Exxon Mobil, owner of the 12-inch Silvertip crude oil pipeline, said about 1,000 barrels of oil spilled into the Yellowstone River near Billings, Mont.
Flooding in the river hampered early remediation work though the Environmental Protection Agency has said crews were getting better access to the area as water levels drop.
The EPA this week said it found numerous piles of debris soaked with oil along the Yellowstone River. The agency said high-pressure water cleaning methods weren't working and burning operations weren't suitable for current conditions.
Montana Gov. Brian Schweitzer announced that he formed a pipeline oversight committee in the state. The purpose of the committee, the governor said in an executive order, was to review the status of all oil pipelines underneath his state's rivers and streams.
Federal regulators have said the Silvertip pipeline met national guidelines in terms of its depth below the Yellowstone River.
A spokeswoman for Exxon Mobil told United Press International in response to e-mail questions that Silvertip had carried oil from tar sands in Canada since the 1980s. But "crude in the portion of the pipeline that failed was a mix of Wyoming Asphaltic (80 percent) and Elk Basin crude (20 percent)," the spokeswoman said.