Oria: backbone of Goierri
It is the main river that crosses the Goierri region, and the largest river in the province of Gipuzkoa. This river also crosses Ordizia. With 882.5 km², it is the largest basin of the Gipuzkoan rivers, part of which extends into the Comunidad Foral de Navarra. It rises on the northern slope of the Aitzkorri mountain range, receiving in its upper part the contributions of the Ursuaran and Estanda rivers and, a little further down, those of the Agauntza, Amundarain and Amezketa. Other important tributaries of its middle and lower part are the Araxes, Zelai and Leitzaran. Its mouth is made in Orio, after describing a sharp curve in the vicinity of Donostia-San Sebastián. The urban and industrial occupation of the basin, which has a population of around 150,000 inhabitants, has occurred mainly around its main axis, in urban centers such as Beasain, Tolosa, Billabona, Andoain and Lasarte-Oria. Historically, moreover, the Oria river valley has been the most important communication route from Gipuzkoa to the south. The humanization of the middle and lower areas of the basin has greatly deteriorated its habitat, which contrasts, however, with other higher areas of the same basin that are sparsely populated and well preserved, such as the Aralar and Aitzkorri mountain ranges and the of the Leitzaran river.