A major dam outside Houston city in the US' Texas state has begun overflowing as storm Harvey pushed the reservoir past capacity, officials said.
Engineers have tried to prevent nearby communities from being inundated by releasing some of the water held by the Addicks dam, BBC reported on Tuesday.
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 in the Houston area while rescuers are trying to reach others that remain stranded.
At least nine persons are reported to have died, including six members of the same family whose van was swept away by rising floodwater, and a man in his 60s who apparently drowned while trying to swim to safety.
While a 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 Houston - - the fourth largest city in the US.
Experts are also concerned about the Barker dam, which also controls the amount of water in the river.
The flood gauges on the dam have themselves been put out of action by the deluge.
Meanwhile, officials in Brazoria County, south of Houston, said a levee at Columbia Lakes had been breached, tweeting: "Get out now!"
US President Donald Trump and First Lady Melania Trump are visiting Texas 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 12 inches (30cm) of further rain on Tuesday.
One neighbourhood in south Houston has seen the heaviest total rainfall from a tropical storm in the US since records began in the 1950s, the National Weather Service says.
A total of 49. 2 inches has fallen at Mary's Creek at Winding Road, beating the previous record of 48 inches.
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 (354 km) 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.