On the flooded Gulf Coast, hundreds of thousands of people are facing a housing crisis that will drag on long after the floodwater recedes.

Many have little information about the homes they fled. Now they must navigate the complex process of filing insurance claims, seeking government assistance and deciding whether to rebuild or pick up stakes.

“We expect a many-year recovery in Texas, and the federal government is in this for the long haul,” Elaine Duke, acting secretary of the Department of Homeland Security, said Wednesday.

The federal agencies in charge of recovery face a task that is complex and massive in scale.

“The life-sustainment mission is huge. It’s going to grow,” said Federal Emergency Management Agency Administrator William B. “Brock” Long, who hours earlier had returned from a trip to Texas, where he had seen the storm damage and flooding firsthand.

Houston’s floodwaters are receding, but they remain dangerously high in many areas

He reissued a call for volunteers and said their help is needed beyond the immediate rescue effort.

“The need to volunteer is going to take place over the next couple of years, okay. And the need to volunteer, let me remind you, is in 50 counties now, not just in Houston but everywhere,” he said.

Retired Coast Guard Adm. Thad Allen, who helped lead the federal government’s response to Hurricane Katrina in 2005, said, “We don’t have complete knowledge of what everybody’s situation is. We know some people went to shelters. Some people went to relatives. It was like a diaspora. What we don’t know is how many housing units were lost.”

“It is insurance claim adjustment on a scale we have never seen before in the history of this country,” he added.

Linda Thompson, a resident of the Robindell neighborhood in southwest Houston, said this week that she had endured smaller floods in 2015 and 2016. This time, the water would have reached over her head had she not evacuated. She managed to salvage one cardboard box containing soggy photographs that she was trying to pat dry with a towel in a hotel lobby. She said Harvey’s floodwater has destroyed at least $50,000 in belongings, including custom-made furniture and original artwork. She plans this time to move rather than rebuild.

“I’m just bracing myself to see if I can do this, because I know: Get the elevation certificates, get your documents for the insurance, and start fighting with the insurance company,” Thompson said. “You’ve got to write all, like, seeing what receipts you have and write every single itemized list.”

Evacuees and volunteers at Purple Heart Recreational Center's ad hoc shelter described the conditions Aug. 29 in Lake Charles, La., where Tropical Storm Harvey continues its downpour. (Tom Moore,Monica Akhtar/The Washington Post)

According to the Texas Department of Public Safety, nearly 35,000 people had taken refuge in 231 shelters as of Wednesday morning. Officials said 294,000 people remained without power from Corpus Christi to Port Arthur.

More than 10,000 rescues have been conducted by state and county agencies, the report said, though that total did not take into account rescues conducted by civilians. FEMA, which has coordinated the federal response, said 12,400 employees from 17 agencies are working to help the disaster survivors in Texas and Louisiana. The agency said its urban search-and-rescue teams had rescued 2,500 people, with another 4,200 people and 1,000 pets rescued by the U.S. Coast Guard.

Houston officials have opened three major shelters: the George R. Brown Convention Center, which quickly filled with 10,000 people; the Toyota Center, where the Houston Rockets basketball team plays; and the NRG Center, a convention center close to the old Astrodome, which was a major refuge 12 years ago for survivors of Katrina who were transported to the city. Dallas is ready to take on another 6,000 storm survivors.

Texas Gov. Greg Abbott (R) said the federal aid package necessary to respond to Harvey “should be far in excess” of the roughly $120 billion spent on the Katrina recovery. U.S. Rep. Sheila Jackson Lee (D-Tex.), who represents central Houston, said the federal response will likely reach about $150 billion.

By Wednesday morning, 195,000 people had already filed for assistance from FEMA, said Alex Amparo, who leads the agency’s recovery directorate. The agency has given out more than $35 million in disaster assistance so far, he said.

He urged people to start the process of recovery by filing an insurance claim if possible. Then they should register for FEMA aid at www.DisasterAssistance.gov.

“Our assistance is not designed to make you whole, which is why it is important to register with your insurance company,” Amparo said.

In the counties declared a federal disaster area, only 16 percent of homes — about 400,000 homeowners — have flood insurance through the federal flood insurance program, said Laura Lightbody, director of the Pew Charitable Trusts’ Flood-Prepared Communities initiative.

“This is an event unlike we’ve ever seen before that no one could have predicted. It forces a national discussion about flood preparedness, the way that we plan and develop and think about living in areas that are prone to flooding,” Lightbody said Wednesday. “Does it make sense to rebuild or build in an area that we know is prone to flooding?”

Long acknowledged that the shelters “are obviously not ideal” but that people will have to be there for a while. So far, he said, 1,800 people have been placed in hotels and motels under FEMA’s Transitional Shelter Assistance program, in which the government seeks to alleviate crowding in emergency shelters by directly paying hotels and motels to put up disaster survivors.

“The next goal is to save houses,” Long said. “This is where the volunteers have to be organized — helping people muck out their houses.”

FEMA is hiring temporary workers to supplement the disaster response. Volunteers can also sign up with the National Voluntary Organizations Active in Disasters.

The U.S. Department of Housing and Urban Development is also charged with assisting in a disaster. Like FEMA, HUD drew criticism for how it handled the aftermath of Katrina.

Before Harvey slammed into Texas late Friday night, Houston had an 11 percent vacancy rate in its rental housing market, according to HUD. But many of those rental units, which could be used to house those displaced by the storm, are likely underwater.

“It’s still early yet, but we’re working to determine if the rental market can absorb the significant number of families forced from their own homes,” HUD spokesman Brian Sullivan said Wednesday.

HUD Secretary Ben Carson said President Trump’s declaration of a disaster in 18 Texas counties will allow the department to offer mortgage and foreclosure relief, among other types of assistance.

“As FEMA begins to assess the damage and respond to the immediate needs of residents, HUD will be there to offer assistance and support the longer-term housing recovery efforts,” Carson said.

In the meantime, there is a lot of uncertainty in the communities where people have not had a chance to return to their homes.

“I don’t think I ever will recover financially from this,” said Linda Oliver in Katy, Tex., as she stood on a soggy golf course near her submerged subdivision. As she spoke, helicopters buzzed overhead, helping with rescues. Her son-in-law had managed to rescue her in his truck.

“I’m certainly just happy to be safe,” she said, clearly shaken.

Eva Ruth Moravec in Austin, Emily Wax-Thibodeaux in Katy, Tex., Avi Selk in Houston, and Wesley Lowery and Peter Whoriskey in Washington contributed to this report.