The Douro is the largest river of the Iberian Peninsula by discharge. It rises near Duruelo de la Sierra in the Spanish province of Soria, meanders briefly south, then flows generally west through the northern part of the Meseta Central in Castile and León into northern Portugal. Its largest tributary is the right-bank Esla. The Douro flows into the Atlantic Ocean at Porto, the second largest city of Portugal.

Castile and León is an autonomous community in northwestern Spain. It was created in 1983 by the merging of the provinces of the historic region of León: León, Zamora and Salamanca with those of Old Castile : Ávila, Burgos, Palencia, Segovia, Soria and Valladolid. The provinces of Santander and Logroño, which until then had been included in the Old Castile administrative division, opted out of this merger and formed the new Autonomous Communities of Cantabria and La Rioja respectively. Condado de Treviño and La Puebla de Arganzón, the two municipalities that make up the Treviño enclave, are geographically surrounded by the neighboring Basque Country Autonomous Community, but belong to Castile and León.