The basin drained by the Douro is the most extensive of all those of the rivers in Spain. Including the portion in Portugal, it comprises 35,000 square miles; the length of the river is about 500 miles; the average rainfall is stated at twenty inches. The chief affluents of the Douro descend from the north from the mountains of Burgos and the Cantabrian range. The largest are the Pisuerga, which rises not far from the sources of the Ebro among the Picos de Europa, and flows almost directly south by Palencia and Valladolid until it joins the Douro, some miles above Tordesilla; the Esla, which also rises from the western flanks of the same chain, not far from Covadonga, takes a somewhat more westerly direction, and after receiving several smaller streams unites with the Douro below Zamora. These two rivers supply water for two of the most successful canals in Spain, especially that along the Pisuerga, for over ninety miles from Alar del Rey to Valladolid. There is a considerable traffic on it, especially for passengers. It was planned in 1753 by Ensenada, but completed only in 1832. The canal of the Esla, for purposes of irrigation, begun by English engineers in 1864, and finished in 1869, has hardly been so successful. The latest report (June, 1880) states that the peasant proprietors, notwithstanding examples of the great utility of irrigation, obstinately refuse to use it. The principal affluents of the Douro on the west and south are the Tormes, which flows by Salamanca, and joins it about midway in its course as a frontier of Portugal; and the Agueda, which runs in just where it takes its final departure for the west.