You need to seal the gaps and edges of the terrain/mountains on each side where they meet the riverbed so that, when you do the water pour, the product will not escape or bleed under the terrain, but stay in the riverbed. Not sure how you are incorporating the scene into your layout, but you probably also need to seal the ends of the riverbed as well so the water pour doesn't run out the ends.
There are a number of ways you can do that sealing along the edges - paintable caulking, sealant, flexible joint compound, etc.
Not sure if the base wood you are using is waterproof but, assuming it is not, you also need to seal it. I use a latex paint on the riverbed which seals it all along the flat part and up to where you have sealed the river edges and, at the same time, provides the color I want for my riverbed. Dark greens, browns and blues are typically used for riverbed colors, but the choice is yours, and multiple colors can be blended from the edges to the center. Some water effects can also be tinted before pouring which may also effect how you paint the riverbed.
Don't forget to glue into the riverbed some rocks, stones, tree branches and whatever else you think looks good before your water pour.
