Well, recently I found out a court document has been released that tells a more-complete version of events. I encourage everyone to read it. It's a dry document, but readable.
starring The Standing Rock Sioux Tribe, The Army Corps of Engineers, and Energy Transfer Partners
I have read the court document. It includes a timeline of events from the pipe's early planning phase up through the present day.
The tribe alleges it was not consulted about the pipeline, but the court document states the company did try for almost 2 years to consult Standing Rock. Time and again from 2014 through the end of 2015 representatives attempted to get in touch with the Standing Rock tribe, but did not receive a response.
I wouldn't call it "stonewalling" on part of the Tribe, as some heavily-editorialized news sites claim, rather mismanagement. From 2014 to the end of 2015, the name Young is mentioned as the primary contact, and then in 2016, there seems to have been a change in leadership, and Young's name is nowhere to be found. Finally, the Tribe got someone in charge who actually responded to the company about the proposed pipeline and was willing to negotiate and work with the company.
Publicly, the Tribe's main objection to the pipe is the integrity of its water supply, but that apparently is not enough of a concern to halt construction, as the water supply in question is not actually on the Tribe's land, and the evaluation thereof is not in the Army Corps of Engineers' jurisdiction.
Much of the DAPL follows an already-excavated utility line, including the section that crosses the tribal land, so the company argues that there is no basis for objection, as the land had already been disturbed before. To be fair, a natural gas pipe is not the same as an oil pipe, so just because permission was granted in the past does not automatically grant permission for the DAPL. An oil pipe leaking will cause far more harm in the long term than a gas pipe breaking.
The only way the Standing Rock tribe can legally block the pipeline is for potential harm it may do to its sacred sites. Only a very small part of the pipe's path crosses tribal land, so this objection does not hold much weight. The company responsible for the pipe did consult with the Tribe multiple times in 2015 and 2016 and alter the course to avoid places of importance to Standing Rock.
The Corps did survey the land owned by the Tribe and identify places where the pipe would impact cultural sites, and when the pipe was rerouted, the Corps approved. According to the court document, the company has been accommodating and flexible, but that's not the point the Tribe is making. The real danger is off Tribal land, to the water supply, and the Tribe wanted the Corp to evaluate the environmental impact along the entire pipe, which it does not have authority to do.
Legally, the Tribe has no grounds to object to the pipeline, as almost all of it crosses private land, and the Corps does not have jurisdiction there. Only a few water crossings off tribal land fell under federal jurisdiction and needed Corps approval, which they received, and no evidence can be presented that harm may befall the water system by a potential leak at sites where permits were granted.
Though the court document does tell the legal side of the story, it hardly condemns the Tribe for objecting to a pipe that is already 90% complete. I think it shows what the real point of the battle is: Standing Rock does not want an oil pipeline going through its land, or anywhere near its water supply, but apparently that option is not on the table.
A reading of the court document makes it obvious that harm to sacred sites is only the legal objection to the pipe. The real objection is potential harm to their water supply, but it cannot object to that because it is not on the tribe's land, and most of it is not federally regulated anyway.
Pipes break. It's inevitable. Sooner or later, the pipe will leak, and oil will contaminate the water and soil for years. The land may never recover. The environmental concerns are still real, even if the legality of the claims is flimsy.