The tribes had asked Boasberg to direct the Army Corps of Engineers to withdraw permission for Dallas-based developer Energy Transfer Partners to lay pipe under Lake Oahe in North Dakota, which the Corps manages for the U.S. government. The stretch under the Missouri River reservoir is the last piece of construction for the pipeline.
The company is wrapping up pipe work under the lake and has said oil could start flowing between Monday and Wednesday.
The tribes fear the pipeline could harm their water supply and their right to practice their religion, which relies on clean water. ETP disputes that.
The tribes' appeal rests on the religion argument. Boasberg has said he doesn't think the tribes have a strong case on appeal. He also said ETP would be "substantially harmed" by a delay in pipeline operations.