A semi tractor trailer rolled over near mile-marker 3, near Marco Flats on Highway 200 east of Bonner this morning, spilling an undetermined amount of chemicals into the Blackfoot River.
Missoula County Sheriff's Department spokeswoman, Paige Pavalone, says officials don't yet know much about the specific chemicals, only that, "if mixed with water, there can be unknown risks."
A hazardous materials team is on scene and a boom is in the water to contain the spill.
Traffic flow was reduced to single lane and Pavalone says the highway will soon be closed in both directions as the truck wreckage is removed.
It's a cool, cloudy morning in Missoula which has reduced the number of floaters and boaters in the Blackfoot, but people are being asked to pull out of the river downstream of the spill.
Fish Wildlife and Parks will make the final decision on whether or not to fully close the Blackfoot.