The Spanish River is a term used to refer to four rivers in Spain: the Ebro, Rio, Douro, and Esla. The Ebro is the longest river in Spain, stretching over 800 kilometers from its source in Cantabria to its mouth in the Mediterranean Sea. The Rio is a tributary of the Ebro, originating in the Sierra de Guadarrama mountain range and flowing into the Ebro near Zaragoza. The Douro is a river that flows through northern Portugal and northwestern Spain, eventually emptying into the Atlantic Ocean. Finally, the Esla is a tributary of the Douro, originating in the Cantabrian Mountains and flowing into the Douro near Zamora.