... that the reason there are so few rivers of any length in M&M -- and indeed in all 3d games -- is that they are incredibly challenging to create.
3D maps are built as terrain and then covered in tiles. A riverbed can be filled with a 'volume' of water and the surface can be covered with an animated tile to represent waves and flow, but the surface will be flat. The best rivers I've seen consist of a series of small pools.
Waterfalls can be done nicely, as can slow moving large rivers, but anything with a gentle but visible downward flow can't be done effectively in any 3D worldbuilding kit I know of.