River Oria. It is the river of Guipúzcoa, like the Zadorra in Alava and the Ibaizábal in Vizcaya. It is the most important river due to its central location, its flow, length and the number of towns it flows through. It is also the longest river in Gipuzkoa, some 80 kilometres. It runs in a north-easterly direction as far as Teresategui, where it heads west to its mouth. It rises at the San Adrián pass, at an altitude of 660 metres. It is formed by the Latsaen and Otzaurte streams, which join at Zupitxoeta. From here, the two watercourses join together and take the name of Oria. It passes through Cegama and Segura. Ormáiztegui and Beasain it joins the stream that flows down from Zumárraga, which is also called Oria. Receiving tributaries that we will list later, it continues through Ordizia (Villafranca), Isasondo, Legorreta, Icazteguieta, Alegría, Tolosa, Irura, Villabona, Andoain, Oria, Lasarte, Usúrbil, Aguinaga and Orio, flowing into this town Anarri point and Amezti mountain. Its most important tributaries are: the Ursuarán at Idiazábal; the Agaunza at Ataun-Lazcano; the Amundarain at Zaldivia; the Amezketa at the village of the same name; the Araxes at Lizarza-Tolosa; the Berástegui at Ibarra; the Leizarán at Andoain, all on its right bank. The tributaries on its left bank are streams that collect water from the foothills of Murumendi, Ernio, Iturriotz, Andatza and other mountains. The course of the Oria, which runs in a general north-easterly direction as far as the hill of Teresategui, now in the municipal district of Donostia (San Sebastián), turns sharply to the west, towards its mouth in Orio. In this low stretch, it meanders through the Usúrbil-Aguinaga area, where it is navigable for small boats. There are traces of an ancient riverbed that flows into La Concha Bay via Ondarreta, currently blocked by the Teresategui hill (67 m).