Lake Oahe, created when the United States Army Corps of Engineers flooded thousands of acres of Sioux lands in the Dakotas by constructing the Oahe Dam on the Missouri River, provides several successor tribes of the Great Sioux Nation with water for drinking, industry, and sacred cultural practices. Passing beneath Lake Oahe’s waters, the Dakota Access Pipeline transports crude oil from North Dakota to Illinois. Under the Mineral Leasing Act, 30 U.S.C. § 185, the pipeline could not traverse the federally owned land at the Oahe crossing site without an easement from the Corps. The question presented here is whether the Corps
violated the National Environmental Policy Act, 42 U.S.C. § 4321, by issuing that easement without preparing an environmental impact statement despite substantial criticisms from the Tribes and, if so, what should be done about that failure. We agree with the district court that the Corps acted unlawfully, and we affirm the court’s order vacating the easement while the Corps prepares an environmental impact statement. But we reverse the court’s order to the extent it directed that the pipeline be shut down and emptied of oil.