With rain still falling and flooding and the threat of more still real, rescuers were struggling 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.
According to the Associated Press, Boulder County spokesman James Burrus said 17 people were unaccounted for Friday morning. At least three people are dead and another is missing.
In the flood-ravaged city of Boulder, thousands of downtown residents have been asked to evacuate and seek higher ground after a nearby gulch washed out.
Nick Grossman, of the Boulder County Office of Emergency Management, told CBS News early Friday that Emerson Gulch, west of Boulder, washed out due to heavy rain, saturated soil and tumbling debris. The gushing water was filling Boulder Creek, which heads into downtown. That is likely to cause more flooding.
Several rescues were being attempted in Boulder County early Friday as rain continued to fall.
Meteorologist Dave Aguilera of CBS Denver station KCNC was predicting heavy rain in the region through Friday morning.
The towns of Lyons, Jamestown and others in the Rocky Mountain foothills have been isolated by flooding and without power or telephone service 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 hundreds were forced to seek shelter up and down Colorado's populated Front Range.
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.
"There's no way out of town. There's no way into town. So, basically, now we're just on an island," said Jason Stillman, 37, who was forced with his fiancee to evacuate their home in Lyons after a nearby river began to overflow into the street.
Stillman, who sought shelter at a friend's house on higher ground, went back to his neighborhood in the afternoon and saw how fast-moving water had overturned cars and swept away homes at a nearby trailer park.
"Water was just coming up over the bridge," he said. "All kinds of debris and trees were just slamming into the bridge. Just surreal."
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.
Water roaring across U.S. Highway 36 south of Lyons prevented residents from leaving the Crestview subdivision, so Howard Wachtel arranged for someone to meet him at a roadblock for a ride to a gas station. He needed more gasoline to keep his generator running so he could pump water out of his basement.
"This is more like something out of the Bible. I saw one of my neighbors building an ark," he joked, over the sound of the rushing water.
President 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 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 flood waters 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.
In rural Morgan County, authorities urged ranchers to move cattle to higher ground as the mountain rains emptied onto the plains.