Slitherine

The map needs rivers.

I know this is an alien world, but it seems to me that any planet with liquid surface water would have rivers.

Yes. Yes it does... Also I remember having a 2 hex "lake" that had an AI controlled city next to it. That lake had 2 ships in it, in the middle of a continent map. Need to eyeball the code on that one.

In some games, rivers act as roads (that you don't have to build).

Alpha Centauri allows a former to "Drill to aquifer, to trigger a river springing from the source square and running down to the ocean.