Authorities worked to track down residents in saturated Colorado mountain towns on Saturday, where authorities confirmed the death toll at four and feared that more may be stuck without food, water or power.
Officials said that more than 500 people were unaccounted for on Saturday, but stressed that those numbers could rise or fall since communications were so poor in the area. Those people are not necessarily considered missing, officials said.
"We are assuming there may be further loss of life and injuries," Boulder County Sheriff Joe Pelle said on Saturday. "Given the devastation on some of those canyons, it's definitely a high probability."
In total, 1,750 people and 300 pets were rescued through air and ground operations by the Colorado National Guard and U.S. Army division in Fort Carson, Lt. James Goff of the Colorado National Guard Public Affairs Office said Saturday evening.
The flooding had spread over a 4,500-square-mile area, according to Weather.com, almost as large as the state of Connecticut.
"The problem now is relatively little additional rainfall may trigger additional flash flooding," Weather.com senior meteorologist Jonathan Erdman said. "With more areas of rain and scattered thunderstorms expected this weekend, a quick inch or so of rain in less than one hour may cause additional rises on already swollen creeks and rivers."
As crews worked around the clock to rescue the stranded – including 85 fifth-graders who were on a school trip – amazing stories of survival and escape emerged.
"We rescued a young couple yesterday from Lyons," said Boulder County Sherriff Joe Pelle on Saturday. "The young lady is pregnant and her due date is tomorrow."
"There are some personal stories coming out of this that makes all of this very rewarding," he added.
"I had three boat rides, one surfboard, a motorboat and a canoe," Nancy Coleman said of her flight from water-logged Longmont.
A visually impaired man walking in Denver with his service dog was swept into a drainage ditch and pulled out four blocks later by a police officer and paramedic. Emergency workers used a zipline to bring a woman to safety at Big Thompson Canyon.
A father of four who spent two hours in a submerged car held a press conference to thank his saviors.
"Two hours was an eternity," said Roy Ortiz, who got trapped when a bridge collapsed early Thursday, sending his car into Rock Creek, flipped on its roof.
He said while he waited for help, with his head position in a small air pocket, he prayed and thought about his family.
"Everywhere I moved, there was the water," Ortiz said, according to the Broomfield Enterprise.
Ortiz was one of the lucky ones.
A young woman whose car got stuck in floodwaters was found dead Friday morning in Boulder. A man who was with her and got out of the car to help was also killed.
A man was found in a creek in Colorado Springs and a fourth person was killed in a building collapse in Jamestown.
Because the waters obliterated many roads, authorities still don't have a handle on how many people might be stranded or missing.
"The thing that worries us the most are the things that we don't know right now," Pelle said in an afternoon briefing. "We don't know about lives lost, homes lost, people stranded in many, many of the canyon areas in our upper communities."
As a result, he said, small towns in the western mountains are "completely isolated" with "no road access, no telephone information, no power, no water, no sewer."
"We have our hands full simply trying to assess what we have on our hands," he said.
Twelve military aircrafts were being used Saturday "to pull off some of the evacuations and support some of the isolated communities with basic necessities, food water," said Dan Dallas, Incident Commander Rocky Mountain Type 2 Incident Management Team B.
In addition to rescue efforts, Dallas added that the aircrafts would be used for reconnaissance to assess damaged areas.
"We recognize the need that the county has to really understand the full scope of what's been impacted," Dallas said.
Late Friday night, the Boulder County Office of Emergency Management reported a breach about 300 feet long in the pipeline that carries about 90 percent of Boulder's untreated wastewater to a treatment plant, with the wastewater discharged directly into Boulder Creek.
There was no risk to drinking water supplies since all drinking water from the creek is drawn upstream from the breach, the agency said. The city was working on a temporary bypass around the breach.
Four National Guard helicopters were enlisted to drop supplies and begin taking people out of two towns surrounded by raging rapids: Lyons, with more than 2,000 residents, and Jamestown, with about 300 people. Late Friday, an OEM spokeswoman said the Guard had evacuated about 190 people from Lyons and about 130 from Jamestown.
Pelle said as of late Friday there were approximately 50 people who wanted to stay in Jamestown.
He said they would be sending a deputy sheriff to "gather them together and talk to them and explain we may not be able to come back for several days or be able to get a road to them for a while."
Aerials from NBC affiliate KUSA show the aftermath of devastating flooding that killed at least three people in Colorado.
Above Jamestown, 85 schoolchildren and their 14 chaperones were waiting for a ride out. "The children are safe and sound. They're in a good, dry situation. They're well-fed," Pelle said.
Waters were receding, but there was still rain in the forecast and the threat of more flash floods. Authorities worried that stir-crazy residents might venture out into dry areas and get trapped by a surprise storm.
"We were lucky to get out," Steve Flowers said after evacuating to Boulder. "My neighbor two doors down is still missing."
Pelle could not predict how long it would take to reach everyone who could not get out.
"There's really no choice but to hunker down and be patient," he said. "This is an unprecedented event for any of us.'
NBC News' Matthew DeLuca contributed to this report. Reuters and the Associated Press also contributed.