Floodwaters that overtook dams and forced the evacuation of about 10,000 people in Central Michigan now threaten a Dow Superfund site and downstream communities.
The Tittabawassee River crested in Dow's hometown of Midland Wednesday at just over 35 feet, about 3 feet under what officials previously forecasted.
Gov. Gretchen Whitmer had warned the city could end up under about 9 feet of water by Wednesday evening.
Dow says floodwater has begun mixing with on-site containment ponds, prompting the company and U.S. Coast Guard to activate emergency plans.
A city official says no one has been reported hurt or killed in the flooding.
Whitmer says the state will investigate the operators of the Edenville and Sanford dams.