Houston - Harvey sent devastating floods pouring into the nation's fourth-largest city on Sunday as rising water chased thousands of people to rooftops or higher ground and overwhelmed rescuers who could not keep up with the constant calls for help.The incessant rain covered much of Houston in turbid, grey-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, boats buzzed across submerged neighbourhoods and high-water vehicles ploughed through water-logged intersections. Some people managed with kayaks or canoes or swam.Volunteers joined emergency teams to pull people from their homes or from the water, which was high enough in places to gush into second floors. The flooding from Harvey, which made landfall late on Friday as a Category 4 hurricane and has lingered dropping heavy rain as a tropical storm, 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.Residents living around the Addicks and Barker reservoirs designed to help prevent flooding in downtown Houston, were warned on Sunday that a controlled release from both reservoirs would cause additional street flooding and could spill into homes. Rising water levels and continuing rain was putting pressure on the dams that could cause a failure without the release.Harris and Fort Bend county officials said on Sunday that residents around certain areas should be prepared for the influx of water that was scheduled to happen at Addicks around 02:00 on Monday and a day later at Barker. Officials warned residents they should pack their cars Sunday night and wait for daylight Monday to leave.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 for at least two deaths.1.3m of rainAs 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 1.3m of rain. That would be the highest amount yet recorded in Texas.Some areas have already received about half that amount. Since Thursday, South Houston recorded nearly 63cm and and the suburbs of Santa Fe and Dayton got 69cm. "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 1 metre 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.After helping the driver of the submerged truck get to safety, a man floats on the freeway flooded by Tropical Storm Harvey near downtown Houston. (Charlie Riedel, AP) Hospitals evacuated 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.Tom Bartlett and Steven Craig pulled a rowboat on a rope through chest-deep water for 1.6m to rescue Bartlett's mother from her home in west Houston. It took them 45 minutes to reach the house. Inside, the water was halfway up the walls.Marie Bartlett, 88, waited in her bedroom upstairs."When I was younger, I used to wish I had a daughter, but I have the best son in the world," she said. "In my 40 years here, I've never seen the water this high."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 Centre 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 on 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.'Lake'William Cain sought shelter after water started coming inside his family's apartment and they lost power. "I live in a lake where there was once dry land," he 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. Governor 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.Roger Braugh jnr searches for his boat is this boat storage facility in Cove Harbour in Rockport, Texas, in the wake of Hurricane Harvey. (Rachel Denny Clow, Corpus Christi Caller-Times via AP)Nightmare"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.Jesse Gonzalez, and his son, also named Jesse, used their boat to rescue people from a southeast Houston neighbourhood. Asked what he had seen, the younger Gonzalez replied: "A lot of people walking and a lot of dogs swimming.""It's chest- to shoulder-deep out there in certain areas," he told television station KTRK as the pair grabbed a fuel can to refill their boat.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 on 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. On Sunday, it was virtually stationary about 40km northwest of Victoria, Texas, with maximum sustained winds of about 72.42km/h, the hurricane centre said. Harvey was the fiercest hurricane to hit the US in 13 years and the strongest to strike Texas since 1961's Hurricane Carla, the most powerful Texas hurricane on record.