Four days of heavy rain and flash flooding have killed at least three people in Colorado as the population braced itself for more floods on Thursday.
Boulder was hit hardest, with up to six inches of rain falling in the county over a span of 12 hours.
Search and rescue teams were still trying to reach residents and motorists stranded in the city of Boulder and in its surrounding mountain communities as the rain continued.
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"The city of Boulder is just overwhelmed with water," said Barbara Halpin, a spokeswoman for the Boulder Office of Emergency Management. "I've heard from people who say they have lived here for 25 years and have never seen anything like it."
Flooding was also reported along the 130-mile stretch between Colorado Springs and Fort Collins, which closed numerous roads. Those living in the canyons amongst the hills have been warned they are still at risk to more flash floods.
"This is not an ordinary day, it is not an ordinary disaster. All the preparation in the world...it can't put people up those canyons while these walls of water are coming down," Boulder County Sheriff Joe Pelle said.
The University of Colorado Boulder campus was closed on Thursday after several of its buildings were flooded with water from Boulder Creek. Schools were also closed in Boulder Valley and St. Vrain districts.
Many drivers have been rescued from flooded roads and ditches, and a firefighter was reported trapped in a tree in Lefthand Canyon in Boulder County after a 15- to 20-foot "wall of water" crashed through the canyon. A dive team was required for the rescue.
The National Weather Service told residents of Colorado to "move to higher ground now" and to "act quickly to protect your life" as it posted flash-flood warnings, watches and advisories on Thursday morning. It called the situation "extremely dangerous and life-threatening."