The overflowing St. Vrain River cut the town of Longmont in half. Evacuation requests were issued for some neighborhoods, all major roads were closed and several thousand homes and businesses were without power.
"This one's going to bring us to our knees," said Tom Simmons, president and co-owner of Crating Technologies, a packing service that had its warehouse inundated. "It's hoping against hope. We're out of business for a long time."
National Guard troops were working to rescue more people stranded in Lyons and other communities in the foothills after conducting more than 100 evacuations Thursday, spokeswoman Cheresa Theiral said. Four helicopters were on search-and-rescue and reconnaissance operations in Boulder County and around Fort Collins.
About 90 miles of Interstate 25 were closed Friday from Denver to Cheyenne, Wyo., because of flooding from the St. Vrain, Poudre and Big Thompson rivers, transportation officials said.
So far, at least three people have been killed and another was missing. Hundreds more were forced to seek emergency shelter up and down Colorado's heavily populated Front Range, which has received more than 15 inches of rain this week, according to the National Weather Service.
That's about half the amount of precipitation that normally falls in the foothills near Boulder during an entire year.
Boulder County spokesman James Burrus said 20 people were unaccounted for Friday. But, he noted, that doesn't necessarily mean they are missing.
"It means we haven't heard back from them," Burrus said.
Two backpackers became stranded after the weather turned. Suzanne Turell and Connie Yang of York, Maine, last sent a text message Thursday with their GPS coordinates, but their cellphones have since gone dead, said Turell's mother, Barbara.
They have a tent but no cold-weather gear.
"We're very concerned we may be in a long line of people needing assistance," she said.
The National Park Service planned to mount a rescue effort for the hikers.
Late Thursday, warning sirens blared in Boulder, and city officials told about 4,000 people living along Boulder Creek to head for higher ground.
Debris and mud coming off the mountainsides had backed up water at the mouth of Boulder Canyon, causing the creek to rise rapidly, authorities said.
The creek began to recede after midnight, but the conditions remained dangerous, authorities said.