Hi Tom, What you have inquired about is right up my waterway, so I will offer you my experience at crafting water.
I used products that I have purchased from Jim Elster's Scenic Express. They are as follows:
+Enviro Tex Resin(s)
+transparent dye(s) marked by Jim Elster for use with the EnviroTex: resin : colors: green; blue; amber.
These waterways I am showing you here have existed for a decade and have not changed color nor "yellowed" at all.
They look exactly as they did the day I:
+painted the plywood base with acrylic paints, the kind artists' shops sell in tube form, the ones you would us to create a painting. I gave a day+ for those paints to dry very thoroughly, so as to be a base of color(s) for the resin mixture, not mix-in with it and possibly cloud it up. Where I wanted the "water" to appear deeper, such as on the inside of the "elbow" of a meander of the waterway, like in real-life, I darkened the mix of colors, as well as lightened my use of real water with the paints: the more opaque the color, the deeper the faux water would appear.
+Mixed the EnviroTex Resin and mixed the colors (Dye set) in as the urge inspired me to do so, desirous of waterways I had seen in-person on trips
conducted specifically to get inspiration and knowledge of how such waterways could or should look.
+Let it all stand and cure, and kept myself from the urge to stick a finger on any of it to see whatever.
+Rocks and gravel(s) were added to the edges once the resin had gotten pretty solid. I didn't want the resins mixture to lap-up onto the rocks too far and look phony.
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I let the entire process be fun. It was a wonderful creative exercise and adventure.
FrankM