BELFIELD, N.D. - A pipeline leak caused a significant oil spill in a tributary of the Little Missouri River this week, but the volume of oil released was still unknown Tuesday, Dec. 6, as a winter storm affected response efforts.

The incident was discovered by a landowner, raising questions about whether the oil pipeline in western North Dakota had a monitoring system to detect leaks.

The pipeline owner, Belle Fourche Pipeline Co., is part of True Companies of Wyoming, which also owns Bridger Pipeline that had a major oil spill in 2015 in Montana that contaminated the Yellowstone River and affected the water supply for the city of Glendive.

The spill discovered Monday morning about 16 miles northwest of Belfield contaminated Ash Coulee Creek with the release contained about 2½ miles downstream, said Bill Suess, spill investigation program manager for the North Dakota Department of Health.

"It is a significant spill," Suess said.

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The company shut down the pipeline and crews have been working to recover oil since Monday, including constructing a dam about 4 miles downstream, according to an incident report.

"It sounds like they've got the creek pretty well protected from migration going downstream," Suess said.

Winter weather was affecting the company's ability to estimate how much oil had been released, said Wendy Owen, spokeswoman for True Companies.

"We do have a crew on the ground that is doing their best to assess the entire situation and what the next step will be," she said.

The incident was reported by a landowner who saw oil leaking from the 6-inch pipeline into the creek, Suess said.

A labor union, which opposed a True Companies pipeline under consideration by the North Dakota Public Service Commission in 2015, issued a statement Tuesday urging regulators to have stronger oversight over the company.

The Laborers District Council of Minnesota and North Dakota, which has members working on the Dakota Access Pipeline, said True Companies has a poor track record of spills and environmental incidents. Dakota Access protesters camped in south central North Dakota often cite the company's 30,000-gallon oil spill in the Yellowstone River as an example of why they oppose a pipeline crossing the Missouri River north of the Standing Rock Sioux Reservation.

"Our members take pride in their work, and we won't just stand by and allow an irresponsible pipeline operator to harm North Dakota's natural resources or damage reputation of our industry," said spokesman Evan Whiteford, a career pipeliner who lives in Ray. "We think it's time for state officials to step in and force the True organization to clean up its act."

An investigator from the federal Pipeline and Hazardous Materials Safety Administration is scheduled to arrive on scene Wednesday, the agency said.

Inspectors from the state health department have been on site investigating and monitoring the cleanup. Suess said True Companies has a good track record of working with the health department.

"They do have a pretty good record of getting things cleaned up," he said.

Winter weather was affecting the cleanup, but Suess said crews stayed on site most of Monday night because roads were closed and they couldn't get back to their hotel.

The creek flows into the Little Missouri River, but the river is several miles away from the spill site and there was no indication Tuesday the oil had reached that far, Suess said.

Many facts were unknown Tuesday, including the age of the pipeline and whether it is a transmission line or a gathering pipeline, which have different levels of regulatory oversight. The North Dakota Public Service Commission was not involved with the siting of the pipeline, Chairwoman Julie Fedorchak said.