"We are pleased to be able to resolve this environmental compliance issue with the State of Montana," Jeffers wrote of the settlement.

Only about 10 barrels of crude were recovered by cleanup crews, federal officials have said. That's less than 1 percent of the total spilled.

The cause of the spill remains under investigation.

The 12-inch Silvertip pipeline was buried just a few feet beneath the riverbed when it was installed 20 years ago. High water last spring and summer eroded that cover, which officials have speculated could have exposed the line to damaging debris.

Thursday's settlement came after more than three months of negotiations between attorneys for Exxon and the state.

The agreement contains provisions to shield the company against any future lawsuits from state agencies, although it will not become final until after a 30-day comment period.

"It was a significant violation. There were hundreds and hundreds of acres of land affected and it was a major oil spill," Opper said. He added the penalties likely would have been "a lot higher" if Exxon had not cooperated on the cleanup.