On Sunday, the US Army Corps of Engineers said it would not grant a needed easement that would allow the Dakota Access Pipeline to cross the Missouri River at Lake Oahe — putting the controversial project on hold indefinitely while officials explore alternative routes.

The decision is a major victory for the Standing Rock Sioux, who have been protesting the proposed path of the $3.8 billion pipeline through southern North Dakota for months — arguing that it would endanger both their water supplies and nearby sacred sites.

The Army Corps announced its decision in a statement: "Although we have had continuing discussion and exchanges of new information with the Standing Rock Sioux and Dakota Access, it's clear that there's more work to do," said Jo-Ellen Darcy, the Army's assistant secretary for civil works. "The best way to complete that work responsibly and expeditiously is to explore alternate routes for the pipeline crossing."

The Dakota Access pipeline, which would run from the Bakken oil fields in North Dakota down to a terminal in Illinois, is nearly complete — save for a small, crucial portion that would cross underneath the Missouri River. The company behind the project, Energy Transfer Partners, was awaiting approval of easements from the Army Corps of Engineers, which manages the land around the reservoir at Lake Oahe.

The Standing Rock Sioux, whose reservation sits just south of the proposed route, had opposed the granting of these easements, arguing they were never properly consulted over their concerns with the nearby pipeline. The tribe has sued the Army Corps in federal court over the process that led to the pipeline originally being permitted (that lawsuit is still ongoing).

Back in September, the Obama administration stepped in and ordered the Army Corps to review its decision to issue the easements at Lake Oahe. Now, the agency has officially declined to approve this particular route and will consider alternatives, including a previously rejected path that would travel north of Bismarck. This process will require an in-depth Environmental Impact Statement and further public consultation. (Read the full Army Corps memo here.)

US Secretary of the Interior Sally Jewell endorsed the decision in a statement: “The thoughtful approach established by the Army today ensures that there will be an in-depth evaluation of alternative routes for the pipeline and a closer look at potential impacts, as envisioned by [the National Environmental Policy Act]. The Army’s announcement underscores that tribal rights reserved in treaties and federal law, as well as Nation-to-Nation consultation with tribal leaders, are essential components of the analysis to be undertaken in the environmental impact statement going forward.”

The big question now: What will Donald Trump do?

The pipeline isn’t officially dead. But a fresh environmental review to examine new routes could take at least a year or more, and Energy Transfer Partners, the company behind the pipeline, has said that its contracts for oil delivery would expire if the project was delayed past January 1, 2017. (If the pipeline is not built, the oil will continue to be shipped out of the Bakken by rail, which is more expensive.)

This decision is certainly a major setback for Energy Transfer Partners, which claims it has already lost $100 million from delays in the pipeline. In a statement, the company denounced the decision as a “purely political action,” noting that the Army Corps had previously granted the company every permit it needed.

It’s possible that the companies behind the pipeline could ask federal courts to enjoin (or block) the Army Corps’ decision in the coming days, says Carl Tobias, the Williams Chair in Law at the University of Richmond. Back in September, the DC Circuit Court sided with the companies in refusing to block the pipeline via injunction.

Another looming question is how Donald Trump might respond when he finally gets into office. Trump has said he’s in favor of finishing the Dakota Access Pipeline, but today’s decision won’t be easy for him to overturn through executive action alone.

“If one agency makes detailed fact findings about how a project is not in the public interest, the next administration can’t just come in and rip it up right away,” Jan Hasselman, an attorney at Earthjustice who is representing the Standing Rock Sioux, told me on Friday. “They could seek to undo it, but it would be subject to judicial review.”

One possibility is that the GOP-controlled Congress could somehow get involved and fast-track the project. House Speaker Paul Ryan denounced the decision on Twitter Sunday night:

This is big-government decision-making at its worst. I look forward to putting this anti-energy presidency behind us. https://t.co/Qu0nFTmGZv — Paul Ryan (@SpeakerRyan) December 5, 2016

But it’s a little early to say know this might all play out.

Protestors are cheering the decision

In Standing Rock, the Army Corps’ decision was met with jubilant celebration by the hundreds of Native Americans, environmentalists, and other activists who have come from around the country to protest the pipeline:

Celebration circle / chat here after #DAPL permit nixed by Obama Admin. #standingrock pic.twitter.com/ybNLMuMkdI — Ernest Scheyder (@ErnestScheyder) December 4, 2016

"We wholeheartedly support the decision of the administration and commend with the utmost gratitude the courage it took on the part of President Obama, the Army Corps, the Department of Justice and the Department of the Interior to take steps to correct the course of history and to do the right thing," said Standing Rock Sioux Tribal Chair Dave Archambault II in a statement.

The decision came right as some 2,000 US military veterans arrived at Standing Rock to help protest the pipeline as cold weather set in. Over the past few weeks, local law enforcement officers have been using particularly harsh methods to try to disperse the protesters — including using water cannons in freezing weather. Last week, the Army Corps of Engineers ordered all protesters to vacate the area north of the Cannonball River by December 5.

Through it all, the protestors insisted they would not leave. And now they’ve scored a major win.

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