Former Oklahoma Attorney General Drew Edmondson brought the case against the poultry farms in 2005. On behalf of the people of Oklahoma, Edmondson alleged that water tainted with waste from those companies’ chickens and turkeys was running downstream into Oklahoma, polluting the Illinois River and Lake Tenkiller with phosphorus and bacteria.

“As late as the 1960s, its waters were crystal clear,” Judge Gregory K. Frizzell wrote in his 219-page decision. “But that is no longer the case. The river is polluted with phosphorus.”

Found in agricultural runoff, phosphorus can cloud waters, harm fish and foster the growth of blue-green algae.

“The Environmental Protection Agency has recognized that nutrient pollution caused by phosphorus is one of America’s most widespread, costly, and challenging environmental problems,” Frizzell wrote.

Last week’s decision found the poultry corporations guilty of violating two of Oklahoma’s anti-pollution laws and inflicting irreparable harm on the Illinois River Watershed.

Arkansas regulators have questioned the appropriateness of Oklahoma’s limit for phosphorus in scenic rivers, calling it “unachievable.” But regardless of its own state water standards, Frizzell wrote, “Arkansas cannot ‘permit’ nonpoint source pollution of Oklahoma’s waters.”

Denise Deason-Toyne is the president of Save the Illinois River, an advocacy group formed in the 1980s to preserve the river, its tributaries and Lake Tenkiller. She said the decision made her feel hopeful about the watershed’s future.

“I am cautiously optimistic that this decision will cause people to kind of wake up and realize that, hey, here's a federal judge who has said that this watershed needs to be protected and this industry, among others, was a big factor in the pollution,” Deason-Toyne said.

Current Oklahoma Attorney General Gentner Drummond said in a statement that he’s also optimistic the pollution can be curbed.

“This is a great and historic day for Oklahoma,” Drummond said. “While this decision has been a long time coming, it is important to note that in the intervening years since the filing of the suit, the poultry industry has made, or is willing to make, strong improvements in waste disposal to ameliorate the extent of the problem.”

It’s up to Oklahoma and the poultry farms to agree on a remedy for the pollution, which Deason-Toyne said might not be an easy task.

“The poultry companies probably think, OK, we'll just throw some money at Oklahoma and we'll walk away,” Deason-Toyne said. “But the remediation is likely to take years. It's going to be something that needs to be continual, it needs to be sustainable, and it isn't going to be inexpensive.”

Oklahoma and the poultry companies have until mid-March to agree on a solution, or the court will determine its own.