A major dam outside Houston has begun spilling over as Storm Harvey pushes the reservoir past capacity, Texas officials say.
Engineers have tried to prevent nearby communities from being inundated by releasing some of the water held by the Addicks dam.
But flood control official Jeff Lindner says water levels are now over the height of the reservoir edge.
Harvey has brought huge floods to Texas and is starting to affect Louisiana.
Unprecedented rainfall has forced thousands of people to flee their homes while rescuers are trying to reach others that remain stranded.
At least nine people are reported to have died in the Houston area. Six members of the same family died trying to flee rising floodwaters, relatives told US media.
While spillover would not cause the Addicks dam to fail, it would add to flooding in areas close to the Buffalo Bayou, the main river into the fourth largest city in the US.
Flood officials are also concerned about the Barker dam, which also controls the amount of water in the river.
President Donald Trump and First Lady Melania Trump are visiting Texas on Tuesday to see the devastation caused by Hurricane Harvey, now downgraded to a tropical storm.
Meanwhile, rain is continuing to fall. In Houston, forecasts suggest that some areas in and around the city could see up to 12in (30cm) of rain on Tuesday, bringing the total rainfall from Harvey to about 50in.
Harvey was the most powerful hurricane to hit Texas in more than 50 years when it made landfall on Friday near Corpus Christi, 220 miles (354km) south-west of Houston.
The slow-moving storm – currently over the Gulf of Mexico – will continue to dump huge amounts of rain in the coming days over already flood-hit areas.
By James Cook, BBC North America correspondent, Houston
All day in the suburbs to the west of Houston, hundreds of families waded cautiously along a road they usually take to the shops, to school and to work.
It was an exhausting journey of well over a mile in waist-high flooding.
All night, as water lapped into their homes, people had been calling for help, they said, but no-one came.
Eventually a lone police officer arrived but by then people had taken matters into their own hands, forming a makeshift armada of little boats, jet skis, blow-up mattresses and even rubber rings.
All day long they streamed out in the teeming rain.
Occasionally someone would collapse and have to be helped up. Some children cried but most sat in shocked, soaking silence as they bobbed along on their inflatable rafts, teeth chattering in the cold.
Wading through the water, Anthony Rogers, 54, summed up the mood. The authorities, he said, had been “useless”.
Water in the Addicks reservoir exceeded 108ft (32.9m) on Tuesday, geological data shows, causing it to flow over the top of defences.
Officials said the overspill could cause further flooding in nearby communities but the exact impact was unknown.
“This is something we’ve never faced before, so we’re trying our best to wrap around what exactly this water is going to do,” Jeff Lindner said.
“Luke 21:26 Men’s hearts failing them for fear, and for looking after those things which are coming on the earth: for the powers of heaven shall be shaken. 21:27 And then shall they see the Son of man coming in a cloud with power and great glory. 21:28 And when these things begin to come to pass, then look up, and lift up your heads; for your redemption draweth nigh.”