A man takes photos of the damage from heavy rains in Boulder, Colorado.
More heavy rain is expected on Saturday in Colorado where rescue workers are battling to reach residents cut off by the worst floods in decades, which have killed at least four people and left 172 still unaccounted for.
Search and rescue teams have used boats and helicopters to pull stranded residents to safety in areas where flash floods toppled buildings, washed out roads and inundated farmland.
The flooding began overnight Wednesday. It was triggered by unusually heavy late-summer storms that soaked Colorado's biggest urban centers, from Fort Collins near the Wyoming border south through Boulder, Denver and Colorado Springs.
Boulder and a string of other towns along the Front Range of the Rockies north of Denver were especially hard hit as water poured down rain-soaked mountains and spilled through canyons that funneled the runoff into populated areas.
Overnight, rescue workers took advantage of a break in the weather to reach residents still stranded in their homes by rampaging floodwaters that turned creeks into raging torrents that burst their banks.
"Quite a bit of the water has receded in the city ... and rescue crews will work throughout the night," Ashlee Herring, a spokeswoman for the Boulder Office of Emergency Management said of the recovery effort.
The National Weather Service in Boulder warned of scattered showers and thunderstorms later on Saturday and into Sunday that could trigger further flash flooding in the already drenched area.
Lyons, a town north of Boulder, was virtually cut off when floodwaters washed out U.S. Route 36, stranding residents without water and power for 48 hours.
At least four people were killed, including a couple swept away in floodwaters after stopping their car northwest of Boulder. The man's body was recovered on Thursday and the woman had been missing and feared dead before her body was found on Friday.
Also killed were a person whose body was found in a collapsed building near Jamestown, an evacuated enclave north of Boulder, and a man in Colorado Springs, about 100 miles to the south, officials said.
On Friday, Governor John Hickenlooper declared a disaster emergency for 14 counties, reaching from the Wyoming border south to Colorado Springs. The declaration authorizes $6 million in funds to pay for flood response and recovery.
In neighboring New Mexico, where floods forced the evacuation of hundreds of people in Eddy, Sierra and San Miguel counties, Governor Susana Martinez declared a state of disaster on Friday making funding available to state emergency officials for recovery efforts.
The Boulder Office of Emergency Management listed 172 people as unaccounted for following the floods, stressing that while they were not yet considered missing or in danger, relatives and authorities had not been able to contact them.
In rural Weld County, where the South Platte River has overflowed its banks and virtually cut the county in half, aerial TV footage showed large stretches of land covered in brown water on Friday. Many homes and farms were largely half-submerged.
Weld County sheriff's spokesman Steve Reams said nearly every road in and around a cluster of towns that includes Greeley, Evans and Milliken had been closed by flooding, including bridges that were washed out.
The flooding was the worst in the state since nearly 150 people were killed in Larimer County in 1976 by a flash flood along the Big Thompson Canyon.
The size and scope of property losses remain unquantified, with county assessment teams unlikely to begin preliminary evaluations of the damage at least until early next week, once water has receded, said Micki Frost, spokeswoman for the Colorado Office of Emergency Management.