The Águeda is a river tributary of the Douro River, that springs from the Sierra de Mesas in Spain, in the autonomous community of Castile and León. It flows 130 kilometers until it reaches the Douro River near Barca de Alva, Portugal. The Águeda makes a natural border between Portugal and Spain for much of its path.
The origin of the Agueda River is the source of the Puente de los Llanos in Navasfrías (Salamanca), between the hills "Tables" and "Fat Rocks". Its water is impounded by the dam Águeda up to municipalities Zamarra and Pastors . Receives water from its tributary, the river Badillo.
Goes through Ciudad Rodrigo ( Salamanca ), by the uranium mine in Saelices the Chico and the reservoir rock of Siega Verde . It is the natural border between Spain and Portugal for 40 miles and empties into the left bank of the Duero near Barca de Alba , in the international border with Portugal .