The Oregon Department of Fish and Wildlife says the fish probably died at the time of the fire.
Samples collected the same day found a high pH both near the mill and just downstream.
Investigators say it’s also likely the fire raised the water’s temperature enough to kill the fish.
More than 90 percent of the fish that died were native species.
The advisory to stay out of the water was lifted on Sunday. Recent tests showed there was no threat to public health.