Lon,
There's a couple of ways . The simplest is just make them in the same way as the marshes.
Only real differences being you paint the backing paper as series of light and dark stripes(graduating the contrast between the two.)
Glue the plastic on top . Then just add the grass tufts in broken/ punctuated line over the top of the lighter zones . Plus all around the edge
You can cut down tuft dimensions by chopping through them on the backing paper. (So you don't need to buy as many tufts
)
The other means a thicker base and more work. Which is the method I'll be double checking with a build.