Sawtooth National Forest officials say the Kelley Fire was sparked by lightning during Saturday's thunderstorms in the area southwest of Baumgartner Campground.
Officials say the fire has grown to 250 acres and they currently have 180-200 people working on fighting it.
The fire is currently threatening campgrounds, but officials believe it will eventually threaten structures in the area as well.
The Forest Service says it plans to close the river corridor between Featherville and Baumgartner Campground starting Tuesday morning.
A pre-evacuation order goes into effect Tuesday at 8 a.m. for homes and campgrounds between Willow Creek and Skelton Creek.
The Elmore County Sheriff's Office says deputies will go door-to-door Tuesday and to camp grounds in the area to notify people of the order.
The sheriff's office says they are encouraging people who parked camping trailers in the area for the upcoming Labor Day weekend to remove them Tuesday.