Four deaths have been confirmed and two people in Larimer County -- a 60-year-old woman and an 80-year-old woman -- are missing and presumed dead, CNN reported.
The Colorado Office of Emergency Management said Sunday flooding has destroyed at least 1,500 homes and damaged more than 17,000 others.
Sunday's weather prevented emergency crews from using aircraft to help as many as 1,000 people evacuate, CNN said.
Incident commander Shane Del Grosso told reporters if crews get a window of opportunity Monday, "which it sounds like we might get, we have the horsepower to hit it hard."
The area had received 15 inches of rain before more rain fell Sunday and authorities were concerned any more could cause additional mudslides or debris flows.
Officials said many of the unaccounted for were probably safe, but did not have telephone service.
The White House said in a statement President Barack Obama declared a state of emergency late Saturday, making federal aid available to supplement recovery efforts in areas of Colorado affected by severe storms, floods, landslides and mudslides.
"Assistance can include grants for temporary housing and home repairs, low-cost loans to cover uninsured property losses, and other programs to help individuals and business owners recover from the effects of the disaster," the statement said. "Federal funding also is available to state and eligible local governments and certain private nonprofit organizations on a cost-sharing basis for emergency work in Boulder County."
Colorado Gov. John Hickenlooper said he had been assured during a telephone conversation Saturday with U.S. Transportation Secretary Anthony Foxx that "the $5 million that was released Friday was just the beginning" of federal assistance to the region.
Boulder County Transportation Director George Gerstle said it will cost about $150 million to fix 100-150 miles of roadway and 20-30 bridges, CNN reported.
"We have lost a great deal of infrastructure," Hickenlooper said Saturday.
The missing 60-year-old woman -- whose name was not reported -- was presumed dead after she was swept away by floodwater that destroyed her home. Larimer County Sheriff's Office Executive Officer Nick Christensen said neighbors had tried to rescue the woman before she was swept away.