Curious how your river turned out,
@ChessieFan I'm late to the party, as usual, but I have a lot of step by step pictures of my process.
Whatever base I use, I prime it first with a flat latex. This seals the surface, and makes your paint application more even if your riverbed uses more than one material. (I'm cheap, and will use scraps of just about anything for this purpose, including foam core, old ceiling tiles, EPS foam or plywood.)
Once the basic terraforming is done, I'll lay down a coat of base color, which is selected and blended from personal observation, color photos, satellite imagery etc as others have described. I use cheap craft acrylics such as one may find at a craft store, usually blending Hunter Green, Raw Umber, Burnt Umber, Black, and whatever shade of blue seems appropriate. This gets brushed across the entire river bed, then blended so there are darker bits in the middle or around what might be deeper bits (a good swimming hole, for instance) and more of the green/blue as an overall shade, then darker and lighter browns along the banks.
To this I'll add bits of stone (ideally harvested from the area being modeled... lots of nice crumbly mud shale along the Potomac) to create some shoals and shoreline effects. I follow that with some sandy soils to fill in around the rocks along the banks, and ground foam and other scenery flocking to give it all some life. Carefully sweep out any loose stuff, and give it all a good soaking with diluted white glue to keep everything in place, then once it's all cured, start brushing on gloss Mod Podge medium. You'll be applying a number of coats to get an illusion of depth, so tint the first couple of coats, especially around those deep channels, and let that dry. With each coat, go back with a stiff bristle brush and tease the surface a bit to get your ripples.
I use white poly fiber that's stretched over rocks and soaked in mod podge to get the waterfalls looking pretty good, which is a handy trick when your river has to flow over hidden trackage below the scenery!
Then, like all scenery, it's a matter of adding more bits and pieces of downed logs, underbrush, fallen leaves, smaller rocks, shopping carts and old television sets... whatever works for you to give the scene the depth you're after.
Looking forward to seeing your progress.
Lee
