The Colorado Department of Transportation re-opened Highway 14 later in the evening.
Residents from Mishawaka on the west to Gateway Park on the east are now under a pre-evacuation alert.
Falls Creek Road was completely covered and impassable at one point.
"[There is] tremendous amount of debris coming down from the Hewlett side," said Jennifer Hillman with the Larimer County Sherriff's Office.
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The area was affected by the High Park fire, which has burned across about 136 square miles since June 9.
Charred hillsides are vulnerable to erosion and flooding during downpours because they have less vegetation to soak up rain, and burned soils can repel water.
The Associated Press contributed to this report.
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