Guadarrama (river)  The Guadarrama is a river in Spain. A tributary of the Tagus, the longest river on the Iberian Peninsula, Guadarrama has its source in the Siete Picos, part of the Sierra de Guadarrama, in the Community of Madrid, in the central part of the country, at an altitude of . The Guadarrama flows from north to south for through the autonomous community of Castilla-La Mancha and the Province of Toledo, where it empties into the Tagus. Its middle course is a protected area within the Regional Park of the Middle Course of the Guadarrama River and Its Surroundings (), which is one of the three regional parks within the Community of Madrid. The basin area is about Its main tributary is the Aulencia River, which flows entirely within the Community of Madrid and supplies water to the reservoir of Valmayor.