Thousands more people in Colorado were ordered to evacuate as water rose to dangerous levels amid a storm system that has been dropping rain for a week.
Rescuers struggled to reach dozens of people cut off by flooding in mountain communities.
At least three people were killed.
Boulder County spokesman James Burrus said 20 people were unaccounted for Friday.
"Unaccounted for doesn't mean missing. It means we haven't heard back from them," he said.
Two backpackers were said to be missing as well after they were hiking in the mountains and the changing weather caught them by surprise, a family member said.
President Barack Obama signed an emergency declaration Thursday night, freeing federal aid and allowing the Federal Emergency Management Agency to coordinate disaster relief efforts.
Some towns in the Rocky Mountain foothills have been isolated by flooding and without power or telephone.
Late Thursday, Boulder city officials said they sent a notice to head to higher ground to about 4,000 people living along Boulder Creek, according to a report in Boulder's Daily Camera newspaper.
Boulder County spokesman James Burrus told The Associated Press that about 8,000 telephone numbers were called with the message to evacuate.
"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 after a nearby river began to overflow into the street.
One person was killed when a structure collapsed. Another man drowned in flood waters, and a woman who was with him was missing after swept away when the vehicle she was riding in got stuck.
The man died after getting out of the vehicle to help her, Commander Heidi Prentup of the Boulder Sheriff's Office said.
Elsewhere, Colorado Springs police conducting flood patrols found the body of a man.
Some of the flooding was exacerbated by wildfire "burn scars" that have spawned flash floods all summer in the mountains. The flames strip away vegetation that normally helps absorbs excess water and leave a residue behind that sheds water.