The largest part of Central Mexico today is occupied by the drainage basin of River Lerma, with a surface area of about 52,000 km2. The total discharge of this river from its drainage basin into Lake Chapala fluctuates around 60 m3/sec (about 1,900 million m3/year), while the discharge from this lake into River (Río Grande de) Santiago is about 32 m3/sec (1,000 million m3/year). The present drainage of Central Mexico is the result of the Neogene-Quaternary geodynamic developments off the western and southern coasts of Mexico and coeval volcanic activity, which impeded the earlier southward-flowing drainage. This region became obstructed and pluvial lakes were formed. The largest of those lakes, Lake Xalisco, had an estimated surface area of about 70,000 km2. This pluvial lake became a sedimentary basin whose deposits are the principal aquifers and aquitards of this region, some of which are being exploited. The area of River Lerma's drainage basin is provided with about 14 m3/sec (442 million m3/year) of water for domestic, agricultural, and industrial use. Of this amount, about 11 m3/sec (332 million m3/year) are discharged into River Lerma, and thence to Lake Chapala, as highly polluted residual waters. Therefore, a part of the 3 m3/sec (110 million m3/year) of the polluted residual waters infiltrates, and eventually contaminates, some of the aquifers. The remaining part evaporates, leaving behind the pollutants. In an attempt to reduce contamination, a series of water-treatment facilities has been put in operation, and others are planned. These facilities, however, represent only one aspect of the solution, since they will not stop the contamination of the aquifers or Lake Chapala. A more thorough and lasting solution to this problem is proposed from the geological and environmental viewpoints, involving the construction of a 1,500 km-long canal system to channel residual waters of River Lerma's drainage basin directly to River (Río Grande de) Santiago, bypassing Lake Chapala, into the Pacific. This would allow in part restoring the Neogene–Quaternary drainage and, at the same time, gradually upgrading Lake Chapala's environmental quality. This combined geologic and civil engineering task has an estimated cost of approximately 1,400 million dollars or, for comparison, one sixth of the amount (8,500 million dollars) estimated for the environmental cleanup of the border region between the United States of America and Mexico.