The compound, which also houses the chemical giant’s world headquarters, lies on the banks of the Tittabawassee River in Midland, where by late Wednesday rising water had encroached on some parts of downtown. Kyle Bandlow, a Dow spokesman, said that floodwaters had reached the Dow site’s outer boundaries and had flowed into retaining ponds designed to hold what he described as brine water used on the site.

The Superfund cleanup sites are downriver from the century-old plant, which for decades had released chemicals into the nearby waterways. The concern downriver, according to Allen Burton, a professor of earth and environmental sciences at the University of Michigan, is that contaminated sediments on the river floor could be stirred up by the floodwaters, spreading pollution downstream and over the riverbanks.

“You worry about the speed of the current, this wall of water coming down the river,” he said. “It just has a huge amount of power.”

Mr. Bandlow did not provide information on the status of the cleanup sites.

There is also a tiny nuclear research reactor on the site, used to create material that can be used in product experiments. Overnight, Dow filed an “unusual event” report with the federal Nuclear Regulatory Commission warning of potential flooding at the site. But the reactor had already been shut down because of the coronavirus crisis, and there were no indications of flood damage on Wednesday.

A federally funded Superfund cleanup of the Tittabawassee River began in 2007, and was slated for completion next year. Cleanup of other contaminated waterways is set to take longer.

“I would hate to see 13 years of work literally go down the drain if this flood wipes away the effort,” said Terry Miller, chairman of the local environmental group, Lone Tree Council, which has for years campaigned for a cleanup. “We were almost nearing the end.”

A former lawyer for Dow who oversaw the cleanup for more than a decade, Peter C. Wright, now runs the federal government’s Superfund cleanup program within the Environmental Protection Agency. A 2018 New York Times investigation found that while Mr. Wright led Dow’s legal strategy relating to the cleanup, the chemical giant was accused by regulators, and in one case a Dow whistle-blower, of submitting disputed data, misrepresenting scientific evidence and delaying cleanup.

Mr. Wright has pledged to recuse himself from cleanups related to his former employer, and was not involved in the government’s response to the flooding, said Francisco Arcaute, a Chicago-based spokesman for the E.P.A.

The agency was prepared to assist Michigan “in assessing and responding to any public health and environmental impacts from the Tittabawassee River Superfund Site and Dow’s Midland facility due to the ongoing flooding,” Mr. Arcaute said, including dispatching emergency personnel to the area.

Dow has not reported chemical releases into the river, Mr. Arcaute added. He said that the company’s Superfund agreement with the agency would require the company to survey for recontamination or any other effect on cleanup efforts after the flooding.

Dow agreed last year to pay another $77 million to fund projects that would attempt to restore nearby fish and wildlife habitats to compensate for decades of pollution from its plant. Signs along the river warn locals not to eat fish caught there, and to avoid contact with soil and river sediment.

The threat to the Dow complex highlights the risks to Superfund and other toxic cleanup sites posed by the effects of climate change, which include more frequent and severe flooding. A federal report published last year found that 60 percent of Superfund sites overseen by the E.P.A., or more than 900 toxic sites countrywide, are in areas that may be affected by flooding or wildfires, both hazards that may be exacerbated by climate change.

The Trump administration rejected the report’s recommendation that the federal government provide more clarity on how it intends to incorporate climate research into readying these sites to withstand a changing climate.