The Pisuerga is a river in northern Spain, the Duero's second largest tributary. It rises in the Cantabrian Mountains in the province of Palencia, autonomous region of Castile and León. Its traditional source is called Fuente Cobre, but it has been discovered that the real source is a glacier higher in the mountains. The river flows south into the Douro river shortly after passing through the city of Valladolid.