A warm, moist storm system has been dropping rain on the region for much of the week. Up to 8 inches fell in an area spanning from the Wyoming border south to the foothills west of Denver. Flooding extended all along the Front Range mountains, including the cities of Colorado Springs, Denver, Fort Collins, Greeley, Aurora and Boulder.
Numerous roads have been washed out or made impassable by floods, and water has poured into homes. Some buildings have collapsed in the torrent. Parts of several interstate highways in the Denver area were temporarily closed because of standing water.
Boulder County appeared to be hardest hit. Sheriff Joe Pelle said the town of Lyons was completely cut off because of flooded roads, and residents were huddling together on higher ground. Although everyone was believed to be safe, the deluge was expected to continue into Friday.
Search vehicles and rescue helicopters were standing by, but many were unable to get to mountain communities because of flooding and fog. Residents were asked to drink bottled or boiled water because of possible contamination to water supplies.
"It is not an ordinary disaster," Pelle said. "All the preparation in the world ... it can't put people up those canyons while these walls of water are coming down."
In a neighborhood northwest of Boulder, Dave Finn said he had to knock down a fence to release water that had backed up behind it. He said he destroyed his fence to save his house.
"I've never seen it like this," he said. "You know, we sort of roll our eyes when they say you have to be prepared for the 100-year flood, so here we are."
Firefighters performed a daring rescue of two men trapped in vehicles in Rock Creek, east of Boulder. After rushing water collapsed a section of road, rescuers used a raft to reach the men, broke the car windows and lifted them to safety.
Some of the flooding was exacerbated by wildfire "burn scars" that have spawned flash floods all summer in the mountains. That was particularly true near Jamestown in an area scarred by fire in 2010 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 National Weather Service warned of an "extremely dangerous and life-threatening situation" throughout the region as flooding forced people from their homes and caused mud and rockslides.
At the University of Colorado, about 400 students in a dorm were evacuated, and administrators canceled classes at least through Friday. About a quarter of the school's buildings have some kind of water damage.
One person was killed when a structure collapsed in the tiny town of Jamestown northwest of Boulder. Another person drowned in northern Boulder, authorities said.
To the south, Colorado Springs police conducting flood patrols found a body in Fountain Creek on the west side of the city.
Weather service meteorologist Bob Kleyla said a 20-foot wall of water was reported in Left Hand Canyon north of Boulder, and a firefighter radioed he was trapped in a tree. He said rescuers were trying to get through, but were blocked by debris.
The creek is usually "just a trickle," said nearby resident Carm Say. "You can walk across it and have fun. Now, as you can see, it's hitting houses."
In Broomfield, U.S. Highway 287 collapsed when a culvert washed out, dumping three vehicles into the rushing water. Three people with minor injuries were rescued.
At least one earthen dam gave way southeast of Estes Park, the gateway to Rocky Mountain National Park, and water levels could rise downstream as authorities release more water to keep more dams from giving way.
Flash flood warnings were issued for multiple counties. Downstream farming areas including Fort Lupton, Dacono and Plateville were also at risk as debris piled up near bridges.
In rural Morgan County, authorities urged ranchers to move cattle to higher ground as the mountain rains emptied onto the plains.
Rain showers and thunderstorms were expected through the night, with some storms capable of dumping an inch of water in 30 minutes, the weather service warned.