a river in northwestern Spain, a right tributary of the Douro. The Esla is 285 km long and drains an area of approximately 15,700 sq km. It rises on the southern slopes of the Cantabrian Mountains and flows through the Old Castile Plateau. High water occurs from November to May. The mean flow rate is approximately 250 cu m per sec. A reservoir and a hydroelectric power plant are located near the mouth of the Esla. The river is used for irrigation.