(Adds color from Boulder; more on flood-related deaths, breached dam)
By Keith Coffman
BOULDER, Colo., Sept 12 (Reuters) - Flash flooding unleashed by torrential downpours in Colorado has killed at least three people and forced hundreds to flee to higher ground as rising water toppled buildings and stranded motorists in their cars, officials said on Thursday.
Heavy rains drenched Colorado's biggest urban areas, stretching 130 miles (210 km) along the eastern slopes of the Rockies from Fort Collins near the Wyoming border south through Boulder, Denver and Colorado Springs.
Among the hardest-hit areas was Boulder County, where the National Weather Service issued a flash-flood warning through 10 p.m. local time (0400 GMT). Flood watches were also posted for several counties in central and north-central parts of Colorado.
"There is water everywhere," said Andrew Barth, the emergency management spokesman in Boulder County. "We've had several structural collapses. There's mud and muck and debris everywhere. Cars are stranded all over the place."
At least 6 inches (15 cm) of rain has fallen on the city of Boulder and up to 8 inches (20 cm) were measured in the foothills west of town, said Kari Bowen, a Weather Service meteorologist in Boulder, northwest of Denver.
The rains transformed Boulder Creek, which runs through the heart of the city and the University of Colorado's Boulder campus, into a raging torrent that overran its banks and flooded adjacent parking lots and streets.
Water gushed over sidewalks, roads and bike paths throughout the downtown area as sirens wailed and public-address loudspeakers urged residents to stay clear of high water: "Warning: Flash flood. Please proceed to higher ground. Do not cross standing or running water. Do not cross Boulder Creek."
The university campus was closed for the day, as were Boulder-area public schools and all municipal office buildings.
More than 400 students were evacuated from ground-floor campus housing overnight, campus police spokesman Ryan Huff said. Roughly 40 buildings on campus were believed to have sustained some flood-related damage, Huff said.
Ron Cobbley, 49, a homeless man who had been camping along the Saint Vrain River west of Boulder near the town of Ward, said he decided to leave the woods and head into town after three days in the rain.
"It was raging with whitewater," he said of the river.
EXTREMELY DANGEROUS CONDITIONS
Steady rain which began on Monday grew more intense late Tuesday and into Wednesday. Roads across the region were flooded out and standing water throughout Denver snarled morning rush-hour in the state capital.
Barth, the Boulder County emergency management spokesman, said conditions were "extremely dangerous" and that up to 4 inches (10 cm) of additional rain was expected to fall in the area on Thursday before tapering off.
All 200 residents of Jamestown, just north of Boulder, were forced to flee overnight, while the town of Lyons, further to the north, was cut off as floodwaters washed out U.S. Route 36 linking Lyons to Boulder, county Sheriff Joe Pelle said.
He said Lyons had reported that its residents had no fresh running water and its sewage treatment plant had been knocked out.
"We're trying desperately to get to them," Pelle said.
At least two people died in flooding in Boulder County, one whose body was found in a collapsed building by emergency crews searching door to door for victims in and around Jamestown and another who drowned elsewhere in the county, Commander Heidi Prentup of the Boulder County Sheriff's Office said.
The body of a third victim, a man, was found by police on flood-watch patrols in Colorado Springs, about 100 miles (160 km) to the south, officials said.
Pelle said it was possible that more flood-related fatalities could emerge as emergency crews reached areas cut off by high water.
Heavy rains also breached an earthen dam on Meadow Lake in a remote corner of Larimer County north of Boulder, releasing about 100 acre feet of water - roughly enough to cover an area the size of a football field in 100 feet (30.5 meters) of water, sheriff's spokeswoman Jennifer Hillman said.
Nearby homeowners were warned by telephone of possible flooding but the dam release dissipated without causing major damage and no injuries were reported, she said.
In Broomfield, a small town just northwest of Denver, three people were rescued from an upside-down car swept into a washed-out culvert with two other vehicles, the Boulder Emergency Management Office said. (Writing by Steve Gorman; Additional reporting by Chris Francescani; Editing by Cynthia Johnston, Grant McCool and James Dalgleish)