An old (1980's) idea for simulating water is Artist's Gloss Medium. Available at Hobby Lobby, art or craft stores. Being water-based, it is compatible w/ foam and everything else. Comes ready to use in a bottle. Paint the stream bed on the base- painting darker in the middle simulates a deeper stream, gradually lighten until edge matches color of the surrounding"dirt".
Then just pour, let it run over the terrain as would a stream. It takes a day or two (if a thicker pour) to cure.