With the Houston area under water for the third straight day, emergency crews rescued thousands of people with boats and helicopters as suburban leaders ordered massive evacuations over fears that levies would give way under the force of flooded waterways.

Much of Fort Bend and Brazoria counties were evacuated as the rain-swollen Brazos River was expected to approach historic flood levels Tuesday, and two subdivisions in northern Harris County were evacuated late Monday amid similar concerns over aging levies.

Rains, sometimes heavy, are expected to continue throughout the area perhaps into next week, reaching up to 50 inches over the upper Texas coast, including the Houston-Galveston area.

"This is a landmark event," said Federal Emergency Management Agency Administrator Brock Long after making a plea for help from the public. "We have not seen an event like this. You could not dream this forecast up."

Across the state, up to 30,000 people may need shelter and perhaps 450,000 could be eligible for federal flood victim assistance, Long said. The other numbers from Monday were equally stunning: 8,000 rescues in the Houston area and 12,000 guardsmen deployed statewide by Monday.

NOAA radar shows Hurricane Harvey moving over the coast. A hurricane warning is in effect for the counties shaded in red on the map.

Dramatic rescues across the Houston region stretched from Dickinson near Galveston — where people were pulled by boat then airlifted to safety — across the Houston area and into the northwest Harris County, where officials began releasing water from two reservoirs into Buffalo Bayou.

Up to 200 homes were already flooding upstream from the Barker and Addicks reservoirs in northwest Harris County Flooding was reported late Monday.

At least eight people are believed to have died in Hurricane Harvey and its aftermath, and a Houston family of six was reported missing after driving into floodwaters near Greens Bayou.

The days of turmoil took their toll on Houston.

"I'm angry. I'm stressed. I'm outraged," said Luis Morales, who waded through water almost up to his neck pulling a boat carrying his wife and two young sons. "Everything."

RESERVOIRS: Harvey flooding forces release from Addicks and Barker

As the San Jacinto River overflowed into homes and the Brazos River rose, officials from as far north as Humble and as far south as Galveston County cleared out neighborhoods, warning that those who did not heed orders might be on their own as floodwaters deepen in coming days.

In Humble, police went door to door in the Northshire subdivision, urging stunned residents to get out before they opened the floodgates at Lake Conroe, sending water downstream. Conroe cleared out the McDade Estates neighborhood and recommended evacuations for a half-dozen more.

By the afternoon, Brazoria County officials were warning that imminent, widespread flooding expected there Wednesday likely would be significantly worse than in 2016, when the Brazos River spilled over and left swaths of the region under water.

By 3:30 a.m. Monday in Fort Bend County, officials already were sounding the alarms. Voluntary evacuations announced for the Sienna Plantation neighborhood were made mandatory Monday morning.

"If people need to get out, now is definitely the time," said Alan Spears, deputy emergency management coordinator of Fort Bend County. "If the water is down, take that opportunity and leave."

People tried to leave Houston on its vast network of highways, but many ran into floodwaters lingering from days of torrential rains. Although some made it out, others turned back to the homes they abandoned hours earlier, planning to wait out the storm.

Houston's emergency response network remained strapped to the point that officials asked for anyone with a boat to come help people stranded in high water.

"Please don't give up on us," Houston Police Chief Art Acevedo said. "None of us will give up."

Harvey hit the Gulf Coast late Friday as a Category 4 hurricane and then stalled into a tropical storm, sending bands of heavy rain toward the Houston area. Torrential rains late Saturday into Sunday inundated the city, and brief respites Monday allowed some streets to clear.

Still, more than 130,000 CenterPoint customers were without power by Monday night, with crews unable to get to some affected areas. Houston TranStar was reporting 343 high water locations along major roads in the area, with hundreds more residential streets submerged.

Most rivers, creeks and bayous tracked by regional flood management officials were showing historic water volumes.

Spring Creek, running along north Harris and south Montgomery counties, had reached a major flood stage Sunday evening and was expected to crest late Monday, putting at risk residents of more than two dozen neighborhoods in The Woodlands and Spring areas on either side of Interstate 45. Voluntary evacuations were recommended.

RESOURCES: Stay up-to-date with forecasts, traffic reports, water levels, etc.

No point on the compass was spared. Most everyone in the greater Houston area and points beyond woke up Monday to yet another day of life-threatening conditions.

So many people went to the Greenspoint-area shelter at the M.O. Campbell Education Center that they ran out of space and supplies and started busing them to the larger George R. Brown Convention Center in downtown Houston. Shelters in Austin and San Antonio also took in some Houston residents.

Dickinson, in Galveston County, was among the hardest hit.

Debbie Barlow, however, refused to budge from the second floor of her elevated home on Deats Road. The city of Dickinson had put out a mandatory evacuation order but Barlow said she wasn't going anywhere.

"We don't feel invincible, but we feel like we're going to be okay," the 46-year-old said. "We have the third floor."

Pat Deeves, 63, spent Monday cleaning out her four-bedroom home in the Maplewood South neighborhood south of Brays Bayou.

After spending Sunday getting out the 6.5 inches of water she and her husband, Sheldon Bloch got in their home, they spent Monday pulling items out of closets.

"A lot of it is just a wadded up mess," she said.

But it wasn't been all bad. She found two handwritten letters from her grandmother – one from 1979 and the other from 1982 – that she didn't know she had. And they were still dry.

The luckier area residents included countless thousands who had to cope with one more round of flooded cars and carpets. Even those for the moment dry or mostly so were not confident they'd been spared.

Whatever happens in the next day or two, Harvey has hit the record books, surpassing Tropical Storm Allison which inundated the city in 2001.

Cody Pendergraft, who lives a few miles west of the Brazos River in West Columbia, fled his home Monday morning for refuge at his aunt's house in Angleton. He packed a camper with clothes and hauled it to the Brazoria County Fairgrounds, where about 50 trailers were parked in an area expected to stay dry.

"It shows no mercy when it starts coming down here," Pendergraft, 31, said. "I don't think anybody really knows how bad it's going to be."

POWER OUTAGES: More problems reported as flooding continues

Reporters Emily Foxhall, Lomi Kriel, Mike Morris, Jacob Carpenter, St. John Barned-Smith, Lydia DePillis, Mike Glenn, Mike Tolson and Lindsay Ellis contributed to this story.