Ebro (ēˈbrō, āˈbrō) [key], Catalan Ebre, longest river entirely in Spain, c.575 mi (925 km) long, rising in the Cantabrian Mts., N Spain, and flowing SE between the Pyrenees and the Iberian Mts. past Logroño and Zaragoza. It empties through a wide delta into the Mediterranean below Tortosa. The river is of little use for inland navigation because of varying volume. In its middle course the waters are canalized for irrigation. The Jalón, the Gallego, and the Cínca-Segre are the main tributaries. Large hydroelectric power plants in the Ebro system supply c.50% of Spain's hydroelectricity.