By Keith Coffman
DENVER (Reuters) - Colorado authorities warned on Sunday that the death toll likely would rise from historic flooding in the state as a second person was confirmed missing and presumed dead, in addition to four deaths previously verified.
An 80-year-old woman whose home was washed away by floodwaters in Larimer County was the latest victim feared dead from the week-long rains, said sheriff's spokesman John Schulz.
"The woman 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 way in the same area of the Big Thompson Canyon likely perished as well, Schulz said.
Officials have confirmed the deaths of four people in the flooding: Three in Boulder County, and one in Colorado Springs, about 100 miles south of Boulder.
President Barack Obama declared the area a federal disaster over the weekend.
More than 500 people remain unaccounted for in Boulder and Larimer counties, authorities said.
U.S. National Guard and U.S. Army 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.
Driving rain and low cloud ceilings on Sunday grounded the air resources until the weather clears, officials said.
Meanwhile, residents 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.
Forty miles northeast in Logan County, authorities have ordered evacuations as the river crest was forecast to exceed historic levels.
Emergency Manager Bob Owens warned residents to prepare for sustained flooding over the next several days.
"This is going to be severe," he said in a statement.
Micki Trost, spokeswoman for the Colorado Office of Emergency Management, said that until rescue efforts were complete and the floodwaters recede, officials would not be able to assess the scope of the damage.
(Writing by Jonathan Allen; Editing by Scott Malone and Maureen Bavdek)