The river rises in the
Ligurian
Province of Savona at an elevation on
Monte Ermetta, to the west of
Monte Beigua. Having passed through the territory of
Sassello, the river enters the
Piedmontese
Province of Alessandria at Ponte Erro (
Malvicino). Its course takes it past
Cartosio, Rivere,
Castelletto d'Erro, Arzello, and
Melazzo before joining the Bormida at
Terzo, a little upstream from
Acqui Terme at an elevation of . ==Sources==