The incessant rain covered much of Houston in turbid, grey-green water and turned streets into rivers navigable only by boat.
Helicopters landed near flooded freeways, airboats buzzed across submerged neighbourhoods and high-water vehicles ploughed through water-logged intersections. Some people managed with kayaks or canoes or swam.
Volunteers joined emergency teams to pull people from their homes or from the water, which was high enough in places to gush into second floors.
The flooding from Harvey, which made landfall late on Friday as a Category 4 hurricane and has lingered dropping heavy rain as a tropical storm, was so widespread that authorities had trouble pinpointing the worst areas.
They urged people to get on top of their houses to avoid becoming trapped in attics and to wave sheets or towels to draw attention to their location.
Rising water levels and continuing rain was putting pressure on the dams that could cause a failure without the release.
Judging from federal disaster declarations, the storm has so far affected about a quarter of the Texas population, or 6.8million people in 18counties. It was blamed for at least two deaths. As the water rose, the National Weather Service issued another ominous forecast.
Before the storm is gone, some parts of Houston and its suburbs could get as much as 1.3m of rain. That would be the highest amount ever recorded in Texas.
Some areas have already received about half that amount. Since Thursday, South Houston recorded nearly 63cm, and the suburbs of Santa Fe and Dayton got 69cm.
“The breadth and intensity of this rainfall is beyond anything experienced before,” the National Weather Service said.
Rescuers had to give top priority to life-and-death situations, leaving many affected families to fend for themselves. And several hospitals in the Houston area were evacuated because of the rising waters.
It was not clear how many people were plucked from the floodwaters. Up to 1200 people had to be rescued in Galveston County alone, said Mark Henry, the county judge.
Some people used inflatable beach toys, rubber rafts and even air mattresses to get through the water to safety. Others waded while carrying trash bags stuffed with their belongings and small animals in picnic coolers.
Houston Mayor Sylvester Turner said authorities had received more than 2000 calls for help, with more coming in. He urged drivers to stay off roads to avoid adding to the number of those stranded.
The Coast Guard deployed five helicopters and asked for additional aircraft from New Orleans.
The White House announced that President Donald Trump would visit Texas tomorrow.
He met yesterday by teleconference with top administration officials to discuss federal support for response and recovery efforts.
The rescues unfolded a day after Harvey settled over the Texas coastline. The system weakened on Saturday to a tropical storm.
Yesterday, it was virtually stationary about 40km north-west of Victoria, Texas, with maximum sustained winds of about 72.42km/* , the hurricane centre said.
Harvey was the fiercest hurricane to hit the US in 13 years and the strongest to strike Texas since 1961’s Hurricane Carla, the most powerful Texas hurricane on record.