BOULDER, Colorado (CNN) -- Clear skies allowed for more evacuations and rescues in flood-devastated Colorado Friday, but the forecast through Sunday called for more heavy rain.
And even after the last of the storms, authorities can't say how long it will take to reach residents who will remain isolated by devastated roads.
The confirmed death toll reached four when Boulder County officials recovered the body of a woman who had been swept away after getting out of her vehicle Thursday, Sheriff Joe Pelle said. Authorities already had recovered the body of a man who left the same car and tried to save the woman. One other death had been reported in Boulder County and one in El Paso County.
President Barack Obama declared an emergency for Boulder, Larimer and El Paso counties, FEMA announced Friday. The declaration allowed FEMA to bring in four rescue teams, the largest ever deployment in Colorado, officials said.
More heavy rain is forecast through Sunday for the region, on top of the 15 inches some parts of the state have already received.
"This isn't over," CNN meteorologist Chad Myers said.
On Friday, National Guard troops using "high-profile" trucks to wade through water were evacuating the entire Boulder County town of Lyons, which had been cut off since the flooding began Wednesday night.
"It just really felt like God came down and saved us. It felt great," Melinda Villa said of the National Guard's arrival at the Lyons apartment where she was stranded with her 1-month-old infant. She said, "I just felt like I was trapped. No phone, no water, barely any formula for my baby, barely any food for us."
Four helicopters were being used for rescues in Boulder County Friday, Pelle said. The sheriff said helicopters also may have to be used indefinitely to deliver food and water to residents along damaged rural roads.
"Please know we're working hard," Pelle told residents who might see his televised news conference. "We're concerned about you. But you're going to have to be patient. Please know this is an unprecented event."
About 80 people in Boulder County have been reported missing or "unaccounted for" by relatives, Sheriff's Cmdr. Heidi Prentup said.
The University of Colorado canceled Saturday's scheduled football game against Fresno State.
In Larimer County to the north, Sheriff Justin Smith surveyed the heavily damaged Big Thompson Canyon by air Friday. Some people remain stranded in homes there, he said, adding, "How we're going to get them out -- it's going to take a damn long time."
However, he said the break in the rain Friday allowed school buses to begin evacuating students who had been stranded at a school.
Smith described widespread damage to roads. He estimated 17 miles of Highway 34, a major artery, will need to be rebuilt.
As other Lyons evacuees arrived at a shelter set up in a church in nearby Longmont, they told stories of houses ripped swept off their foundations as the St. Vrain Creek turned into a violent river, CNN affiliate KMGH reported.
KMGH reporter Theresa Marchetta said evacuees also described homes dangling off cliffs.
Some people in Lyons still were awaiting rescue, evacuees said, and some residents had chosen to stay. Marchetta said evacuees told her there had been a town meeting and residents were checking on each other to ensure no one was missing.
To the north, in Larimer County, Sheriff Justin Smith said many people remained stranded, although a break in the weather was allowing the National Guard to reach some residents by air, KMGH reported.
State transportation officials issued an emergency alert to residents in some of the hardest-hit counties, warning them to stay off roads because many are unstable and could give way without notice. They also closed Interstate 25 from the Wyoming line south to Denver. Part of Interstate 70 also was shut down.
In Fort Collins, some residents had been urged to leave their homes. And in Denver, police responded when a man was swept into a drainage pipe with his dog. Both were saved after traveling two blocks in the water, police said on Twitter.
The rains sent virtually every waterway in Boulder County coursing out of its banks, and massive water flows washed away roads and bridges, flooded homes and stressed numerous other bridges.
The National Guard effort to get residents out of Lyons began shortly after daybreak. About 100 troops in 21 heavy vehicles able to ford high waters streamed into the city to begin moving residents out, Gov. John Hickenlooper said.
Residents had been entirely cut off, without water or sewer service, in many cases without electricity, facing what Fire Chief J.J. Hoffman said in a Facebook posting was a "very large disaster."
It was unclear when the evacuation would be complete.
"I encourage all of you -- stay strong!" Hoffman wrote on the fire department's Facebook page. "We will make it through this, we are here for you and doing the absolute best we can with the resources we have to get to each and every one of you!"
Lyons follows fellow Boulder County towns of Jamestown and Eldorado Springs to be evacuated as a result of the storm, which began around 6 p.m. Wednesday, Boulder County Sheriff Joe Pelle said.
Up to another half inch of rain an hour is possible this afternoon, authorities said.
While the forecast called for less rain Friday than the region had received the last few days, meteorologists warned that any rainfall would add to the flooding potential in the region, thanks to waterlogged ground unable to absorb any more water. A flash-flood warning remained in effect through noon.
Overnight, flood sirens sounded in Boulder County as Colorado emergency officials feared that debris-caked canyons might give way and send another wall of water crashing through the city of Boulder and neighboring communities.
"All residents are warned to go to higher ground immediately due to the potential for flash flooding along the creek," Boulder's Office of Emergency Management said.
Emergency management warned that "there are mudslides at the mouth of Boulder Canyon 400 feet long and four feet deep as the sides of the canyon give way due to the saturation from the days-long rain."
Gov. John Hickenlooper warned an extensive recovery is ahead.
"This is not going to get fixed in a week," he said. "We have lost a great deal of infrastructure."