The river rises at the south of the municipality of Zegama near the hamlet of Otzaurte and the San Adrian tunnel, harvesting the waters of the north-eastern side of the mountain range Aizkorri, so the river results from the merging of several streams. The first town it crosses is the very nucleus of Zegama, with other major towns of Gipuzkoa being located on the river, e.g. Beasain, Ordizia, Tolosa, Andoain, Lasarte-Oria, Usurbil and finally Orio at the mouth. From Lasarte-Oria to Orio, the sinuous and traffic laden coastal road N-634 winds along.