What popped into my head was a 'water table' used for play - not that you would use one of those, but you could make your own. Use some plastic/pvc for the water areas and some good waterproof adhesive designed for this and you could incorporate it into a diorama base of some type. Some textured exterior paint on the pvc should help make it look realistic (textured as in just adding some sand and small stones (ie fish tank gravel/stones - try to get the pre-tumbled ones that are smooth as they would look so much more realistic as water-worn rocks)).
Now, how easy is all this going to be to move, store, drain!, etc, I do not know.
