About 130 firefighters and two helicopters were battling the blaze, which started in the foothills west of Fort Collins, The Denver Post reported.
The fire has resulted in two injuries; both firefighters who were treated at a local hospital, the Larimer County Sheriff's Office said in a statement. However, no structures have been destroyed or damaged.
Lory State Park has been closed indefinitely to allow Colorado Parks and Wildlife time to assess the damage, the agency said in a statement. Numerous pedestrian bridges in the park have been damaged, the statement said.
Authorities are investigating the cause of the fire, but officials said Monday it was human-caused.