Houston - Officials released more water from Houston-area reservoirs overwhelmed by Hurricane Harvey early on Monday in a move aimed at protecting the city's downtown from devastating floods, but that could still endanger thousands of homes, even as the nation's fourth-largest city braced for more rain.Harvey, which made landfall late on Friday as a Category 4 hurricane and has lingered dropping heavy rain as a tropical storm, sent devastating floods pouring into Houston on Sunday. The 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.Federal Emergency Management Agency Director Brock Long said during a news conference on Monday that 50 counties in Texas are affected by the flooding and that a tremendous amount of rainfall is in the cards for southwest Louisiana. The rain has been blamed in at least two deaths. Vehicles sit stuck in high water from Hurricane Harvey in Houston. (Erich Schlegel, AFP) Residents living near the Addicks and Barker reservoirs - that were designed to prevent flooding in downtown Houston - were warned on Sunday that a controlled release from both reservoirs would cause additional street flooding that 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 advised residents to pack their cars on Sunday night and wait for daylight to leave."The idea is to prepare ... pack up what you need and put it in your vehicle and when the sun comes up, get out," said Jeff Lindner, a meteorologist for the Harris County Flood Control District. "And you don't have to go far, you just need to get out of this area."The Army Corps of Engineers started the reservoir releases before 02:00 on Monday - ahead of schedule - because water levels were increasing at a rate of more than 15cm per hour, Corps spokesperson Jay Townsend said. The timetable was moved up to prevent more homes from being flooded, Townsend said.'800-year flood level'Meanwhile, officials in Fort Bend County, Houston's southwestern suburbs, late on Sunday issued mandatory evacuation orders along the Brazos River levee districts. A submerged car on Houston’s Interstate 610 North. (Thomas B Shea, AFP) County officials were preparing for the river to reach major flood stages late on Sunday. County Judge Robert Herbert said at a news conference that National Weather Service officials were predicting that the water could rise to 18, three metres 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, 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, airboats 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 in pulling people from their homes or from the water. Authorities 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.DrainagePolice Chief Art Acevedo said on Monday that drainage remains a concern."I'm not sure where the water is going because it's just so much that we can't really absorb more in the ground at this point. ... We have way too much water and not enough places for it to drain," Acevedo told MSNBC's Morning Joe. A boy stands in what had been his cousin’s yard that was flooded when Hurricane Harvey hit Bayside. (The Victoria Advocate via AP) And on the possibility of the rain subsiding, he said: "We're just keeping our fingers crossed"As the water rose, the National Weather Service issued another ominous forecast: Before the storm that arrived on 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.FEMA's 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.SheltersHouston'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 moved some belongings upstairs then grabbed her grandchildren."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 and urged drivers to stay off the roads."I don't need to tell anyone this is a very, very serious and unprecedented storm," Turner said.GovernorThe 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, citing the risk of sending the city's 2.3 million inhabitants onto the highways at the same time."If you think the situation right now is bad, and you give an order to evacuate, you are creating a nightmare," Turner said.The Coast Guard deployed five helicopters and asked for additional aircraft from New Orleans.The White House said 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 on Saturday to a tropical storm. By early Monday, Harvey had shifted a little closer to the coast, hovering about 30km east of Victoria, with sustained winds of about 65km/h. The National Hurricane Centre said it continued to edge in a southeasterly direction at 4.8km/h. 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.