The Haight Creek Fire burning near Alma, southwest of Eugene, is estimated to be at 100 acres on Oregon Department of Forestry-protected Bureau of Land Management land.
ODF officials say the fire is not threatening any structures or power lines.
In a Wednesday night update, ODF officials said 140 firefighters were working the fire, with night-shift crews continuing to build a line around the fire's footprint.
Heavy fire activity was reported Wednesday, especially in the north end of the fire. Fire managers say it's burning in challenging, steep terrain with very heavy fuels, which makes it difficult to reach and fight.
Eight air tankers and seven helicopters were working the fire Wednesday. Water scooping aircraft were dipping into Fern Ridge Reservoir to dump on the flames.
The cause of the Haight Creek Fire is under investigation.
