Livingston Parish, Louisiana (CNN) The catastrophic flood devastating Louisiana is now the worst natural disaster to strike the United States since Hurricane Sandy four years ago, the Red Cross said.

"Thousands of people in Louisiana have lost everything they own and need our help now," said Brad Kieserman, the Red Cross' vice president of disaster services operations and logistics.

"This disaster is the worst to hit the United States since Superstorm Sandy, and we anticipate it will cost at least $30 million -- a number which may grow as we learn more about the scope and magnitude of the devastation."

The calamity struck quickly and ferociously. In one part of Livingston Parish, more than 31 inches of rain fell in 15 hours

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"I bought enough food to last for a week in case we were flooded in, but I wasn't prepared for this much devastation," said Jo Lee Misner , who lives in the Livingston Parish community of Colyell. "Local stores are running low on everything from food to fuel."

At least 36 of her relatives have lost their homes -- all 13 of them.

"It's unbelievable what we've been through," she said. "We never imaged this would happen."

And it's not over. At least 13 people have died across five parishes. And with more rain forecast, the destruction could mount.

National response

A road in Baton Rouge was relatively dry Saturday but was inundated by Sunday.

US Homeland Security Secretary Jeh Johnson visited flood-stricken areas Thursday, a day after the director of the Federal Emergency Management Agency arrived.

President Barack Obama had directed FEMA Director Craig Fugate to "utilize all resources available to assist in the response and recovery," the White House said. Obama has declared at least 20 parishes as disaster areas.

But the President has been criticized for continuing his vacation at Martha's Vineyard instead of visiting the flood zone and victims.

The US Coast Guard, National Guard, local emergency responders and even neighbors have helped rescue more than 30,000 residents and 1,400 pets.

Help desperately needed

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On Thursday, some residents returned to their homes, only to find their belongings soaked and destroyed. Heaps of drenched furniture, mattresses and toys were piled up on lawns as owners struggled to find anything salvageable.

Nathan Teaford said many residents have no protection from floods.

"There's areas that are not in a flood plain. They've never required flood insurance and never thought that something like this would happen because they were above elevation," he said.

Teaford left his suburban Baton Rouge home in a truck. He came back in a boat.

The costliest US floods Hurricane Katrina in 2005: $16.3 billion Superstorm Sandy in 2012: $8.3 billion Hurricane Ike in 2008: $2.7 billion Hurricane Ivan in 2004: $1.6 billion Hurricane Irene in 2011: $1.3 billion Source: Federal Emergency Management Agency

But he was one of the lucky ones; the floodwater stopped at his foundation. Many of his neighbors, though, were not as fortunate.

Gov. John Bel Edwards said at least 40,000 homes have suffered at least some damage. It's not clear how many are uninhabitable.

He called on volunteers to help clean out mud from homes.

"Not everyone can do this on their own," the governor said. He said anyone interested in helping can visit VolunteerLouisiana.gov

The Red Cross has also asked for donations to help more than 7,000 people in emergency shelters.

"Thousands more are without power in hot, humid conditions," the aid group said.

