Akhelóos or Achelous, river, 137 mi (221 km) long, rising in the Pindus Mts., NW Greece, and flowing generally south, traversing many mountain gorges, and emptying into the Ionian Sea opposite Keffallinía. It is a source of hydroelectric power. It formed a part of the boundary between ancient Aetolia and Acarnania and was formerly called Aspropotamos.