The Esla is a river in the provinces of León and Zamora in the northwest of Spain. It is a tributary of the Duero River that starts in the Cantabrian Mountains and is 275 kilometres (171 mi) long. Its direction of flow is from north to south. It is the largest tributary of the Duero in terms of discharge; in fact, at its mouth at the confluence with the Duero, it has a greater discharge than the volume in the main river.