The valley of Mexico is a great basin, which although seven thousand five hundred feet above the level of the sea, and of course subject to constant and rapid evaporation, is yet exceedingly humid for so elevated a region. No stream, except the small _arroyo_, or rivulet of Tequisquiac, issues from the valley, whilst the rivers Papalotla, Tezcoco, Teotihuacan, Guadalupe, Pachuca and Guautitlan pour into it and form the five lakes of Chalco, Xochimilco, Tezcoco, San Cristoval and Zumpango. "These lakes rise by stages as they approach the northern extremity of the valley; the waters of Tezcoco, being, in their ordinary state, four Mexican varas and eight inches lower than the waters of the lake of San Cristoval, which again, are six varas lower than the waters of the lake Zumpango, which forms the northernmost link of this dangerous chain. The level of Mexico in 1803 was exactly one vara, one foot and one inch above that of the lake of Tezcoco,[36] and, consequently, was nine varas and five inches lower than that of the lake of Zumpango; a disproportion, the effects of which have been more severely felt because the lake of Zumpango receives the tributary streams of the river Guautitlan, whose volume is more considerable than that of all the other rivers which enter the valley combined. "In the inundations to which this peculiarity in the formation of the valley of Mexico has given rise, a similar succession of events has been always observed. The lake of Zumpango, swollen by the rapid increase of the river Guautitlan during the rainy season, forms a junction with that of San Cristoval, and the waters of the two combined burst the dykes which separate them from the lake of Tezcoco. The waters of this last again, raised suddenly more than a vara above their usual level, and prevented from extending themselves to the east and south-east, by the rapid rise of the ground in that direction, rush back towards the capital and fill the streets which approach nearest to their own level. This was the case in the years 1553, 1580, 1604 and 1607, in each of which years the capital was entirely under water, and the dykes which had been constructed for its protection destroyed."[37] Such is a topographical sketch of the country accurately given by a careful writer; and to protect an important region so constantly menaced with inundation, the viceroy now addressed himself. Accordingly he commissioned the engineer Enrique Martinez, in 1607 to attempt the drainage of the lake of Zumpango, by the stupendous canal now known under the name of the DESAGUE DE HUEHUETOCA.