Anciently, Acarnania was reckoned the most westerly province of Greece, bounded on the north by the Ambracian Gulf, on the northeast by Amphilochia, on the west and southwest by the Ionian Sea, and on the east by Aetolia. It contained about . Under the Romans, or probably a little earlier, the river Achelous formed the boundary between Acarnania and Aetolia; but in the time of the Peloponnesian War, the territory of Oeniadae, which was one of the Acarnanian towns, extended east of this river. The interior of Acarnania was covered with forests and mountains of no great elevation. Between these mountains there are several lakes, and many fertile valleys. The chief river of the country is the Achelous, which in the lower part of its course flows through a vast plain of great natural fertility, called after itself the Paracheloitis. By the 19th century, the plain was covered with marshes, and the greater part of it appeared to have been formed by the alluvial depositions of the Achelous. Owing to this circumstance, and to the river having frequently altered its channel, the southern part of the coast of Acarnania has undergone numerous changes since antiquity. The chief affluent of the Achelous in Acarnania is the Anapus (Ἄναπος), which flowed into the main stream 80 stadia south of Stratus.