painting your riverbed itself, then you simply brush on the gloss medium to add a "wet" sheen and some choppy waves if you want them, kind of like varnish but more substantial.
Woodland Scenics Realistic Water works well enough but at least in my own experience is better suited to smaller projects like thinly poured table-top waterway sections as opposed to major rivers or coastal areas carved out of large terrain boards.
You can add choppy wave texture to the top of already dry resin or medium using more medium or using the other Woodland Scenics product "Water Effects" which can also be used to create waterfalls or ice. You can dry-brush a highlight coat of white paint onto the wave texture after it dries.
I'm also a fan of the Terrain Tutor and recommend checking out his many and varied Youtube tutorials.
Another great resource -- for building terrain boards in particular -- is Clarence Harrison's Quindia Studios site, which has a great tutorial for building a river using EnviroTex Lite. You can visit it here:
link
@LordNth: yes, you can indeed "pull" waves, though it's more like "drag" waves by drawing a plastic utensil through the semi-dry resin. If you go at the resin too soon it will have no effect, and if you go too late it will create chunky gaps more suitable to an earthquaking surface, so timing is everything! With EnviroTex you are supposed to wait 6-8 hours after pouring before "dragging" your waves.
