From the fact that the lower part of the course of the great rivers of the plateau—the Douro, the Tagus, and the Guadiana—flow through Portugal, their streams are hardly at all available as a means of communication or of navigation for Spain; and from the nature of the deeply cut beds which the waters have worn through the soil, flowing, especially as they approach the frontiers of Portugal, through gorges approaching in length and depth the cañons of North America, the rivers are little available for irrigation, although far more use might be made of them for this purpose than is actually done. Owing to the prejudices of the Spanish husbandman, and to his reluctance to accept any change, however profitable, in his ancient routine, neither the little that has been done in the present century, nor the remains of a wiser agriculture in former times are used by the peasantry. In the province of Zamora, for instance, both the ancient "acequias" and the modern canal of the Esla are equally neglected. The rich results that have followed the employment of the waters in the few cases in which they have been intelligently directed, stirs no one up to follow the example. It is one of the many contrasts between different parts of Spain, that the value of irrigation should be so well understood in some parts and so utterly neglected and under-valued in others. But we shall have more to say of this when we treat of the eastern and southern streams: at present let us return to the Douro, and to the other rivers of the plateau.
The Douro takes its rise in the Lago Negro, or Black Lake, on the southern flanks of the Mount Urbion, in the north-western angle of the province of Soria. It first runs eastward to the city of that name, the ancient Numantia, then turns almost directly south as far as Almazan, whence it runs westward to Portugal, receiving meanwhile the waters of the Esla, below Zamora; at the frontier, again it turns south, through deep gorges which form the boundary between Spain and Portugal, until it receives the waters of the Agueda, where it finally enters Portugal, and after a westerly course thence of about 100 miles, falls into the Atlantic below Oporto.
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.
The Tagus, the central river of Spain, and which divides its territory into two nearly equal portions, rises from a fountain called the Fuente Garcia, or Pié, on the south side of the Muela de San Juan, between the Sierras de Molina, Albaracin, and San Felipe, the knot of mountains which, as we have indicated above, form the great watershed of the peninsula, whence the waters flow northwards to the Ebro, east and southwards to the Mediterranean, and westwards, in the Tagus and its tributaries, to the Atlantic. Were the whole peninsula of Spain and Portugal one kingdom, the Tagus would be perhaps the most important of its rivers; but in the divided state it is of far more value to Portugal than to Spain. Its swift and turbid current, flowing between steep banks, and in a bed broken into rapids and encumbered by rocks, is scarcely navigable above Abrantes. The basin of the Tagus contains an area of nearly 30,000 square miles, and its length is estimated at about 550. The rainfall is less than that of the Douro, being only sixteen inches annually. The river, moreover, runs by no means in the centre of its basin, but far to the southwards of a central dividing line, and consequently the tributaries which it receives from the north or left bank are of much greater importance than those which come from the south or right. After flowing a few miles in a north-westerly direction, the river gradually bends, first westerly, and then in a slightly south-westerly direction, in a deep channel, through a bare rolling country, where everything takes the prevailing colour of red dusty uplands, until it arrives at Aranjuez, situated at the confluence of the Jarama and the Tagus, a royal residence whose abundance of water and of shade make it a true oasis in a desert. The Jarama, which rises in the Guadarama, brings in also the waters of the Henares, and those of the Manzanares, on which Madrid is situated. These streams have been the subjects of many projects and attempts at canalization, either for irrigation or for supplying the metropolis with water. Most of these have failed, but a canal from Porcal to Aranjuez, of seventeen miles and a half, is in working order. The canal of Cabarrus brings the waters of the Lozoya to Madrid. But the great enterprise of the canal of irrigation from the Henares, constructed by the same English company which made the canal of the Esla, and which was to have been twenty-eight miles in length, and to have irrigated 30,000 acres, is suspended by lawsuits as to the ownership of the waters. The Alberche, which rises to the north of the Sierra de Gredos, enters the Tagus near Talavera de la Reyna. The Tietjar, and the Alagon, which joins the main stream just above Alcantara, beside the frontier stream, the Heyas, are the only Spanish waters of importance from the north before the Tagus enters Portugal; and from the south the Salor and the del Monte, both of which have their rise and course in the same province of Caceres alone need mention. In the upper part of its course, however, the smaller tributaries of both the Tagus and the Guadiana often overlap, and but a very few miles separate the Tagus itself from the waters which flow into the Guadiana.
The exact source of the Guadiana has been a subject of much debate and of many fables. Its true origin seems to be in a series of lakes at the junction of the provinces of Ciudad Real and Albacete, near Montiel, in La Mancha. A picturesque stream, the Ruidosa, with many cascades and broken water, connects these lakes; but after running a few miles in a north-westerly direction, it disappears underground near Tomesillo, and is believed to rise to the surface after about twenty miles, in the Ojos (eyes) of the Guadiana, near Damiel. Very soon it receives from the right the united waters of the Zancara and the Giguela, streams whose contributions are much more scanty, especially in summer, than the length of their course on the map would lead one to suppose; thence the river flows in a westerly direction, passing near Ciudad Real, below which the Javalon enters from the left, coming from the Campo de Montiel;