HOUSTON — At least eight people are now dead in Tropical Storm Harvey’s assault on southeast Texas, authorities said Monday, as the nation’s fourth largest city remained awash in murky brown water that submerged highways, houses, shopping plazas and entire neighborhoods.

More than 30,000 people in Houston and across the Gulf Coast were likely to seek temporary shelter as Harvey continued to drench southeastern Texas and Louisiana with heavy rains and surging floodwaters.

“We’re not at recovery yet,” William “Brock” Long, head of the Federal Emergency Management Agency, said at a news conference in Washington, D.C., on Monday, noting that the storm was a “landmark event” that had affected as many as 50 Texas counties, and that more than 450,000 people could end up seeking disaster assistance. “This shelter mission is going to be a very heavy lift.”

As the rain kept pouring, as many as 13 million people, from Houston to New Orleans, were under flood watches and warnings.

The death toll by midday Monday had risen to eight. Officials in Harris County, which includes Houston, have reported six “potentially storm-related” fatalities. One person died in the small coastal town of Rockport, near where Harvey made landfall. A 52-year-old homeless man was also found in the La Marque, a small city near Galveston.

“It’s the hurricane that won’t die,” said William Patzert, an oceanographer at NASA’s Jet Propulsion Laboratory. “The trap is close enough to the Gulf that it keeps siphoning off energy and moisture from the very, very warm waters of the Gulf. Until the large-scale pattern changes, it will continue to soak Texas.”

The National Weather Service said Monday that parts of Harris County, which includes Houston, had seen 30 inches of rain. An additional 15 to 25 inches are expected across the upper Texas coast, with isolated storm totals as high as 50 inches.

By Monday morning, 911 operators had received 56,000 calls, but the backlog that left residents hanging on the telephone, calls unanswered, was almost resolved, city officials said. Houston Police Chief Art Acevedo said officers had rescued 2,000 people from flooding in the city and 185 critical requests for help remained pending.

“Our goal is to complete the rescues of all critical missions today,” Acevedo said.

“It’s still a very dangerous situation out there,” Houston Fire Dept. Chief Samuel Pena said, noting that there had been 290 water rescues since midnight and his department also had pending calls. “We’re expecting more rain. We’re expecting the demand for our services is going to increase.”

Already, more than 5,500 residents are in Houston-area shelters. Local officials expect that number to rise, along with the flood water.

“We need to get people to shelters and make sure they are good and have what they need,” Harris County Judge Ed Emmett said at a Monday news briefing, noting that the American Red Cross has had trouble getting supplies to its shelters.

Houston braced for yet more water as the Army Corps of Engineers opened two swollen flood-control reservoirs early Monday. The Corps said it needed to undertake a controlled release of water from the Addicks and Barker reservoirs to limit the scope of the disaster.

Even with the controlled release, the reservoirs were rising at a rate of 4 inches an hour, said Edmund Russo, deputy district engineer for programs and project management for the Army Corps of Engineers Galveston District.

“It could create additional problems, additional flooding,” Houston Mayor Sylvester Turner said at a news conference Monday. “People who were not in a crisis state yesterday may find themselves in a crisis state today.”

Harvey was a Category 4 hurricane at its peak. It has since been downgraded to a tropical storm. But the hurricane scale is based on wind speed, not volume of water, and Harvey has continued to funnel tremendous amounts of moisture into Texas.

As homes across the metro region filled with waist-deep water, rescuers scoured flooded inner-city streets and subdivisions in kayaks, fishing boats and inflatable rafts, plucking families to safety.

“Hurricane Harvey has effectively turned south and central Texas into a lake the size of Michigan,” Brad Kieserman, vice president of disaster operations and logistics for the Red Cross, told NPR. “This is as catastrophic as you could possibly imagine from a Category 4 storm.”

Strong currents proved a challenge Monday morning as half a dozen volunteers with a pontoon boot tried to save 20 people, including children and the elderly, trapped in a flooded neighborhood in Spring, Texas, at the northern edge of Houston.

People called out for help from the upper levels of two-story homes. Yet the 40-foot boat could save only a dozen at a time. After they launched to attempt the rescue, a Harris County deputy constable ran up to the crew, frantic. Authorities planned to release more water from Lake Conroe to the north that would overwhelm the creek, he said.

The boat retreated without saving anyone.

“We couldn’t get them,” Mandi Davis, 36, of Spring, said when she landed. “The current was too strong and the water was too deep. They’re going to have to get airlifted out.”

Genesis Rivas, 20, and her family were disappointed to see the volunteers return empty-handed. Seven of her relatives were stranded, including her grandmother and two children ages 4 and 2. She estimated that 200 people were trapped on their street.

“We’re worried about the kids,” she said as the group huddled under an umbrella near the would-be rescuers. “Hay mucha agua — the water is too strong,” she told a relative in a mix of Spanish and English. Her sister watched astonished as an Austin special operations rescue crew arrived, checked the water and departed.

“They’re just going to leave the people there?” said Odaly Ticas, 23. “It’s more than 200 people. There was a cop with a boat just here. I don’t know why they left.”

With forecasters predicting that the Brazos River, which runs southwest of Houston, would crest at 59 feet — topping its historical record of 54.7 feet — local city and county officials on Sunday urged residents in low-lying areas to leave their homes to find safer ground.

“Evacuate immediately,” the city of Rosenberg urged residents on Twitter.

On Sunday night, Fort Bend County Judge Robert Hebert issued mandatory evacuation orders for more districts.

“Fifty-nine feet represents at least an 800-year flood event, and there’s no levee designed to prevent an 800-year flood,” he said at a news conference.

More than 100,000 residents in Fort Bend — roughly 20 percent of the county’s population — were under voluntary and mandatory evacuations, he said.

By early Monday afternoon, the National Hurricane Center in Miami said Harvey’s center was 40 miles east-northeast of Port O’Connor and drifting erratically east-southeast toward the mid-Texas coast at 3 mph. On Tuesday, it is expected to gradually turn northeast toward the shore of the middle and upper Texas coasts.

In Houston, the police chief urged residents to be patient, saying it was still extremely difficult to reach those who were stranded in flooded homes.

“You know, the dams are about to open and that’s not music to my ears, I can tell you that much,” Acevedo said on a livestream video late Sunday as he cruised the city’s southwest freeway in the dark amid torrential rain.

“It’s amazing,” he said. “They said it was going to be a five-day event, and I’m telling you, Harvey’s going to make us sweat every single day. Just continue to pray for our city and the people that we’re serving and the men and women that are trying to save them.”

Meanwhile, the National Weather Service warned the flood threat is spreading east into neighboring Louisiana, bringing up to 25 inches of rain in the southwest part of the state.

President Donald Trump declared “emergency conditions” in Louisiana on Monday after Gov. John Bel Edwards sent a letter requesting an emergency disaster declaration for the state. Trump is planning to visit flooded areas of Texas on Tuesday.

Just a few inches of rain could cause serious problems in New Orleans, which is still recovering from flooding after thunderstorms this month overwhelmed the city’s drainage system.

Hundreds of people who were stranded at Houston’s Hobby Airport arrived in Dallas late Sunday on specially approved “rescue flights.”

David Best, 60, of Cedar Hill outside Dallas, got stuck after a weeklong vacation with friends in Belize. He slept on the floor, ate rationed burritos from the only restaurant open — Pappasito’s — and hoped for relief.

“I felt sorry for the airport employees who were there and got trapped,” he said after his Southwest flight arrived at Dallas’ Love Field. “It doesn’t look like it’s going to be over for some time. They’re talking about that storm coming right up through the center of Houston again.”