Search and rescue teams have used boats and helicopters to pull stranded residents to safety in areas where flash floods toppled buildings, washed out roads and inundated farmland.
The flooding began overnight on Wednesday. It was triggered by unusually heavy late-summer storms that soaked Colorado's biggest urban centres, from Fort Collins near the Wyoming border south through Boulder, Denver and Colorado Springs.
Boulder and a string of other towns along the Front Range of the Rockies north of Denver were especially hard hit as water poured down mountains and spilled through canyons that funnelled the run-off into populated areas.
Overnight on Friday, rescue workers took advantage of a break in the weather to reach residents still stranded in their homes after creeks turned into raging torrents and burst their banks.
"Quite a bit of the water has receded in the city ... and rescue crews will work throughout the night," said Ashlee Herring, a spokeswoman for the Boulder Office of Emergency Management.
The National Weather Service in Boulder warned of scattered showers and thunderstorms into today that could trigger further flash flooding in the already drenched area.
Lyons, a town north of Boulder, was virtually cut off when floodwaters washed out US Route 36, leaving residents without water and power for 48 hours.
At least four people have been killed, including a couple swept away in floodwaters after stopping their car northwest of Boulder. The man's body was recovered on Thursday and the woman was found on Friday.
The body of another victim was found in a collapsed building near Jamestown, an evacuated enclave north of Boulder, and a man in Colorado Springs, about 100 miles to the south, was also known to have died, officials reported.
On Friday, Governor John Hickenlooper declared a disaster emergency for 14 counties, reaching from the Wyoming border south to Colorado Springs.
The declaration authorises $6 million in funds to pay for flood response and recovery.