More rain to come

Photos: Louisiana flooding is 'major disaster,' governor says Photos: Louisiana flooding is 'major disaster,' governor says Baron Leblanc, left, and George Snyder move a refrigerator out of George's flooded home in St. Amant on Thursday, August 18. More than 30,000 people have been rescued in southern Louisiana after heavy rains caused flooding. "This is a major disaster," Louisiana Gov. John Bel Edwards has said. Hide Caption 1 of 46 Photos: Louisiana flooding is 'major disaster,' governor says A Virgin Mary statue is barely visible in front of a flooded home in St. Amant on August 18. Hide Caption 2 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Flooding affects a chicken coop in Sorrento on August 17. Hide Caption 3 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Kade Lewis helps gut his grandparents' home in Walker on Wednesday, August 17, after flooding. Hide Caption 4 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Caskets float in floodwaters near a cemetery in Gonzales on August 17. Hide Caption 5 of 46 Photos: Louisiana flooding is 'major disaster,' governor says A man talks on his phone while helping salvage items from a house in Denham Springs on August 17. Hide Caption 6 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Gary Schexnayder stands in a flooded street in Sorrento as an early morning fog blankets the area August 17. Hide Caption 7 of 46 Photos: Louisiana flooding is 'major disaster,' governor says David Key looks outside his flooded home in Prairieville on Tuesday, August 16. Hide Caption 8 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Raymond Lieteau waits for help to move a refrigerator as his friend Melissa Lockhart helps clean up the living room in his flood-damaged home in Baton Rouge on August 16. Lieteau had more than 5 feet of water in his home. Hide Caption 9 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Danny and Alys Messenger canoe away from their flooded home in Prairieville on August 16. Hide Caption 10 of 46 Photos: Louisiana flooding is 'major disaster,' governor says A pickup truck is partially submerged in Port Vincent, Louisiana, on August 16. Hide Caption 11 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Cattle are driven through a flooded road in Sorrento as they are herded to trucks on August 16. Hide Caption 12 of 46 Photos: Louisiana flooding is 'major disaster,' governor says People sort through water-damaged products outside Jasmine's Beauty Supply in Baton Rouge on August 16. Hide Caption 13 of 46 Photos: Louisiana flooding is 'major disaster,' governor says A man navigates a boat of rescued goats past a partially submerged car in Gonzales on August 16. Hide Caption 14 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Noel Michael holds up a cell-phone photo of her flooded home in Livingston, Louisiana, on Monday, August 15. Hide Caption 15 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Ann Chapman, from the Louisiana State Animal Response Team, carries a dog she helped rescue in Baton Rouge on August 15. Hide Caption 16 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Eli Turnage hands out water to people waiting in line at Carter's Supermarket in Livingston on August 15. Hide Caption 17 of 46 Photos: Louisiana flooding is 'major disaster,' governor says A boy in Baton Rouge rides his bike inside the flood-damaged Life Tabernacle Church on August 15. Hide Caption 18 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Ryan Evans makes his way through Baton Rouge on August 15. Hide Caption 19 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Robert Enfinger and his wife, Michelle, spend the night in a church that had been turned into an evacuation shelter in Walker. Hide Caption 20 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Tracy Thornton walks to his house through a flooded neighborhood in Baton Rouge on August 15. Hide Caption 21 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Floodwaters surround prefabricated houses in Denham Springs on August 15. Hide Caption 22 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Evacuees spend time together at the church shelter in Walker on August 15. Hide Caption 23 of 46 Photos: Louisiana flooding is 'major disaster,' governor says An American flag hangs in floodwaters in Walker on August 15. Hide Caption 24 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Danielle Blount and her 3-month-old baby, Ember, wait to be rescued by the Louisiana Army National Guard near Walker on Sunday, August 14. Hide Caption 25 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Members of the Louisiana Army National Guard help people near Walker on August 14. Hide Caption 26 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Youngsville police Chief Rickey Boudreaux takes a break from helping flood victims to view the damage to his own home on August 14. Hide Caption 27 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Jennifer Bernard and her dog, Shelby, travel by kayak down the flooded streets of Youngsville on August 14. Hide Caption 28 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Floodwaters inundate a street in Youngsville on August 14. Hide Caption 29 of 46 Photos: Louisiana flooding is 'major disaster,' governor says A girl takes her horse out on a dry road near Walker on August 14. Hide Caption 30 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Emergency personnel rescue flood victims near Walker on August 14. Hide Caption 31 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Those displaced by the flooding wait to board National Guard trucks at a Denham Springs church on August 14. Hide Caption 32 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Residents evacuate Hammond, Louisiana, with food in ice chests on Saturday, August 13. Hide Caption 33 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Floodwaters overwhelm businesses in Hammond on August 13. Hide Caption 34 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Jeff Robinson lowers a ladder from a National Guard truck as his wife wades through floodwaters near their home in Baptist, Louisiana, on August 13. Hide Caption 35 of 46 Photos: Louisiana flooding is 'major disaster,' governor says An aerial view of flood-damaged homes in Hammond on August 13. Hide Caption 36 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Tammie Wise holds her dog, Mikey, after Jeffrey Lesage helped rescue them in Central, Louisiana, on August 13. Hide Caption 37 of 46 Photos: Louisiana flooding is 'major disaster,' governor says A resident wades through floodwaters at apartments near Louisiana State University in Baton Rouge. Hide Caption 38 of 46 Photos: Louisiana flooding is 'major disaster,' governor says A firefighter brings luggage to hotel guests during an evacuation of the Fairfield Inn in Lafayette, Louisiana, on August 13. Hide Caption 39 of 46 Photos: Louisiana flooding is 'major disaster,' governor says People ride an all-terrain vehicle through a flooded neighborhood in Carencro, Louisiana, on August 13. Hide Caption 40 of 46 Photos: Louisiana flooding is 'major disaster,' governor says The Tangipahoa River overflows near the towns of Amite, Independence, Tickfaw and Robert on August 13. Hide Caption 41 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Officials close down an exit off Interstate 55 south in Independence as floodwaters creep closer to the highway on August 13. Hide Caption 42 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Jeremy and Chelsea LeMieux paddle through floodwaters in Carencro on August 13. Hide Caption 43 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Rescue personnel and civilians team up to pull people from their homes along the Tangipahoa River on August 13. Hide Caption 44 of 46 Photos: Louisiana flooding is 'major disaster,' governor says Cattle huddle together in floodwaters near Hammond on August 13. Hide Caption 45 of 46 Photos: Louisiana flooding is 'major disaster,' governor says A firefighter helps a resident in Baton Rouge on Friday, August 12. Hide Caption 46 of 46

At least a dozen parishes are still under flood warnings Thursday, with scattered storms in the forecast, CNN meteorologist Dave Hennen said.

"River levels are expected to continue to fall, but some will remain in flood (stages) at least through the weekend," he said.

And given Louisiana's topography, this recovery will be exceptionally slow.

"It's flat. So unlike West Virginia, where water just runs into the river and goes down, it goes down very slowly" in Louisiana, CNN meteorologist Chad Myers said.

"This is a very lumbering, slow, slow flood. It's going to be with us still for many, many days."