A major dam outside Houston has begun overspilling as Storm Harvey pushes the reservoir past capacity, a Texas official says, the BBC reports.
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.
Unprecendented rainfall has forced thousands of people to flee their homes. At least nine people are reported to have died in the Houston area.
While spillover would not cause the Addicks dam to fail, it would add more water to the Buffalo Bayou, the main river into the fourth largest city in the US.