By Peter Henderson and Mica Rosenberg

HOUSTON (Reuters) - Floodwaters from Tropical Storm Harvey, which has already killed at least seven people in Texas and was expected to drive 30,000 others from their homes, will likely rise in the coming days, officials warned on Monday as heavy rain continued to pound the U.S. Gulf Coast.

National Guard troops, police officers, rescue workers and civilians raced in helicopters, boats and special high-water trucks to rescue the hundreds of people still stranded in and around Houston, the nation's fourth-largest city.

The storm was the most powerful hurricane to strike Texas in more than 50 years when it came ashore on Friday near Corpus Christi, 220 miles (354 km) south of Houston.

It is believed to have killed at least six people in Harris County, where Houston is located, according to Tricia Bentley, a spokeswoman for the county coroner's office, including a man who died in a house fire and an elderly woman attempting to drive through flooded streets on the city's west side.

A 60-year-old woman died in neighboring Montgomery County when a tree fell on her trailer home while she slept, the local medical examiner said on Twitter.

As stunned families surveyed the wreckage of destroyed homes and roads flooded or clogged with debris, Texas Governor Greg Abbott warned Houstonians to brace for a long recovery.

"We need to recognize this is going to be a new and different normal for this entire region," Abbott said after touring Corpus Christi.

Harvey was expected to linger over Texas' Gulf Coast for the next few days, dropping another 10 to 20 inches (25 to 51 cm) of rain, with threats of flooding extending into Louisiana.

MORE FLOODING TO COME

In scenes evoking the aftermath of Hurricane Katrina in 2005, police and Coast Guard teams have rescued at least 2,000 people so far, plucking many from rooftops by helicopter, as they urged the hundreds more believed to be marooned in flooded houses to hang towels or sheets outside to alert rescuers.

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Residents with boats pitched in, including 26-year-old Kayla Harvey who used social media to locate stranded neighbors.

"This is just what we do for our community. We don't wait for someone to come and help. We just go out and do it," she said.

Harvey's center was 85 miles (137 km) south-southwest of Houston on Monday afternoon and forecast to arc slowly toward the city through Wednesday, with the worst floods expected later that day and on Thursday.

Schools and office buildings were closed throughout the metropolitan area, home to 6.8 million people, as chest-high water filled some neighborhoods in the low-lying city.

The U.S. Army Corps of Engineers said Monday that it was releasing water from the nearby Addicks and Barker reservoirs into Buffalo Bayou, the primary body of water running through Houston.

"The more they release it could go up and it could create even additional problems," Houston Mayor Sylvester Turner warned. But the release was said to be necessary to prevent an uncontrolled surge of water, which Turner said "would be exponentially worse."

Torrential rain also hit areas more than 150 miles (240 km) away, swelling rivers and causing a surge that was heading toward the Houston area, where numerous rivers and streams already have been breached.

'ALL THE ROADS ARE CLOSED'

The high floodwaters made it hard for some residents who had fled their homes to find shelter. Christe Fletcher, 37, fled her house after it flooded to waist-deep, but was struggling to find a safe route to a nearby hotel.

"It's kind of hard to get there because all of the roads are closed," she said. "It's the worst experience you can go through."

About 5,500 people were in shelters as of Monday morning, city officials said, with Federal Emergency Management Agency director Brock Long forecasting that 30,000 would eventually be housed temporarily in shelters.

Regina Costilla, 48, said she and her 16-year-old son had been rescued from their home by a good Samaritan with a boat. She worried until she was reunited with her husband and large dog, who had been left behind because they did not fit into the boat.

"I'm not complaining; we're alive," said Costilla. "When I saw the forecast of the storm I said I'll be happy if we get out with our lives."

Houston did not order an evacuation due to concerns about people being stranded on city highways now consumed by floods, Turner said.

Abbott, who had suggested on Friday that people leave the area, declined to second-guess the mayor on Monday, telling reporters, "Decisions about evacuations are something that are behind us."

U.S. President Donald Trump plans to go to Texas on Tuesday to survey the damage. He said he might return on Saturday and could stop in Louisiana where the storm is now dumping rain.

Trump, facing the biggest U.S. natural disaster since he took office in January, has signed disaster proclamations for Texas and Louisiana, triggering federal relief efforts.

Almost half of the U.S. refining capacity is in the Gulf region. Shutdowns extended across the coast, including Exxon Mobil Corp's facility in Baytown, the nation's second largest refinery.

The floods' path of destruction could destroy as much as $20 billion in insured property, making it one of the costliest storms in history for U.S. insurers, according to Wall Street analysts.

(Additional reporting by Marianna Perraga, Erwin Seba, Nick Oxford and Ernest Scheyder in Houston and Steve Holland in Washington; Writing by Scott Malone and Dan Whitcomb; Editing by Tom Brown and Lisa Shumaker)