The waters of the rain-swollen Rio Grande push against International Bridge 1, which remained closed on Friday in Laredo, Texas. ((Eric Gay/Associated Press))

A strong storm dumped rain on the already high Rio Grande River with those living downstream on both sides of the U.S.-Mexico border bracing for a torrent of water.

That flood has already led to the closure of two international bridges and evacuations on both sides of the border further up the river.

Residents of some neighbourhoods in the downstream towns of Roma and La Grulla were being evacuated in anticipation of floodwaters.

Shelters were readied for residents of low-lying neighbourhoods in several communities. And the last hand-drawn ferry on the U.S.-Mexico border was hauled out of the Rio Grande's swift and rising waters for the second time this week.

A tropical depression that came ashore just north of the mouth of the Rio Grande on Thursday was expected to add 15 to 20 centimetres of rain to rivers and reservoirs in south Texas and northern Mexico already swollen from the heavy rains dropped by Hurricane Alex last week.

In Rio Grande City in Starr County, Texas, the river reached 16 metres early Friday. A flash flood warning was scheduled to remain in effect for the lower Rio Grande Valley until Friday night.