all these measures the city was again submerged in 1607, and in a council held by Velasco drainage was agreed upon as indispensable.
The valley of Mexico lies, as is well known, more than seven thousand feet above the sea-level, in a vast basin enclosed by porphyritic ranges, from whose slopes a number of rivers unite to form four groups of lakes, the Chalco-Xochimilco, Tezcuco, Cristóbal, and Zumpango. The first was a fresh-water body, lying two varas higher than the salt Tezcuco, above whose level the last two also rose to the north in their terrace beds four and ten varas respectively. Zumpango received the two largest streams, notably the Quauhtitlan, which contributed a larger volume than that of all the other valley rivers combined. During the rainy season the excess of water overflowed into the Cristóbal lake, which again discharged into the Tezcuco, causing its waters to rise considerably. At certain periods, once in twenty-five years on an average, this overflow proved destructive, especially to the capital, whose main square lay barely four feet above the lake. Taught by experience, the Aztecs had sought to stem the waters with dikes, not only round the city but on the northern lakes. Both of these were, besides, divided into two sections by transverse causeways. Although strengthened and extended under Spanish rule the barriers proved ineffective, as we have seen, and drainage was at last declared to be the only means.
One natural outlet from the valley existed in the small stream of Tequisquiac, but measurements showed that the cost of making it available for drainage would be too great, and that the only practicable point for an outlet was near the village of Huehuetoca, as demonstrated already in 1580 by Licenciado Obregon
i. 728-9. Among the works attributed to him is the presa de Oculma. Inund. de Mex., MS., 356 Cepeda, Rel., 10, is less exact; and so is Humboldt, Essai Pol., i. 209. On the Mexicalzinco branch of the southern causeway two flood-gates were added. Nevertheless this construction proved prejudicial to Xochimilco and adjoining towns. He also began a new aqueduct. Cavo, Tres Siglos, i. 243-4, 264-5; Beltrami, Mexique, ii. 62-3.
