Passage 1.38.1
οἱ δὲ Ῥειτοὶ καλούμενοι ῥεῦμα μόνον παρέχονται ποταμῶν, ἐπεὶ τό γε ὕδωρ θάλασσά ἐστί σφισι· πείθοιτο δὲ ἄν τις καὶ ὡς ἀπὸ τοῦ Χαλκιδέων Εὐρίπου ῥέουσιν ὑπὸ τῆς γῆς ἐς θάλασσαν κοιλοτέραν ἐμπίπτοντες. λέγονται δὲ οἱ Ῥειτοὶ Κόρης ἱεροὶ καὶ Δήμητρος εἶναι, καὶ τοὺς ἰχθῦς ἐξ αὐτῶν τοῖς ἱερεῦσιν ἔστιν αἱρεῖν μόνοις. οὗτοι τὸ ἀρχαῖον, ὡς ἐγὼ πυνθάνομαι, πρὸς Ἀθηναίους τοὺς ἄλλους ὅροι τῆς γῆς Ἐλευσινίοις ἦσαν, καὶ διαβᾶσι τοὺς Ῥειτοὺς πρῶτος ᾤκει
The streams called Rheitoi provide only a current of rivers, since their water is actually seawater. Indeed, one might be persuaded also that they flow underground from the Euripus channel at Chalkis, emptying into a deeper sea. The Rheitoi are said to be sacred to Kore and Demeter, and only the priests are permitted to catch the fish from them. As I learn, these Rheitoi streams, in ancient times, formed the boundary between the territory of Eleusis and the rest of Athens, and the first settlement beyond the Rheitoi was inhabited...