In 2015 I made a series of modular terrain boards. Several of them included river features. That was some of my very first attempts at using Mod Podge for doing miniatures rivers.
Just this past week, I finished up a new set of Mod Podge rivers, but this time it was a modular river that was made in free sections that were not locked into an actual terrain board itself.
These were mounted on 1/8" thick PVC plastic sheet cut to purpose. To my surprise, I discovered that you can actually cut PVD sheet with an ordinary hobby knife. It takes some effort, but when you get part way down, you can snap off the excess, leaving behind the finished shape you need.
The river banks are rectangular sections of pink insulation foam that were glued to the PVC with Liquid Nails. I then sanded down the foam sections to smooth them out.
Then it was just a simple coat of craft paint for the river beds. A mixture of green, blue, and gray gave me the surface color.
And then it was a matter of simply gluing a coat of sand onto the river banks and then painting those brown, with some tan drybrushing over the base coat.
I gave these two coats of gloss Mod Podge, and that achieved a fairly decent "piling" of the waves and ripples. Sometimes a single coat of Mod Podge can shrink down to the point where you lose a lot of the ripples and waves, and you need a second coat piled upon the first to get a bit more depth.
And that was pretty much it!
…and then some pics. of my old river terrain boards from ten years ago: