Sir Richard C. Jebb, Commentary on Sophocles: Trachiniae

Ἀχελῷον. The Acheloüs rises at the centre of Pindus, in Mount Lacmon, the great watershed of northern Greece, and, after a course of some 130 miles from N. to S. , flows into the Ionian Sea. Its lower waters formed the boundary between Acarnania on the west and Aetolia on the east. The modern name, ‘White River’ (Aspropotamo), is due to the yellowish colour which the stream derives from a clayed bed.

To the Greeks, Acheloüs was the king of rivers ( Il.21. 194“κρείων Ἀχελώϊος”). He was the ‘eldest son of Oceanus and Tethys’: Acusilaüs fr. 11 a (Müller Frag. Hist. I. 101) “Ὠκεανὸς δὲ γαμεῖ Τηθὺν ἑαυτοῦ ἀδελφήν: τῶν δὲ γίγνονται τρισχίλιοι ποταμοί: Ἀχελῷος<*> δὲ αὐτῶν πρεσβύτατος καὶ τετίμηται μάλιστα”. The oracle at Dodona,—which was not far west of the river's sources,—‘enjoined sacrifice to Acheloüs in all its responses’ (schol. Il. 21. 194). In Acarnania “ἀγῶνες” were held in his honour (schol. Il.24. 616). The cult of this river-god was, however, not merely local, but Panhellenic. Such pre-eminence is enough to explain how he became a type of “πηγαῖον ὕδωρ” generally, without assuming the more than doubtful kinship of “ἀχ” with aqua. For Greek, it should rather be “ἀπ”, as in “Μεσσάπιοι”.