LYONS, Colo. -- With rain still falling and the flood threat still real, authorities called on thousands more people in the inundated city of Boulder and a mountain hamlet to evacuate as nearby creeks rose to dangerous levels.
The late-night reports from Boulder and the village of Eldorado Springs came as rescuers struggled to reach dozens of people cut off by flooding in Colorado mountain communities, while residents in the Denver area and other downstream communities were warned to stay off flooded streets.
The towns of Lyons, Jamestown and others in the Rocky Mountain foothills have been isolated by flooding and without power or telephone since rain hanging over the region all week intensified late Wednesday and early Thursday.
At least three people were killed and another was missing, and numerous people were forced to seek shelter up and down Colorado's populated Front Range.
Late Thursday night, warning sirens blared in Boulder and city officials sent notice to about 4,000 people living along Boulder Creek around the mouth of Boulder Canyon to head for higher ground, according to Boulder's Daily Camera newspaper.
The alert was prompted by rapidly rising creek levels caused by water backing up at the mouth of the canyon because of debris and mud coming off the mountainsides, the city Office of Emergency Management said.
The creek began to recede after midnight, but the conditions remain dangerous and a surprising amount of water is still flowing into the city's streets, Police Chief Mark Beckner told the Daily Camera after touring the damage.
Early Friday, Boulder County spokesman James Burrus said the entire hamlet of Eldorado Springs, about 500 people, was urged to evacuate because of a flash flood and mudslide threat along South Boulder Creek.
In Fort Collins, neighborhoods along the Cache La Poudre River were evacuated overnight, with the river expected to rise to nearly 2 feet above flood stage Friday morning, according to the National Weather Service.
City officials in Fort Collins closed bridges after water began topping Seaman Reservoir in the Poudre Canyon, The Denver Post reported. The city warned residents to stay clear of the river.
In Lyons, residents took shelter on higher ground, including some at an elementary school. Although everyone was believed to be safe, the deluge was expected to continue into Friday.
The Colorado National Guard began trucking people out of Lyons on Thursday evening.
To the north, residents along the Big Thompson Canyon in Larimer County, scene of the deadliest flash flood in state history, were also evacuated. The Big Thompson River flooded in 1976 after about a foot of rain fell in just four hours, killing 144 people.
Early Friday, the National Weather Service warned of more flash flooding in Loveland, according to the Post. NOAA reported that the Big Thompson River at Drake was more than 4 feet above its flood stage of 6 feet.
President Barack Obama signed an emergency declaration Thursday night, freeing federal aid and allowing the Federal Emergency Management Agency to coordinate disaster relief efforts.
The rain has been produced by a low pressure system that has been stationed over Nevada since late Sunday, said National Weather Service meteorologist Todd Dankers in Boulder.
The low has drawn subtropical moisture from the Mexican mainland over New Mexico and into the Rockies' foothills in Colorado -- and it's been trapped by a stationary upper level ridge over the Great Plains and another system over the Great Lakes, Dankers said. The moisture becomes rain when it hits the mountains, the end result of a system he described as a monsoon conveyer belt.
So-called monsoon rains common to Colorado usually occur in late July and August and are typically brief events that provide welcome moisture to a normally sunny, arid state.
Some of the flooding was exacerbated by wildfire burn scars that have spawned flash floods all summer in the mountains. That was particularly true in an area scarred by fire in 2010 near the tiny community of Jamestown and another near Colorado Springs' Waldo Canyon that was hit in 2012.
Rain is normally soaked up by a sponge-like layer of pine needles and twigs on the forest floor. But wildfires incinerate that layer and leave a residue in the top layer of soil that sheds water. A relatively light rain can rush down charred hillsides into streambeds, picking up dirt, ash, rocks and tree limbs along the way. Narrow canyons aggravate the threat.
The University of Colorado canceled classes at least through Friday after a quarter of its buildings were flooded. Students in family housing near Boulder Creek were also forced to leave.
One person was killed when a structure collapsed in the tiny town of Jamestown northwest of Boulder. Another man drowned in floodwaters north of Boulder early Thursday and a woman who was with him was missing.
The woman was swept away after the vehicle she was riding in got stuck in water. The man died after getting out of the vehicle to help her, Commander Heidi Prentup of the Boulder Sheriff's Office said.
A firefighter that had been trapped in a tree in Lefthand Canyon by flooding was rescued and treated, she said.
To the south, Colorado Springs police conducting flood patrols found the body of 54-year-old Danny Davis in Fountain Creek on the west side of the city.
At least one earthen dam gave way southeast of Estes Park, the gateway to Rocky Mountain National Park. Water levels could rise downstream as authorities release more water to ease pressure on dams. With debris piling up near bridges, downstream farming areas including Fort Lupton, Dacono and Plateville were also at risk.
Associated Press writers Colleen Slevin, Steven K. Paulson, Mead Gruver and Thomas Peipert contributed to this report.
Storify on Colorado Flooding: