A potamodromous fish, like an anadromous or catadromous fish, is a migratory fish.  Unlike anadromous or catadromous fish, a potamodromous fish spends its whole life in fresh water.  Generally, these migrations are for spawning purposes and cover short distances: from an upstream tributary to a mainsteam river or between connected lake and river systems.  

The endangered Colorado Pikeminnow migrates long distances only in fresh water.

Some species, for example the endangered Colorado Pikeminnow (Ptychocheilus lucius) have very extensive fresh water migration routes – reportedly up to 300km.  Like salmon, the pikeminnow uses homing to reach very specific spawning locations with a high degree of fidelity. (fishionary.fisheries.org/potamodromous)