Did you coat the inside of the streambed with anything? If not, try to 'seal' the streambed area with a mix of 75%-25% white glue to water. Put a couple of coats on if you feel it is necessary. Paint all the way up to the sides and top of the banks of the stream; it'll add to the wet effect at the top of the bank. Then, you should have a smooth, almost plastic-feeling surface. Paint the streambed with a dark brown color for the bottom once it's dry. Block off the ends of the streambed as best as possible, and try adding the Future again. Do it in layers if about a mm or two at a time and let each layer dry before you add the next one. Then, as these layers dry, you can always tint them with clear browns, yellows and greens to give it a 3-D stream effect that will add to the look of the stream. Each color above the lower one should be a bit more light and diluted than the one below.
Good luck with this. I hope it helps. If it does, please post pics so we can all see how it turns out.