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 canons 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.