With more than a foot of rain falling in parts of Houston by Monday afternoon and much of the surrounding areas shut down, Texas Gov. Greg Abbott declared a state of disaster in nine counties.
At a late afternoon press conference, Abbott said there had been over 1,000 water rescues. Authorities also were investigating two fatalities to see if they were weather related.
Crews watching the high water on a Houston freeway service road saw an 18-wheeler truck drive directly into high water, Harris County Precinct 5 Sgt. Herbert Martinez said. Workers later found the driver's body inside the truck.
Officials also were trying to determine if the storm was responsible for the death of a contractor working for the city's airport system whose body was found in a submerged vehicle.
The floods turned commuting into a nightmare, forced schools in the nation's fourth-largest city to close and knocked out power to thousands of people who were urged to shelter in place.
Mayor Sylvester Turner told people to stay home to fend off a weather system he called "stubborn." More rain was projected over the next two to three days although heavy downpours had subsided somewhat by midday and only another half-inch was expected through Monday night, he said.
Rain gauges in parts of Harris County, which includes most of Houston, showed water levels approaching 20 inches since late Sunday night. Ten-inch and higher rainfall amounts were common, particularly in west and north sections of the Southeast Texas area.
The Harris County Flood Control District reported 13 bayous and creeks out of their banks, several of them in Houston.
"There is water all over the place, there is flooding all over the place," Turner said.
The counties where Abbott declared a state of disaster were Austin, Bastrop, Colorado, Fort Bend, Grimes, Harris, Montgomery, Waller and Wharton.
Several shelters were established for people forced from their homes. At least 100 people taken from apartment complexes in the north part of the city were being sheltered at a shopping mall.
Harris County Judge Ed Emmett, the county's chief administrator, said more than 1,000 homes were flooded.
"This is a rain event that's very significant, no question about it," he said. "Many of those homes haven't flooded before."
About 1 million students got the day off, including the Houston Independent School District's 215,000 students, Texas' largest public school district. Most colleges and universities also closed because of the bad weather.
No other major injuries were reported.
Dozens of Houston subdivisions flooded. At least two interstates -- I-10, the main east-west freeway, and I-45, the major north-south freeway -- were under water near downtown.
"We've seen those go under water before and they're under water again," Emmett said.
Other major freeways, plus some feeder roads leading to the highways, were blocked by high water.
"I was trying to get to work," Marcel Gwinn said as he was stranded for more than 90 minutes on an overpass in west Houston. "It kills me because my boss just told me that work's closed for the day."
Fire Department spokesman Jay Evans said the agency had made "numerous, numerous rescues."
"When you get off the freeways and off the main thoroughfares, you could be in water 10 to 15 feet deep," he said. "You do not want to trap yourself in these vehicles."
The storms were part of a wide weather system that left warnings and watches through Tuesday morning for Houston, Austin, San Antonio, Dallas, Fort Worth, Tyler-Longview and as far east as Texarkana.
One TV reporter in Houston helped to rescue a man who drove his car into a flooded underpass.
In the incident captured on video Monday, KTRK reporter Steve Campion yells, "Dude, you've got to get out of the car!"
The man opens the passenger door and crawls out into the water as the reporter yells: "Leave the car! Swim!"
The driver swims toward Campion, who wades out into the waist-deep water and extends his hand.
As the car slowly sinks under water, the driver tells Campion that he's OK and that he didn't think the water was so deep.
Houston, at near sea level and known for its "gumbo" soft soil, is no stranger to flooding from torrential rains, tropical storms and hurricanes. Last Memorial Day, heavy rains caused severe flooding in the southwest parts of the city. Bayous there quickly rose and the mayor urged residents to prepare for another round of floods.
"A lot of rain coming in a very short period of time, there's nothing you can do," Turner said. "I regret anyone whose home is flooded again. There's nothing I can say that's going to ease your frustration. We certainly can't control the weather."
George Bush Intercontinental Airport in Houston reported more than 500 flights canceled. A ground stop was in effect nearly all morning Monday, meaning all traffic in and out was halted. William P. Hobby Airport, the city's other major airport, canceled about 150 flights.
Just north of the airport, emergency crews were trying to rescue several horses trapped by high waters at a stable. Video images showed the animals struggling to keep their heads above water as people driving by on a nearby road yelled encouragement. The fate of the horses wasn't immediately known.
The Associated Press contributed to this report.