Footage showed bridges, roads and cars washed away by floodwaters
Thousands of people have been warned to evacuate the Boulder area of Colorado and a mountain hamlet as flooding swells creeks to dangerous levels.
Storm rains have killed at least three people in the state and caused severe damage to property.
Water levels in the Boulder Canyon are reportedly rising rapidly because of debris and mud blocking its mouth.
President Barack Obama has signed an emergency order approving federal disaster aid for Boulder County.
Towns such as Jamestown, Lyons and Longmont are said to have been reduced to islands by the swirling floodwaters.
'Biblical' Officials set up road blocks to prevent residents fleeing in their vehicles on to flooded roads.
The raging torrent - dubbed a "100-year flood" by officials - has hampered rescue crews trying to reach communities stranded downstream.
Some 4,000 people living along Boulder Creek were sent notices to move to higher ground late on Thursday, reported Boulder's Daily Camera newspaper.
Another 500 inhabitants of Eldorado Springs were urged to leave due to a threat from South Boulder Creek, an official told the Associated Press early on Friday.
Low-lying areas beyond the Rocky Mountains are also at risk, with up to 3,000 people previously ordered to leave the Commerce City district of Denver.
'Staggering rainfall' Emergency notifications have been issued to 8,000 telephone numbers in areas along Boulder Creek.
Governor John Hickenlooper told local radio there had been "a staggering amount" of rainfall.
Floodwaters erupt out of a sewer on Canon Avenue in Manitou Springs, Colorado.
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"Given the drought situation we've had, it was almost a year's worth of rain," he said on the KBCO radio station.
The rain was forecast to continue until midday on Friday (18:00 GMT).Emergency officials have reported at least three deaths: a man in Colorado Springs; one when a building collapsed in Jamestown; and a man in Boulder, who died after getting out of a car to help a woman who was swept away from the same vehicle. She was still missing.
"There is water everywhere," Andrew Barth, emergency management spokesman in Boulder County, told Reuters news agency. "We've had several structural collapses. There's mud and muck and debris everywhere. Cars are stranded all over the place."
National Guard troops have reportedly been dispatched north of Boulder to the cut-off town of Lyons, which is said to be without fresh water, power or phone lines.
Footage showed rescuers saving one man from his swamped car
A 20ft (6m) wall of water was reported in Left Hand Canyon, north of Boulder, according to National Weather Service meteorologist Bob Kleyla.
Two areas south-east of the town of Estes Park were ordered to evacuate after an earth dam gave way in the area.
Classes at the University of Colorado, Boulder, were cancelled until Friday at least, while schools in the area are also closed.
Standing water on a road caused some traffic delays into Denver International Airport, but not to flights.
Officials told NBC that water reached as high as first-floor windows in some parts of Boulder, while cars were seen floating in the streets.
The prolonged rain has been blamed on a low-pressure system stationed over Nevada, which is drawing moist air out of Mexico into the Rockies' foothills.
