Mexico City teeters on 'unprecedented' water shortage - Reuters

[róisín_dubh]

7. You heard correctly.

Because the original lake system had brackish sections, the Mexica not only had to figure out a system (the aqueduct) to get adequate water to the city, they also developed an intricate dike system to prevent flooding of brackish water into the city during the rainy season.

Xochimilco is the only part of the lake that's left, but for how long indeed.