KMGH reporter Theresa Marchetta said evacuees also described homes dangling off cliffs.
Some people in Lyons still were awaiting rescue, evacuees said, and some residents had chosen to stay. Marchetta said evacuees told her there had been a town meeting and residents were checking on each other to ensure no one was missing.
State transportation officials issued an emergency alert to residents in some of the hardest-hit counties, warning them to stay off roads because many are unstable and could give way without notice. They also closed Interstate 25 from the Wyoming line south to Denver. Part of Interstate 70 also was shut down.
In Fort Collins, some residents had been urged to leave their homes. And in Denver, police responded when a man was swept into a drainage pipe with his dog. Both were saved after traveling two blocks in the water, police said on Twitter.
The rains sent virtually every waterway in Boulder County coursing out of its banks, and massive water flows washed away roads and bridges, flooded homes and stressed numerous other bridges.
In the early hours Friday, flood sirens sounded in Boulder County as emergency officials feared that debris-caked canyons might give way and send another wall of water crashing through the city of Boulder and neighboring communities.
"All residents are warned to go to higher ground immediately due to the potential for flash flooding along the creek," Boulder's Office of Emergency Management said.
Emergency management warned that "there are mudslides at the mouth of Boulder Canyon 400 feet long and four feet deep as the sides of the canyon give way due to the saturation from the days-long rain."
Authorities continued to warn of the danger of mudslides Friday night.
Hickenlooper warned an extensive recovery is ahead.
"This is not going to get fixed in a week," he said. "We have lost a great deal of infrastructure."