(Updates with latest damage figures)
By Keith Coffman
DENVER, Sept 15 (Reuters) - The death toll from historic flooding in eastern Colorado stands at four and will likely increase, authorities said on Sunday, as rescuers struggled to reach hundreds of people stranded or displaced by fierce floodwaters.
At least four people are confirmed dead after six days of rain that flooded an area nearly the size of Delaware and destroyed 1,500 homes, according to Colorado's Office of Emergency Management officials.
About 1,000 trapped people in Larimer County were awaiting rescue by helicopter after driving rain and low cloud ceilings grounded all rescue aircraft on Sunday, said Shane Del Grosso, commander of the federal Rocky Mountain Incident Management Team.
"It's going to take months or a year to rebuild those roads to their previous standards (in Larimer County)," Del Grosso said.
President Barack Obama declared the area a major disaster over the weekend, freeing up federal funds and resources to aid state and local governments.
In Boulder County, search and rescue crews conducted house-to-house searches for stranded people, said Gabrielle Boerkircher, a spokeswoman for the Boulder Office of Emergency Management.
"We drive where we can, but where we can't, the crews are on foot," Boerkircher said.
An 80-year-old woman whose home was washed away by flooding in the Big Thompson Canyon in Larimer County was the latest victim feared dead from the week-long rains, said sheriff's spokesman John Schulz.
She "was injured and couldn't get out of her house, and when neighbors went back to help her, the house was gone," Schulz said.
A 60-year-old woman whose home was swept away by flood waters in Larimer County was also feared dead, police said.
Drenching rains have washed out several roads in the waterlogged region, in some cases cutting off entire communities.
U.S. Army and National Guard troops have rescued 1,750 people cut off by washed-out roads in the mountain canyons of Boulder and Larimer counties, Army spokesman Major Earl Brown said in a statement.
On Saturday, troops airlifted to safety 85 fifth-graders who were on a school trip in Boulder County when heavy rains collapsed roads, stranding them near Jamestown.
Moderate rain was expected through the night over the region before tapering off by Monday afternoon, said Kyle Fredin, a meteorologist with the National Weather Service.
Some areas along Colorado's urban corridor have measured 17 inches (43 cm) of rain in the six days the rainstorm has stalled over an area known as the Front Range.
Those totals exceed average annual rainfall by more than five inches in the semi-arid region, he said.
People in the farming communities of northeastern Colorado braced for a surge from the north-flowing South Platte River.
Morgan County Sheriff James Crone said all eight bridges spanning waterways in the county were impassable from the rising river.
"Our county is cut in half," he said.
Crone said there would be significant crop damage from standing water in the corn, hay, millet and sugar beet fields that dot the agricultural county.
"There is no way for the water to drain, so come November when it freezes, it's going to be one huge ice cube," he said.
In Logan County, authorities have ordered evacuations to in light of the expected river crest beyond historic levels, and Emergency Manager Bob Owens warned people to prepare for sustained flooding over the next several days.
"This is going to be severe," Owens said.
State officials would be unable to assess the overall damage until rescue efforts were complete and the floodwaters receded, said Micki Trost, a spokeswoman for the Colorado Office of Emergency Management. (Editing by Chris Francescani, Daniel Trotta and Jackie Frank)