Midland — Dow Inc. activated its emergency operations center at its Midland facilities Wednesday after flood waters reached historic levels and mixed with chemical plant containment ponds, raising fears of an environmental disaster. Another fear: That the deluge will stir up cancer-causing dioxins in the river downstream of the federal Superfund site.

The multinational chemicals giant headquartered in Midland temporarily shut down its operations in the Michigan Operations industrial park as the failure of two dams on the Tittabawassee River forced thousands of residents to evacuate. The company’s operating units were shut down except for facilities needed for safely managing chemical containment, company spokesman Kyle Bandlow said in a statement.

The potential release of any chemicals in the containment ponds is just one concern that could necessitate environmental remediation once the water recedes. The historic flood also has stirred up the contaminated sediment laying at the bottom of the Tittabawassee River put there by the Dow plant that helped build the city around it. The 1,900-acre facility abuts the river, and for years dangerous dioxins contaminated it, pushing the federal government to create a Superfund site there.