Both Lowes(blue) and Home Depot(pink) have foam insulation. Start with a two inch thick base and carve the river with surform tools(spelling), available from most hardware stores. They look like an industrial strength cheese grater. Be sure to have the shop vac handy and running as you shape the foam.
Cut the foam into rough shapes with a surated knife or key hole saw and glue it together to form the mountains. Liquid nails for projects and foam is very good for gluing the stacked foam together, but it can take a few days to setup. Once the adhesive has set up, do fine shaping with the surform tools.
I've seen references to foam scenery looking like stacked foam, but if you take your time doing the final shaping it can look really good.
With the mountain shaped, I like to cover it with Sculptamold. It's a plaster product with paper fibers mixed in. It makes a rock hard surface when dry. For flat areas, latex paint works fine as a base for your ground cover. Use a color that will blend with your ground foam incase any shows through.
Once the Sculptamold is applied, I'll spray it with brown Rit dye mixed with water. Applying the color while the Sculptamold is still wet allows the color to penetrate. Diluted India ink is great for gray shading to the surface.
You can even brush white glue onto the wet surface and start adding ground foam.
I did all my landscape scenery this way, here are a few pictures to give you an idea of the finished product.
I get all my foam from construction sites. I watch for when there is a stack on sight. I ask the forman if I can have the scrap. I tell him I am a model railroader and after he tells me about the train set he had as a kid, I usualy get the scrap. I hit the mother load last week and got the equivelant of about 8 4x8x2in sheets, some in 2x8 size.
I cut it with a steak knife and cover it with my version of ground goodp (1/3 premixed drywall seam compound, 1/3 saw dust/ 1/3 house paint for color, a squirt of Lysaol and water to get the texture right. I paint it on with a brush.
Or, you could consider using screen wire and drywall mud or using cardboard strips covered with WS plaster cloth. Having used these two methods as well as stacking and shaping foam, it is my opinion that foam provided me with the poorest results with the most amount of effort.
The first volume of Design Plan Build DVD series has an excellent demonstration of how to use WS plaster cloth over cardboard strips. I can tell you that it is VERY EASY to do and the results can be wonderful.
Regardless of what technique you use, you'll probably and eventually be using a plaster or plaster like compound of some type, whether it be Sculptamold, Drywall Mud or Hydrocal.
I've spent a lot of time looking at pictures of mountains on the web and at train shows and I cannot tell you how many 'mountains' looked more like stacked foam than they did mountains.
Just my 2 cents worth nothing more
Joe
For a good example of how to use screen wire with drywall mud, visit "FeatherRiverRoute" On Photo bucket. John Warren's work is unbelievable.