The Zadorra is a 78 km long river tributary of the Ebro in the Basque Country in the north of the Iberian Peninsula. The river flows primarily across Álava province (flowing briefly through the Treviño enclave at La Puebla de Arganzón) finally reaching the Ebro near Miranda de Ebro in Burgos province, Castille and León. The rivers water volume is the largest in Álava, with its basin being the most extensive in the province. Nowadays it provides by means of the Zadorra Reservoir System (comprising reservoirs Uribarri-Ganboa, Urrunaga and Albina) water supply for Vitoria and half of the Basque Autonomous Community.
The river rises in the slopes of the Entzia Plateau at the spring known as Los Corrales (municipality of San Millán/Donemiliaga), meandering thereafter across the Alavan Plains to the west (loops around Agurain/Salvatierra) past Vitoria by the north, where it takes a turn to the south heading to the Ebro through La Puebla de Arganzón.
