[466] ἐν ποταμῷ may mean ‘apud fluvium,’ as “πόλιν οἰκουμένην ἐν τῷ Εὐξείνῳ πόντῳ” Xen. Anab.4. 8. 22, or perhaps actually in the torrent-bed, or at any rate within its banks. Cp. Il.18. 520“οἱ δ᾽ ὅτε δή ῤ̔ ἴκανον ὅθι σφίσιν εἶκε λοχῆσαι”
“ἐν ποταμῷ”.νύκτα is accusative of duration, as in Od.22. 195“νύκτα φυλάξεις” “εὐνῇ ἐνὶ μαλακῇ”.
φυλάσσω, intransitive = ‘keep watch;’ cp. Il.10. 192.