A pair of reservoir dams that protect central Houston and a suburban levee have started overflowing, adding to the rising floodwaters from Storm Harvey that have crippled the city after five days of rain.
Engineers began releasing water from the Addicks and Barker reservoirs on Monday to ease the strain on the 70-year-old dams, but the releases were not enough to relieve the pressure after one of the heaviest downpours in US history, the Army Corps of Engineers said.
Both reservoirs are at record levels and the release of water means more homes and streets will flood, with some homes inundated for up to a month, said Jeff Lindner of Harris County Flood Control District.
The county is trying to determine where the water will go, Mr Lindner added.
Meanwhile, Brazoria County authorities posted a message on Twitter warning that the levee at Columbia Lakes, south of Houston, had been breached and telling people to leave the area.
Authorities said residents had been warned that the levee would overflow at some point and a mandatory evacuation order was given on Sunday.
More than 17,000 people are seeking refuge in Texas shelters, the American Red Cross said, and the number looks certain to grow.
Houston officials said they will open two or three more mega-shelters to accommodate people who continue to arrive at the overflowing George R Brown Convention Centre seeking refuge from the flooding.
The centre already held more than 9,000 people, almost twice the number officials originally planned to house there, said mayor Sylvester Turner.
"We are not turning anyone away. But it does mean we need to expand our capabilities and our capacity," Mr Turner said. "Relief is coming."
City officials have made a formal request with the Federal Emergency Management Agency for more supplies for an additional 10,000 people, which it is hoped will arrive no later than Wednesday, the mayor said.
Calls for rescue have so overwhelmed emergency teams that they have had little time to search for bodies, and officials acknowledge that fatalities could soar once the floodwaters start to recede from one of America's most sprawling metropolitan centres.
More than four days after the storm ravaged the Texas coastline as a Category 4 hurricane, authorities had confirmed only three deaths - including a woman killed when heavy rains dislodged a large tree on to her trailer home in the small town of Porter.
"We know in these kinds of events that, sadly, the death toll goes up, historically," Houston Police Chief Art Acevedo said. "I'm really worried about how many bodies we're going to find."
One Houston woman said she presumes six members of a family, including four of her grandchildren, died after their van sank into Greens Bayou in East Houston.
Virginia Saldivar said her brother-in-law was driving the van when a strong current took the vehicle off a bridge and into the bayou. The driver got out and urged the children to escape through the back door, Ms Saldivar said, but they could not.
"I'm just hoping we find the bodies," she added.
A spokeswoman for a Houston hotel said one of its employees disappeared while helping about 100 guests and workers evacuate the building.
The disaster is unfolding on an epic scale, with the nation's fourth-largest city mostly paralysed by the storm that parked itself over the Gulf Coast. With nearly 2ft more of rain expected on top of the 30in that has fallen so far in some places, authorities worried the worst might be yet to come.
The Houston metro area covers about 10,000 square miles, and is crisscrossed by about 1,700 miles of channels, creeks and bayous that drain into the Gulf of Mexico, about 50 miles to the south east.
Forecasters expect the storm to linger over the Gulf before heading back inland east of Houston some time on Wednesday. The system will then head north and lose its tropical strength.
Rescuers continued plucking people from inundated Houston neighbourhoods. Mayor Turner put the number saved by police at more than 3,000, while the Coast Guard said it had also rescued more than 3,000 by boat and air and was taking more than 1,000 calls per hour.
Nearly 6,000 inmates displaced by flooding have been moved from prisons in the Houston area to other facilities in south and east Texas, according to the state Department of Criminal Justice.
A mandatory evacuation was ordered for the low-lying Houston suburb of Dickinson, home to 20,000, and questions swirled in Houston itself about why the mayor did not issue a similar evacuation order.
Mr Turner has inisisted that a mass evacuation of millions of people by car was a greater risk than enduring the storm.
By Tuesday morning, more than 9,000 people were at the city's largest shelter set up at the George R Brown Convention Centre - which originally had an estimated capacity of 5,000.
The Red Cross said volunteers made more space in part by pushing some camp beds closer together. A shortage of beds means some people will have to sleep on chairs or the floor.
Before the storm is gone, up to 20in more rain could fall, the National Weather Service said, meaning the flooding will get worse in the days ahead and the water will be slow to recede once Harvey finally moves on.