I decided to make the dry riverbed into a wet one. I did some research on the subject of modelling water and settled on this product. It is cheaper than the stuff sold by Woodland Scenics and a lot easier to work with.
The big thing to remember is to pour thin layers, maybe 1/16" to 1/8" thick. The polycrylic dries in a couple of hours and then you can pour another layer.
It starts out milky, but dries clear.