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Opponents of the Dakota Access Pipeline erupted in cheers on Sunday after U.S. regulators rejected a final permit needed to complete the controversial pipeline.

The U.S. Army Corps of Engineers said it won't grant an easement that would've allowed the pipeline's builders to run the conduit under Lake Oahe, a reservoir near the Standing Rock Sioux Reservation in North Dakota.

SEE ALSO:Amnesty condemns 'excessive' violence at Standing Rock, calls for Obama to act

In an interview with Mashable, former Vice President Al Gore said he was "very excited" by what he called "an extremely important victory" for the Standing Rock Sioux and the 100 other tribes that have joined the anti-pipeline effort in solidarity.

"Their iconic teaching during this struggle -- water is life -- will take its place with some of the iconic statements from some of the great reform movements in history," Gore said Sunday. He also commended tribal leaders for emphasizing the pipeline's climate change risks throughout their campaign.

"They used the power of prayer, and they have objected to the label of 'protesters' and accurately used the term 'water protectors,'" he said.

The decision is an enormous, though not necessarily permanent, victory for the thousands of people camped near the disputed construction site.

Native American activists and their allies have insisted that the $3.8 billion project would threaten the region's water supplies and damage sacred sites. Critics also noted the 1,170-mile pipeline would boost U.S. greenhouse gas emissions by allowing for increased oil production in North Dakota's shale region.

Activists participate in an art project conceived by Cannupa Hunska Luger, from the Standing Rock Sioux Tribe, at Oceti Sakowin Camp on the edge of the Standing Rock Sioux Reservation, Dec. 3, 2016.Credit: Getty Images

"The Standing Rock Sioux Tribe and all of Indian Country will be forever grateful to the Obama Administration for this historic decision," David Archambault II, the Standing Rock Sioux's tribal chairman, said in a statement on Sunday.

He urged President-elect Donald Trump to uphold the decision when Trump takes office in 2017.

U.S. Secretary of Interior Sally Jewell applauded the Corps' decision in a statement on Twitter.

Around 2,000 U.S. veterans had traveled to Cannon Ball, North Dakota, over the weekend to join demonstrations and serve as a protective shield for activists against an increasingly hostile law enforcement.

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Police in recent weeks have used tear gas and rubber bullets on protesters and blasted crowds with water in subfreezing weather. Organizers said more than a dozen people were hospitalized following the water cannon incident for hypothermia.

In an effort to end the four-month-long standoff, the Corps and the North Dakota governor's office last week ordered demonstrators to evacuate their sprawling protest camps by Monday. But pipeline opponents said they would dig in their heels, while authorities said they wouldn't forcibly remove them.

On Sunday, what began as a tense morning transformed into a scene of jubilation, according to reporters on the ground.

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Archambault said his tribe and their supporters would celebrate in "wopila," or in thanks, in the coming days. A victory pow-wow is planned for Tuesday near the tribe's pavilion in Cannon Ball.

Still, the opponents' victory isn't certain.

President-elect Trump could potentially reverse the Corps' decision or take other steps to revive the pipeline. Some political observers have also noted that Trump owns stock in Energy Transfer Partners, the company building the pipeline.

Achambault in his statement urged the incoming Trump Administration to respect Sunday's decision.

The missing piece

The Dakota Access pipeline, if finished, would bring oil from North Dakota's shale patch through South Dakota and Iowa into Illinois.

Most of the pipeline has already been built, save for the now-blocked segment underneath Lake Oahe, a reservoir on the Missouri River. Since the river is federally protected, Energy Transfer Partners needed the Corps to give it an easement to drill underneath the waterway.

Jo-Ellen Darcy, the Corps' assistant secretary for civil works, said her decision to withhold the easement was based on the need to "explore alternative routes" for the pipeline's crossing.

Most of the Dakota Access Pipeline is already built, save for a river crossing in southern North Dakota.Credit: energy transfer partners

Her office in mid-November said it was delaying its decision to allow for additional discussions with the Standing Rock Sioux Tribe, whose reservation lies just half a mile south of the proposed crossing.

"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," Darcy said in a statement. "The best way to complete that work responsibly and expeditiously is to explore alternate routes for the pipeline crossing."

Energy Transfer Partners and Sunoco, an investor in the Dakota Access pipeline, blasted the Corps' decision to not grant the easement as "a purely political action," according to a statement issued on Sunday night.

"For more than three years now, Dakota Access Pipeline has done nothing but play by the rules," the pipeline companies said.

They added that the Corps' call for alternative routes "is just the latest in a series of overt and transparent political actions by an administration which has abandoned the rule of law in favor of currying favor with a narrow and extreme political constituency."

This story has been updated to include a statement from Energy Transfer Partners.