Overnight, helicopters set to work evacuating 295 people and their pets from Jamestown, isolated by flooding in a canyon. In the hard-hit town of Lyons, the National Guard took advantage of a break in the torrential rains that have unleashed floodwaters up and down the state to sweep through in trucks, going door to door to pull out up to 2,000 trapped residents.

"These individuals are not only coming with just themselves, but with their suitcases and their precious household items along with their pets and everything, all getting loaded in the back of these vehicles," said Colorado National Guard spokesman First Lieutenant Skye Robinson.

The floods started Wednesday and cover 4,500 square miles (11,655 square kilometers). The days-long rush of water from higher ground has killed four people and turned towns on Colorado's expansive eastern plains into muddy swamps.

Boulder County officials announced 172 unaccounted people Friday, according to local television and newspaper reports. Thousands of evacuees sought shelter in cities nearly surrounded by rivers spilling over their banks.

'The largest storm'

US President Barack Obama approved federal assistance to release funds to help with emergency protection. And Colorado Governor John Hickenlooper declared a disaster emergency for 14 counties, authorizing $6 million (4.5 million euros) in funds to pay for flood response and recovery.

"It's got to be the largest storm that I can imagine in the state's history," Hickenlooper said, advising people to stay out of sand-filled floodwaters he called "almost like liquid cement."

Property losses will likely remain unquantified until water has receded, perhaps early next week, said Micki Frost, spokeswoman for the Colorado Office of Emergency Management. She said that an unknown number of homes and other structures had collapsed in the state but that "we have no structure count at this point" because some areas remained underwater.

Record rainfall

Boulder has received 14.7 inches (37.4 centimeters) of rain in September, nearly three times the 1940 record of 5.5 inches. Authorities moved 4,000 at-risk residents from their homes late on Thursday. In Longmont, about 14 miles northeast of Boulder, the St. Vrain River burst its banks, cascading across main thoroughfares and cutting the city in two. A city official said that 7,000 households had received mandatory evacuation orders.

"Many communities in our western mountains are completely isolated, no water, no septic, no sewer, we lost every roadway leading to the western end of our county," Boulder County's Office of Emergency Management announced in a Friday afternoon update.

"We don't yet know fully about homes and lives lost in canyons and mountain communities," the agency reported. "This will go on for several days."

mkg/msh (Reuters, AFP, dpa, AP)