Governer Andres Granier said opening the dam could release up to 2,000 cubic metres of water per second into the Carrizal and Samaria rivers which are already at critical levels. He added that the real rainy season was just about to start for local residents.
The states of Veracruz, Oaxaca and Tabasco were the worst hit by the floods which swept through entire towns. In the state of Tabasco alone, there has been more than three and half times the annual rainfall for the region.
The heavy rains are partly due the hurricanes which regularly hit south-east Mexico, but also to an increase in the temperature of the sea.
President Felipe Calderon has blamed the situation on climate change.
Some 200,000 hectares of land were submerged, affecting more than 20,000 farmers in the region. The town of Tlacotalpan, a Unesco World Heritage site, is also submerged under water.
The massive flooding has also hit neighbouring Guatemala where the death toll has reached at least 60 with many more still missing. On Tuesday, officials called off the search for more corpses over safety fears.