The pristine snow piled high in the northern Colorado mountains is beginning to melt, and officials worry that under the wrong conditions, it could unleash another ugly torrent through towns and farms still scarred by last autumn's floods.
After a wintry Mother's Day storm, the snowpack is nearly 150 percent of the mid-May average on the slopes that feed the South Platte River, whose tributaries did some of the worst damage in the September floods.
A heat wave or rainstorm could suddenly accelerate the runoff and send water gushing into flood-damaged streambeds that might not be able to contain it, experts say.
So far, the long-term weather outlook isn't definitive.
"Fifty-fifty, could be bad, could be good," said Kevin Klein, director of the Colorado Division of Homeland Security and Emergency Management, which began planning for the runoff almost immediately after September's flood.
Colorado's spring runoff normally doesn't cause floods, even though most of the state's surface water is collected in a 2-month frenzy of melting snow, said Nolan Doesken, the state climatologist.
But some streambeds in northern Colorado are loaded with tons of sand and gravel swept down by the September floods, so they hold less water and are more likely to overflow, said Treste Huse, a National Weather Service hydrologist.
There's also some evidence the water table remains high and the ground is still saturated in areas that flooded, experts said. That could send runoff rushing down canyons instead of soaking into the soil, and it could make hillsides more susceptible to landslides and rock fall.
The September floods killed nine people, and a 10th was killed during recovery operations. A storm backed into the mountains from the east and sat for days, pouring out rain that overwhelmed rivers far out on to the state's eastern plains. Nearly 2,000 homes were damaged or destroyed; total damage was estimated at $2 billion.
It was described as a once-a-century flood or even a once-a-millennium flood, but Klein said researchers are still unsure about where it fits in climate history.
Cleanup and repairs were just getting underway when snow began to fall in the mountains.
In the shallow South Platte valley on the eastern plains, farmers are hoping the runoff doesn't damage newly repaired irrigation systems.
Irrigation networks that watered as much as 470 square miles were damaged by the flood, said John Stulp, a water policy adviser to Gov. John Hickenlooper. Irrigation has been restored to all but about 30 square miles, Stulp said.
State highway officials have been racing to rebuild U.S. 36, a vital link between Lyons and Estes Park, a town at the entrance to Rocky Mountain National Park. The highway was heavily damaged by the flood. Crews hope to complete the blasting and initial paving by early June, said Amy Ford, a state transportation spokeswoman.
The early runoff is already flowing down North St. Vrain Creek in Lyons, a quarry town in the foothills northwest of Denver that was hit hard by the flood. The St. Vrain River destroyed dozens of homes, a trailer park, two town bridges and sections of the only road in and out of the picturesque town of 1,600 framed by sandstone cliffs.
Sandbags line the front walk of Charles Stacy's recently repaired house near the creek, but Stacy said he isn't too concerned about the runoff and took the sandbags only because they were free.
"I don't think it's going to go," he said, adding that most of his neighbors aren't using sandbags.
Pamela Sichel rents a home on the other side of the creek, and although the flood carved away part of the backyard, she too said she's not really worried. The previous tenants cleared out after the house was swamped by the flood, but Sichel was undeterred.
"I thought the odds were in my favor after a 500-year-event," she said.
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