Torrential rain that followed Hurricane Alex has left parts of Mexico underwater with tens of thousands of people evacuated from their homes.

Officials have closed two US-Mexico border bridges with yet another storm on the way.

The small town of Anahuac woke up underwater as the Salado River overflowed, waters rising more than one metre in the early hours of the morning.

Police made their way through the town trying to usher thousands to higher and drier ground.

Mayor Garza Garcia said 20 floodgates had been opened at the Venustiano Dam. He described the situation as "critical" adding that controlled flooding was necessary to avoid losing the town of Rodriguez to flood waters in its entirety.

A plane on inspection tour of the flood zone has also crashed killing the mayor of the border town of Piedras Negras, the state public works director, a municipal civil defence official, a government photographer, the pilot and co-pilot.