The Zadorra is a river tributary of the Ebro in the Basque Country at the north of the Iberian Peninsula. The river flows across province Álava all along (with the exception of Burgos' exclave La Puebla de Arganzon) till it pours into the Ebro near Miranda de Ebro in Burgos' lands. The river's 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 Millan/Donemiliaga), meandering thereafter across the Alavan Plains to the west (loops around Salvatierra/Agurain) past Vitoria by the north, where it takes a turn to the south heading to the Ebro through La Puebla de Arganzon.