κατ' ἀνάγκην

From LSJ

βορβόρῳ δ' ὕδωρ λαμπρὸν μιαίνων οὔποθ' εὑρήσεις ποτόνonce limpid waters are stained with mud, you'll never find a drink

Source

Greek > English (Woodhouse)

necessarily, by necessity