Ok so I need some input, I painted the river bed with acrylic paints and I will be pouring envirotex for the water. Questions are do I need to seal the river base with modge podge or something before the envirotex is poured? I'm thinking if its not sealed and the envirotex hit it that before it has a chance to set the colors may bleed.
Question 2 is I need to put some course sand down along the river banks to simulate the shallows along with some debris , do I need to seal this down before envirotex pour or pour the envirotex right on the sand and logs stones ?

You need to make sure it is sealed and everything glued down nicely before pouring envirotex. You don't want any leaks, and you don't want the riverbank debris to "float" in the resin. White glue should work fine. Acrylic paint is relatively inert when dry, and I would not expect envirotex to dissolve it.

What is "the river base" at present? I would expect the acrylic paint to seal the surface of something like plaster of Paris or hydrocal, or to essentially seal it. If it is painted wood, there would be minimal absorption for the same reason.

But you must provide an impermeable barrier to the epoxy...it will find tiny holes, although it may seal itself in small enough holes and not actually drip. I used painter's masking tape, the green stuff, and taped up the edges of my river where it is open in profile at the bench edge. The tape worked effectively because I had it affixed to a fairly planar surface.