The Army Corps of Engineers started the reservoir releases before 2 a.m. Monday -- ahead of schedule -- because water levels were increasing dramatically at a rate of more than six inches (15 centimeters) per hour, Corps spokesman Jay Townsend said. The timetable was moved up to prevent more homes from being flooded, Townsend said.
 
Meanwhile, officials in Fort Bend County, Houston's southwestern suburbs, late Sunday issued widespread mandatory evacuation orders along the Brazos River levee districts. County officials were preparing for the river to reach major flood stages late Sunday. County Judge Robert Herbert said at a news conference that National Weather Service officials were predicting that the water could rise to 59 feet (18 meters), three feet (90 centimeters) above 2016 records and what Herbert called an "800-year flood level." Herbert said that amount of water would top the levees and carries a threat of levee failure.
 
On Sunday, incessant rain covered much of Houston in turbid, gray-green water and turned streets into rivers navigable only by boat. In a rescue effort that recalled the aftermath of Hurricane Katrina, helicopters landed near flooded freeways, airboats buzzed across submerged neighborhoods and high-water vehicles plowed through water-logged intersections. Some people managed with kayaks or canoes or swam.
 
Volunteers joined emergency teams in pulling people from their homes or from the water. The flooding was so widespread that authorities had trouble pinpointing the worst areas. They urged people to get on top of their houses to avoid becoming trapped in attics and to wave sheets or towels to draw attention to their location.
 
Judging from federal disaster declarations, the storm has so far affected about a quarter of the Texas population, or 6.8 million people in 18 counties. It was blamed in at least two deaths.
 
As the water rose, the National Weather Service issued another ominous forecast: Before the storm that arrived Friday as a Category 4 hurricane is gone, some parts of Houston and its suburbs could get as much as 50 inches (1.3 meters) of rain. That would be the highest amount ever recorded in Texas.
 
Some areas have already received about half that amount. Since Thursday, South Houston recorded nearly 25 inches (63 centimeters), and the suburbs of Santa Fe and Dayton got 27 inches (69 centimeters).
 
"The breadth and intensity of this rainfall is beyond anything experienced before," the National Weather Service said in a statement.
 
Average rainfall totals will end up around 40 inches (1 meter) for Houston, weather service meteorologist Patrick Burke said.
 
The director of the Federal Emergency Management Agency, Brock Long, predicted that the aftermath of the storm would require FEMA's involvement for years.
 
"This disaster's going to be a landmark event," Long said.
 
Rescuers had to give top priority to life-and-death situations, leaving many affected families to fend for themselves. And several hospitals in the Houston area were evacuated due to the rising waters.
 
It was not clear how many people were plucked from the floodwaters. Up to 1,200 people had to be rescued in Galveston County alone, said Mark Henry, the county judge, the county's top administrative post.
 
Houston's George R. Brown Convention Center was quickly opened as a shelter. It was also used as a shelter for Katrina refugees in 2005.
 

Gillis Leho arrived there soaking wet. She said she awoke Sunday to find her downstairs flooded. She tried to move some belongings upstairs, then grabbed her grandchildren.
 
"When they told us the current was getting high, we had to bust a window to get out," Leho said.
 
Some people used inflatable beach toys, rubber rafts and even air mattresses to get through the water to safety. Others waded while carrying trash bags stuffed with their belongings and small animals in picnic coolers.
 
Houston Mayor Sylvester Turner said authorities had received more than 2,000 calls for help, with more coming in. He urged drivers to stay off roads to avoid adding to the number of those stranded.
 
"I don't need to tell anyone this is a very, very serious and unprecedented storm," Turner told a news conference. "We have several hundred structural flooding reports.  We expect that number to rise pretty dramatically."
 
The deteriorating situation was bound to provoke questions about the conflicting advice given by the governor and Houston leaders before the hurricane. Gov. Greg Abbott urged people to flee from Harvey's path, but the Houston mayor issued no evacuation orders and told everyone to stay home.
 
The governor refused to point fingers on Sunday.
 
"Now is not the time to second-guess the decisions that were made," Abbott, a Republican, said at a news conference in Austin. "What's important is that everybody work together to ensure that we are going to, first, save lives and, second, help people across the state rebuild."
 
The mayor, a Democrat, defended his decision, saying there was no way to know which parts of the city were most vulnerable.
 
"If you think the situation right now is bad, and you give an order to evacuate, you are creating a nightmare," Turner said, citing the risks of sending the city's 2.3 million inhabitants onto the highways at the same time.
 
The Coast Guard deployed five helicopters and asked for additional aircraft from New Orleans.
 
The White House announced that President Donald Trump would visit Texas on Tuesday. He met Sunday by teleconference with top administration officials to discuss federal support for response and recovery efforts.
 
The rescues unfolded a day after Harvey settled over the Texas coastline. The system weakened Saturday to a tropical storm. By early Monday, Harvey had shifted a little closer to the coast, hovering about 20 miles (30 kilometers) east of Victoria, with sustained winds of about 40 mph (65 kph). The National Hurricane Center said it continued to edge in a southeasterly direction at 3 mph (4.8 kph).
 
Harvey was the fiercest hurricane to hit the U.S. in 13 years and the strongest to strike Texas since 1961's Hurricane Carla, the most powerful Texas hurricane on record.