River Zadorra. If the Ebro is defined as a marginal river, as a mere collector of almost all the waters of Alava, the Zadorra is the largest and most important, the river of Alava par excellence. In terms of its geographical importance, it plays a similar role to the Ibaizábal in Vizcaya or the Oria in Guipúzcoa. The Zadorra basin is the largest in Alava. Its upper basin is fed by various springs on the northern slopes of Urbasa. The Zadorra rises in the municipal district of Munain, some five kilometres to the east of Salvatierra (Agurain). After passing through the village of Munain, it is joined by the first stream that flows down from Ocáriz. It passes - forming a bend - to the north of Salvatierra. But before this, it has been fed by the Zaraeta stream. In Salvatierra, it is joined by mo; the second by Eguilleta, Erenchun, Añua, Gazeta and The course of the Zadorra runs in a south-east-north-west direction. In Zuazo de Salvatierra it receives the stream that descends from the Iruraitz mountains and the contributions of the Ordeñana and Luzuriaga together. It enters Barrundia and passes through the villages of Heredia, Dallo, Audicana, Guebara and Maturana. The streams of Heredia bring their water through the Sarnostegui. After Audicana and Etura, it receives the Echébarri-Urtupina stream. The River Barrundia, as it passes through the municipalities of Larrea, Hermua and Ozaeta, collects the waters that flow down from the Aránzazu and Elguea mountain ranges, via the small collectors known as Txikito, Errekalde, Ugarana and others. It leaves its course almost parallel to the Zadorra and joins it at Maturana.
The Zadorra enters Gamboa, where the Ulíbarri reservoir is located. Here it heads north-south and adds the contributions of the Urkiola, which has received the Bostibayeta and Santa Engracia, currently dammed in Villarreal. These waters come from the Urkiola and Gorbea crags. In the district of Eskalmendi, opposite Gamarra Menor, it is joined by the Alegria. The tributaries of the Alegria, almost all of them on the left side, come from the Iturrieta and Vitoria mountains. The four important ones take these routes: the first through the village of Gauna to flow into Alegría itself; the second through Eguilleta, Erenchun, Añua, Gazeta and El Burgo (villages); the third through Ijona, Trocóniz, Andollu, Villafranca, Argandoña Cerio and Illarraza (villages); the fourth through Aberásturi and Ascarza (villages), flowing into the River Alegría, opposite Zurbano. It receives the Arcaute on its left bank, coming from the mountains of Vitoria, near Gamarra Mayor, almost dynamos in Vitoria. It passes close to the capital on its westward course. On its right bank, before Trespuentes, it is joined by the Zalla, coming from the foothills of Gorbea. At Trespuentes, after a big bend, it changes course, heading south towards the Conchas de Arganzón gorge.
It passes through Villodas, Nanclares de la Oca, the aforementioned Conchas de Arganzón pass, La Puebla de Arganzón, Armiñón, Ribaguda, Lacorzana and Berantevilla. In this village it receives the Ayuda, which brings the waters of Treviño. It flows into the Ebro five kilometres below Miranda de Ebro.
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