Houston battered by 'mind-boggling' flash flooding

The Texas city of Houston has suffered through its wettest day ever, copping a "mind-boggling" half a metre of rain that has killed four people.

More than 1000 homes have been flooded, 900 people rescued, NBC reports.

Livestock and horses were also caught by the flooding, with a number filmed desperately trying to swim through neck-deep water.

Happily, Houston Office of Emergency Management officials say none of the horses lost their lives.

More than 70 horses were able to be rescued, with people using ropes and a boat to pull them to safety.

The flash flooding came on so strong and fast drivers were caught unawares and forced to abandon their cars.

"Why are you doing that?" local reporter Steve Campion yelled to an elderly man, who tried to drive through a flooded underpass.

The car then starts to float, then sink.

"Dude, you’ve got to get out of the car! Leave the car! Swim!" Campion yells.v The reporter then pulled the man to safety.

His vehicle was eventually swallowed by the water.

"This is a mind-boggling situation," said a spokesman for the Weather Channel.