When the Army Corps of Engineers cleared the way for completing the Dakota Access Pipeline on Tuesday, the agency also canceled something the Standing Rock Sioux have been asking for since the project started: a full report on the environmental effects to the tribe’s main water source.

The Army Corps granted the easement on Wednesday to allow Energy Transfer Partners, the company behind the pipeline, to build on federal lands at Lake Oahe on the Missouri River — a move the Standing Rock Sioux say could contaminate their main water supply and destroy sacred land. The Corps also canceled an environmental impact statement, which had never been completed for the area.

In abandoning the report and granting the easement, the Army Corps referenced one of Donald Trump’s first acts as president: a presidential memorandum expediting the pipeline. But that may not be good enough to clear the legal hurdles, according to environmental experts who say the agency needs to provide a reason for reversing course on completing the report, which the Army had deemed necessary just a few months ago.

Within hours of the Army Corps’ decision, the Standing Rock Sioux vowed to challenge the decision in court. While Trump ordered the completion of the pipeline as soon as possible, the Army Corps’ reversal of its prior decision could set in motion a lengthy legal fight, one that could go all the way to the Supreme Court, according to Zygmunt Plater, a professor of environmental law at Boston College.

“That may work when you’re on ‘The Apprentice,’” he said. “But when you’re in court, the allegations will be that the decision was originally based on fact and has been changed not on fact.”

Reversing the decision

The planned lawsuit will cite two main arguments: that the environmental impact statement was wrongfully abandoned, and therefore, the Army Corps can’t legally grant the easement, according to Jan Hasselman, lead attorney for the Standing Rock Sioux at nonprofit law firm Earthjustice.

“The president doesn’t have the authority to change the law, and the executive order doesn’t support the change,” he said. “[The Army Corps of Engineers is] waning to political pressure and violating the law and the tribe’s rights by abandoning the environmental impact statement.”

Technically, the Army Corps of Engineers reserves the right to change its mind about projects, but the agency needs to give a reason, according to retired Col. John Eisenhauer, a district engineer and commander with the Army Corps of Engineers from 2011 to 2013. He now serves as senior adviser at Dawson & Associates, an environmental regulation consulting firm.

Both the Army Corps’ federal court filing and its letter notifying Congress refer to compliance with Trump’s memorandum about finishing the Dakota Access Pipeline. But the memorandum itself doesn’t offer any explanation as to why the project needed further review in December but not today.

Plater said if he were litigating the case for the Sioux, he’d cite a Supreme Court decision as precedent that the current administration hasn’t provided adequate evidence for the reversal. In the 1970s, the federal government passed and amended an act that required vehicles to include safety features, like airbags and seatbelts. But under President Gerald Ford in 1976, the secretary of transportation suspended those rules entirely. State Farm, an insurance company with a financial interest in keeping its clients safe, challenged the decision.

Strong precedent

The Supreme Court ruled the government didn’t consider the facts and therefore the suspension of the rules was deemed “arbitrary and capricious.” Since Trump didn’t support his memorandum about finishing the pipeline, nor has the Army Corps of Engineers given a clear reason for reversing plans to complete an environmental review, the same standard could apply to the easement, according to Plater.

The case also helped to clarify application of the Administrative Procedures Act of 1946, which governs decision-making at all federal agencies, according to Robert Anderson, law professor and director of the Native American Law Center at the University of Washington School of Law.

“Under the act, agencies get a lot of deference, but they have to say why,” he said. “I think that’s going to be the main argument for the plaintiffs — that the [Army Corps] issued the permit without making some finding about what was wrong with the process that had been previously laid out.”

Under the National Environmental Policy Act, signed by President Richard Nixon in 1970, environmental assessments must be completed for planned infrastructure projects that would cross federal land, like Lake Oahe. And the Army Corps of Engineers evaluates every application for use of federal lands.

Once completed, environmental assessments can go one of two ways: If the environmental assessment determines the project could cause significant environmental effects, a full environmental impact statement must be completed. If not, a finding of no significant environmental impact is issued, and the project progresses to the next stage of planning.

Significant impacts

In July, a finding of no significant impact was released for the Dakota Access Pipeline, leading to the approval of permits allowing construction. And in September, a federal court upheld those permits. The Department of Justice, however, later noted that due to the sensitive nature of the crossing at Lake Oahe, the Army wouldn’t authorize construction there until the agency reaffirmed its decision that the pipeline wouldn’t cause significant environmental problems.

Under President Obama, the Army did reconsider in December when Assistant Secretary of the Army for Civil Works Jo-Ellen Darcy denied the easement due to concerns about risks to the water supply and Sioux treaty rights. In a statement at the time, Darcy suggested a full environmental impact statement be done to consider alternative routes for the pipeline, which could have taken months. The decision was widely celebrated as a victory for the Sioux tribe and “water protectors” at the Standing Rock encampment.

Enter President Trump.

On his fourth day in office, the president issued a memorandum that essentially told the Army to get on with the pipeline already. Citing that executive action, the agency decided Tuesday to cancel the environmental review and grant the easement to complete the project as soon as possible.

Earthjustice and the Sioux contend that in complying with the memorandum, the Army Corps of Engineers bowed to political pressure from an administration much cozier with fossil fuels than the last.

But Eisenhauer, the former district engineer and commander with the Army Corps, gave another reason: It’s easier. If the agency completes an environmental impact statement, that could put the permit allowing construction at risk.

“It’s not only something that’s been granted; it’s been litigated and upheld,” he said. “You know, why would you want to open yourself up to another process?”