I made them by cutting out the segments from sheet styrene (for sale signs), then using caulk to make the river banks. Once the caulk dried, l painted the river with the appropriate colors. Yes, rivers are not really blue, but I gave up after several tries to get a more realistic color using greens and browns. The river was given two coats of gloss Mod Podge.
I have two widths. The wider river segments I can also use for 15mm games. The bottom picture shows a short segment that is supposed to be a ford. It looks a little better than the photo....it look more like peanut brittle here! I am not happy with it, but it will do.
I will probably use painter's masking tape to hold them to the gaming table.
These turned out better than the ones using the foam core and took 1/3rd the time to make them.