It involves using Wood lacquer. Upon finishing the woodwork on furniture, most furniture get a varnish of either shiny wood lacquer or wood varnish. I'm not sure of the actual terminology but the former tends to be more viscous than the latter. In my vignette, I've used both in combo but found the former better due to the shine.
Made the river base using something similar to celluclay. Painted the darkest shade of yucky-brown first based on the kind description of the Thames by DaveCox and Plasticbattle and using the stuff that comes off your nose when you have a bad flu for reference.
The next phase requires a bit of patience. Paint a thin layer of lacquer which has diluted yucky brown over the base. I was actually suprised that with a bit of detergent & water I was able to blend in acrylic with the lacquer (approx a drop of paint to a teaspoon of lacquer). In fact it became an emulsion when stirred giving it a semi-transparent, sediment look.
Wait several hours to dry and continue painting additional layers. Each subsequent layer should have a progressively lighter basecolor. In my case I just added a bit of yellow to the original yucky brown. It helps if you apply the subsequent layers over several days (requires a patience bordering on the speed of snail). The mutiple layers give it a sense of depth not unlike the excellent water making method mentioned earlier by Plasticbattle using CA glue.
The number of layers depends on your satisfaction versus your patience and can range from 5-10 layers. The last 1 or 2 layers should be just lacquer with no paint. Lastly top of with polyurethane glue to create an uneven wave-like water surface. You may have to stir a bit just before the glue dries but be careful not to trap air bubbles.
Don't know if any of you might find the above useful but here's some pics anyway:
