WASHINGTON – States have broad power to regulate the quality of their rivers, the Supreme Court ruled Monday.

By a 9-0 vote, the high court upheld a ruling by the Maine Supreme Judicial Court that allowed the state to set additional conditions on a hydropower dam owner in exchange for renewing his license to operate five dams on the Presumpscot River.

A ruling against Maine would have eliminated a key regulatory tool used by nearly every state to improve the quality of waterways. The state said the operation of the dams had caused long stretches of the natural riverbed to be essentially dry.

“The alteration of water quality … is a risk inherent in limiting river flow and releasing water through turbines,” Justice David Souter wrote. “Changes in the river like these fall within a state’s legitimate legislative business and the Clean Water Act provides for a system that respects the states’ concerns.”

Maine and its supporters said that the federal Clean Water Act gives states the authority over discharges into their waters, including dams that release water to generate power.

S.D. Warren Co., a division of Sappi Fine Paper, contended that it shouldn’t have to get state permits just because water flows through its dams. The company said the water going through the dams did not constitute discharges because the water going in and out of the dams is the same water.

Maine Attorney General Steven Rowe said the case was being watched by dam owners, regulatory agencies and environmental groups nationwide.

There are more than 1,000 licensed hydroelectric projects with more than 1,500 dams in 45 states, he said. There are more than 100 licensed dams in Maine.

“The impact of this decision will be felt well beyond the state of Maine,” Rowe said.

Dusti Faucher, president of the Friends of the Presumpscot River group, said the court’s decision means the group now can move forward to have fish passages installed on the river’s dams for salmon, shad and other anadromous fish.

“The Presumpscot River is the only river in Maine that had sea-run fish that doesn’t have fish passages on its dams,” Faucher said.

The dispute involved the Saccarappa, Mallison Falls, Little Falls, Gambo and Dundee dams.

A call to S.D. Warren officials was not immediately returned.