Last Updated Aug 29, 2017 4:19 AM EDT
HOUSTON -- On so many levels, the deluge of rainunleashed is matched by a deluge of need in the flood zone, CBS News correspondent Mark Strassmann reports from Houston.
FEMA estimates 30,000 evacuees in the city could need shelter within a couple days.
CBS News learned there were nearly 5,000 people inside Houston's convention center -- serving as the largest evacuation shelter -- by Monday. That's almost full capacity, with many more days of catastrophic flooding ahead.
Some Houston flood survivors left rooftops in a basket on Monday, as 20 Coast Guard helicopters plucked more than 300 Harvey victims to safety. But officials admit there aren't enough helicopters, boats or high-water vehicles to reach everyone.
Emergency systems were overwhelmed. By Monday morning, 75,000 had called 911 desperate for help.
Mike Hawthorne, a captain with the Houston Fire Department, said volunteers with boats and trucks have been lifesavers.
"I know of yesterday, we pulled 76 people to dry land," Hawthorne said. "The need is overwhelming. We, as a city entity, and government have resources but we don't have enough. It is neighbor helping neighbor."
Many were tweeting for help in real time. Civilian rescuers were checking social media sites for locations of people in distress.
A group evacuees arrived at shelters by a dump truck battered by the elements, their future uncertain. But they were grateful to get out alive.
"We were happy because it was more than my waist -- the water," one man said.
On Sunday, 4,500 people had crowded Houston's major shelter: its downtown convention center -- and more on Monday kept coming.
Among them was Kanesha Brown, a mother of two. Many like her wish they had been given more warning.
"We was waiting. We called, like, all the numbers. We couldn't get nobody to come get us," Brown said.
But Harvey exploded from a tropical storm to a Category 3 hurricane in 30 hours. Officials believe an evacuation order might have created a different disaster: highways of trapped evacuees fully exposed to the storm's fury.
"It's difficult to evacuate 6.5 million people unless you're providing fuel and all those other things," Mayor Sylvester Turner said.
Police are investigating reports that six drowned Sunday when their van overturned trying to escape through rising waters. A family member, who didn't want to give his name, told Strassmann how the tragic event unfolded.
"The water took 'em, the water took 'em, they went into a ditch. My uncle got out, the National Guard came and tried to get the door open but couldn't. All my nieces and nephews drowned — my grandpa, grandma," the man said.
"Were you close to your nieces and nephews?" Strassmann asked.
"Yes, they're my babies, they're my babies," the man replied.
There's something else to consider: flooding may not crest until Thursday.
Amid the flooding, private citizens heeded the call to use their boats for rescues. CBS News correspondent DeMarco Morgan spoke with one volunteer who used his own boats to rescue others stranded in Dickinson, Texas.
"We were, like, in our garage up to here, and yelling, and we flagged him down, and he came and rescued us," one mother said.
Amid thousands of calls for help, Texas Gov. Greg Abbott announced on Monday afternoon he's activating the entire state's National Guard.
They were doing their best, but there are so many still in need. People have used whatever flotation device they had, even putting strangers on their backs and wading through knee-deep water to help.
"This is our savior," one woman in a wheelchair said.
Joshua Mtanyos is part of the Cajun Navy, a flotilla of volunteers from Louisiana. He came to help the hundreds of residents stranded in Dickinson, which is a small town 30 miles east of Houston.
"We're giving it everything we got — and we won't stop," Mtanyos said.
"Louisiana is in a bind, they come for us, we'll come for them. We'll help them out as much as we can," he said.
The National Weather Service called the event "unprecedented," saying the impacts are unknown and beyond anything experienced. But some are willing to risk it all.
"We're here for a purpose and by the grace of God, we're going to do what we can," Mtanyos said.
The waters in Dickinson were starting to recede Monday, but that was causing a problem for boat owners who brought their boats out to help with the rescues.
A mandatory evacuation is now in place as the rescues continue.
In downtown Houston, the Army Corps of Engineers began releasing water from two reservoirs to prevent additional flooding, but that sent water pouring into some neighborhoods.
CBS News correspondent David Begnaud was in one neighborhood, the Fleetwood subdivision in west Houston, which has become an emergency evacuation zone.
Water was rushing in from the nearby bayou and reservoir. People who refused to leave were forced to call for help -- and they got it from civilian Samaritans.
"How long have you waiting to get out?" Begnaud asked one resident.
"Thirteen hours," the man replied.
Begnaud was on a boat with Truett Allen when a woman yelled "help!" from her second-story window. She walked out of her home, which was filled with three feet of water.
Down the street was Laura Blinten. The water was up to a foot in her house.
"I kept telling people online it's going to be okay," said Blinten, who left behind her husband's ashes, saving her her own life on her birthday.
The urgency of the moment was palpable: people on their roofs, signs saying "help," and children whose faces were unforgettable.
One baby was in the arms of her mother, who was in the arms of a rescuer. A young boy was seemingly alone -- if not for his stuffed animal. Two children were carried away by Harris County Deputy Rick Johnson.
No one, however, seemed angry, just grateful.
As the rain got heavier, and rescuers left, there was one woman in the corner of the neighborhood where the water was rushing in. She was determined to stay behind. She didn't say why she wanted to stay put, but she said she knew the water was rising and emergency responders were overwhelmed with calls for help.
"I'm not leaving," she told Begnaud.
Many others were. Some evacuees in Houston were being bused three hours away to San Antonio, CBS News correspondent Melissa Villarreal reports. Nicholas Tyre and Shelby Burley evacuated Rockport with their two small children.
"We have nowhere for them to go, we don't know what we are going to do we don't have any other resources than what we are getting here," Burley said while fighting back tears.
Most of the shelters were unequipped for children. Families made do with makeshift cribs made out of cardboard boxes, before a nonprofit arrived with real cribs.
"At least now, we are reassured and we can sleep a little better knowing they won't fall off or get hurt in any other way," Burley said.
New arrivals were registered and issued identification tags to keep families together while officials searched for empty beds in shelters. So far, four are open and more are needed. San Antonio is preparing for more than 10,000 victims.
"There's pets, seniors and people in hospitals and the impact is huge," said Michael Guerra, director of the San Antonio Food Bank.
On top of taking in and sorting donations, their crews are preparing 4,000 hot meals and 2,000 emergency food boxes every day.
"We are the largest food bank near them, and we are mobilizing forces in Texas to be able to come to their rescue and come to their support. We are going to mobilize the rest of texas to come to their aid," Geurra said.
Houston's food bank is closed because of the flooding, so other donations are being diverted to the San Antonio shelter. That will continue for at least a week when things will hopefully clear up.
President Trump will fly to the disaster zone Tuesday and possibly again over the weekend. The White House announced late Monday that the president and first lady Melania Trump will depart Tuesday morning for Corpus Christi, where they'll receive a briefing on relief efforts by local leaders and relief organizations.
The couple will then head to Austin for a tour of the Texas Department of Public Safety's Emergency Operations Center.
Mr. Trump insisted Monday that Congress will act swiftly to approve a multi-billion dollar recovery package for Texas.