Back in the old days when I buildt LARP weapons I used to pour acrylic colours into the latex. The colours won't be WYSIWYG, but a lot whiter than what they'll turn out once dry.
I wouldn't recommend it to build rivers though. Just paint the river the colour you want on the flat surface of the Terrains base. Then add transparent silicone and tap it with an old spoon to get waves. Let it dry for three days or so and finally drybrush with white.