Boulder County was hit hardest, with up to 6 inches of rain falling over 12 hours. But flooding was reported all along the Front Range, from Colorado Springs to north of Fort Collins.
Capt. John Burt of the Colorado State Patrol said a storm cell moved over the mountains during the night, headed east over the Plains, then circled back around. The National Weather Service warned of an "extremely dangerous and life-threatening situation" throughout the region as the flooding forced people from their homes and caused mud and rockslides in some areas.
"Move to higher ground now. Act quickly to protect your life," the Weather Service warned throughout the morning.
Boulder Office of Emergency Management spokeswoman Gabrielle Boerkircher said many roads were blocked and volunteers were trying to help stranded people until emergency crews could arrive.
Boerkircher told The Associated Press one person was killed when a structure collapsed in the tiny town of Jamestown. Boulder County Sheriff Joe Pelle said another person was also found dead in northern Boulder.
"We're trying to get to Jamestown," Boerkircher said of a mountain community northwest of Boulder. "A lot of towns need assistance, and we cannot get through."
Two other structures in the area were also damaged and may have collapsed.
To the south, Colorado Springs spokeswoman Kim Melchor said police conducting flood patrols found a body in Fountain Creek on the west side of the city early Thursday.
National 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.
In Broomfield, U.S. Highway 287 collapsed when a culvert washed out, dumping three vehicles into the rushing water. Three people were rescued and had minor injuries.
Near Lyons, about 2 feet of water was standing on U.S. Highway 36 as a normally shallow creek known for trout fishing flooded its banks.
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.
A National Guard helicopter requested to help pull people from their homes hasn't been able to take off because of fog and low cloud cover throughout the area.
Flash flood warnings were issued for multiple counties in the region, including Jamestown and the Fourmile wildfire burn area.
Flash flood emergencies were also issued downstream from the Front Range mountain areas, including Fort Lupton, Dacono, Plateville and other farming areas as debris piled up near bridges.
An evacuation center for the mountain residents has been sent up in nearby Nederland, officials said.
Meanwhile, about 400 students in a dorm at the University of Colorado in Boulder were evacuated and classes were canceled Thursday and Friday because of the flooding.
Mudslides and rockslides were reported in several areas, with parts of U.S. 6, Boulder Canyon, Colorado 14 and U.S. 287 all reporting problems and temporary blockages during the evening Lefthand Canyon was reported blocked by one of the many slides.
Boulder police dispatchers were receiving calls of flooded homes and streets and submerged cars.