I have made a river to go with my fur terrain mat. I'm fairly happy with it apart from the colours which are a bit off.
The basic process was.
The base - flexible sheet (like some ring binders) - cut to shape - The transparency made this easier.
Tine Foil - glue crumpled tin foil over the base
The riverside - Add Glue and various pebbles, flock to the edges, in a few stages.
Paint the river - dark in the middle, blue either side, muddy Green/brown edges
Add two layers of "still water" product to finish (wasn't cheap)