UPDATE 3:45 p.m.:
Environmental health experts say chemicals were in a non-hazardous state and have largely disbursed and diluted. No sulfuric acid reaction. No reports of fish/wildlife kills. Rivers and access sites to re-open shortly.
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Citing an "abundance of caution", Montana Fish, Wildlife and Parks Regional Fisheries Manager, Pat Saffel, says the Blackfoot and Clark Fork rivers from Johnsrud Park, west into Missoula are closed down due to a chemical spill.
Earlier this morning a semi wrecked just east of Bonner, spilling some chemicals into the Blackfoot. The driver was not seriously injured.
Saffel says one of the chemicals was aluminum sulphate. (Updated information: The other chemical is identified as diethanolamine. When combined, they're used to manufacture industrial detergents)
Saffel says the two are inert when separate, but could form an acid when in contact with water. Saffel says so far there are no reports of dead fish or other wildlife on that portion of the river. Boaters and people floating the river are being told to leave the water. Area access sites are being taped-off and closed down.
Avoid bringing dogs to this entire section of river until further notice.