BOULDER, Colorado (CNN) — The water rose and the people fled.
Flood sirens sounded overnight 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."
It's been a struggle for emergency crews, made up of police, firefighters and the National Guard.
Boulder County Sheriff Joe Pelle said the "devastating storm" that started Tuesday dumped more than a half a foot of rain on the region during a 19-hour period. The rain and runoff pushed into Friday morning.
The forecast was not encouraging. The National Weather Service has issued a flash flood warning "until further notice" for Boulder Creek.
Widespread flash flooding washed out roads, pushed dams to their limits and beyond and killed at least three people.
Late Thursday President Barack Obama stepped in, signing an emergency declaration for Colorado, an action that helps allocate federal assistance.
There were dramatic rescues Thursday, including a man pulled from an overturned car in rushing water on live television. But for much of the day, rescuers had to wait at the edge of impassable roads or for skies to clear enough for helicopter flights.
Boulder County takes a beating
The worst of the damage reported Thursday was in Boulder County, where the National Weather Service said a 20-foot wall of water roared down a mountain canyon north of the city.
One death was confirmed and another feared after a car stopped in the rushing water. Witnesses reported a woman emerged from the car and was swept away by the water. A man left the car and tried to reach her and also was overcome, Boulder County Sheriff's Cmdr. Heidi Prentup said. She said the man's body had been recovered, while the woman was missing.
Bodies also were found in a collapsed home in Jamestown and on a roadway in Colorado Springs.
Lyons, a small town of 2,000 near the foothills of the Rocky Mountains, was cut off by what city officials described as a 500-year flood.
National Guard rescuers in high-water vehicles reached Lyons with humanitarian aid Thursday night, Prentup said, but the town remained otherwise cut off. Troops planned to evacuate the town at daybreak.
Elsewhere, homes collapsed onto residents and a dam in Larimer County broke, flooding some houses and leaving three people trapped, a county spokesman said.
Larimer County Sheriff Justin Smith said some residents there face the dilemma of whether to try to move to safer shelters over bridges that may have been damaged. They will "have to use their own judgment," he said.
An emergency message from the sheriff's office to residents of Big Thompson Canyon said, "If you are cut off because of a compromised bridge, you need to stay at your residence but have a plan to get to higher ground at a moment's notice."
Pelle, the Boulder County sheriff, said emergency crews were worried about a Lyons Fire Department crew stuck on a mountainside near a washed-out road. Hypothermia from the cold, wet conditions was of particular concern, he said.
As of Thursday evening, authorities in Boulder had not been able to make contact with the Lyons department to confirm the firefighters had been rescued, Herring said.
Dams threatened, roads washed away
Between 25 and 30 roads were closed Thursday afternoon in Boulder County, Prentup said.
Several roads washed away, including one near Lafayette -- about 12 miles southeast of Boulder -- where rescue crews staged the dramatic rescue of a man from an overturned, partially submerged car.
Lafayette Fire Chief Gerry Morrell said it appeared three cars driving through pre-dawn darkness and heavy rain plunged into the void left when water washed out the road. Two other motorists got out of their cars, at least one with the aid of firefighters.
At first, rescuers feared the last car's occupant may have died after more than 30 minutes in the partially submerged car, Morrell said.
The rescue -- captured live on television -- nearly went awry when the car, which had been pulled up so that it was on its side, turned back onto its roof in the swift water just as the man was preparing to climb through a window broken open by his rescuers.
"I was aghast," said Morrell, who watched the rescue unfold from the riverbank.
All three people were taken to a hospital for treatment of minor injuries, according to fire officials.
A firefighter in Boulder had to climb a tree to escape the initial surge of water. Although injured, the firefighter made it to a nearby home, Prentup said.
In Lyons, water was flowing over the tops of five dams, Boulder County spokeswoman Gabi Boerkircher said. The dams had not broken, but emergency officials were monitoring them very closely, she said.
The extent of the damage was widespread but difficult to pin down, Pelle said.
"We've lost roads, we've lost bridges, cars, homes," he said. "And we are just now trying to assess the scope of the damage."
Ed Payne reported and wrote from Atlanta; Ana Cabrera reported from Boulder, Colorado; CNN's Mike Pearson, David Simpson, Matt Smith, Sara Weisfeldt, Tina Burnside, Shawn Nottingham and Sherri Pugh also contributed to this report.