Colorado farming communities along the South Platte River were ordered to evacuate ahead of a predicted surge in the flooding, which may have claimed a fifth life and has left many still unaccounted for, authorities said on Saturday.
Four deaths have already been confirmed and, the Larimer County Sheriff's Office said in a Tweet, "a 60-year-old woman from Cedar Grove ... is missing and presumed dead."
About 200 people in eastern Colorado were unaccounted for, although authorities said some of those residents might be stranded or cut off from communication in the region's worst flooding in decades.
More heavy rain was expected as search-and-rescue teams used boats and helicopters to pull stranded residents to safety as flash flood waters toppled buildings, washed out roads and bridges and inundated farmland.
"Given the destruction, there is a high probability" of more fatalities, Boulder County Sheriff Joe Pelle said at a media briefing.
Two small farming communities in eastern Colorado were under evacuation orders as a surge from the flooding was headed in their direction on the plains, the Colorado Office of Emergency Management said on Twitter.
"The town of Orchard is in immediate danger. EVACUATE NOW," it said. "The town of Goodrich is under an evacuation order."
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 towns along the Front Range of the Rockies north of Denver were especially hard hit as water poured down rain-soaked mountains and rushed through canyons that funneled the runoff into populated areas.
The National Weather Service in Boulder warned of scattered showers and thunderstorms later on Saturday and into Sunday that could trigger more flash flooding.
North of Boulder, Lyons was virtually cut off when flood waters washed out U.S. Route 36 and left residents without power and water for 48 hours.
Among those confirmed dead were a couple who stopped their car northwest of Boulder and were swept away by flood waters. Both bodies were recovered, the man's on Thursday and the woman's on Friday.
Another casualty was found in a collapsed building near Jamestown, an evacuated enclave north of Boulder, and still another, a man in Colorado Springs, about 100 miles (160 km) to the south, officials said.
On Friday, Governor John Hickenlooper declared a disaster emergency for 14 counties from the Wyoming border to Colorado Springs. The declaration authorizes $6 million in funds to be used 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 on Friday declared a state of disaster, making funding available to state emergency officials for recovery efforts.
In rural Weld County, aerial TV footage showed large stretches of land covered in brown water, with homes and farms half-submerged.
Weld County sheriff's spokesman Steve Reams said bridges had been washed out and nearly every road in and around Greeley, Evans and Milliken was closed.
The flooding was the worst in the state since 1976, when nearly 150 people died in Larimer County in a flash flood along the Big Thompson Canyon.
The size and scope of property losses remain unquantified, with county assessment teams not likely to begin preliminary evaluation of the damage before early next week, once the water has receded, said Micki Frost, spokeswoman for the Colorado Office of Emergency Management.