In MM7, it could be argued that the waters in Barrow Downs, and Erathia (Which the Fort rests over) could be considered rivers.
As for MM6, the closest I could find were ones that went from Frozen Highlands, south into Free Haven, then turned east and into the bay. Also, there's the one that goes from the hills in Silver Cove, and dumps into Bootleg Bay.
Some food for thought; I'll look into this more tonight.
BTW...in "My" thread farther down, I posted an observation about the weather in Might & Magic, if you haven't already seen it...