Fireworks lit the sky at the Oceti Sakowin Camp near Cannon Ball, North Dakota on December 4, 2016, after the Army Corps of Engineers halted the Dakota Access Pipeline route. An executive order by President Donald Trump in January allowed work to resume. Scott Olson/Getty Images

Activists embrace after the December halt of the Dakota Access Pipeline route. The $3.7 billion project that would cross four states and change the landscape of the US crude oil supply. The Standing Rock Sioux tribe says the pipeline would affect its drinking-water supply and destroy its sacred sites. JIM WATSON/AFP/Getty Images

An activist rides down from a ridge on the edge of the Standing Rock Sioux Reservation on December 4. Scott Olson/Getty Images

Activists celebrate at Oceti Sakowin Camp on December 4. An executive order by President Donald Trump in January allows work to resume on the Dakota Access Pipeline, which the activists oppose. JIM WATSON/AFP/Getty Images

Dan Nanamkin of the Colville Nez Perce tribe drums a traditional song by the Cannonball River in Cannon Ball, North Dakota, on Thursday, December 1. David Goldman/AP

A procession makes its way down to the Cannonball River to take part in a Native American water ceremony on December 1. David Goldman/AP

Snow covers the camp on Wednesday, November 30. Scott Olson/Getty Images

A person walks through snow and wind on Tuesday, November 29. David Goldman/AP

People against the construction of the Dakota Access Pipeline speak at a news conference near Cannon Ball on Saturday, November 26. James MacPherson/AP

A sculpture stands at an encampment where protesters of the pipeline have been gathered for months. James MacPherson/AP

People protest along Highway 1806 as they walk past a sprawling encampment on Thursday, November 24. Boston Globe/Getty Images

A man stands along Highway 1806 on November 24. Boston Globe/Getty Images

In this image provided by the Morton County Sheriff's Department, law enforcement and protesters clash near the pipeline site on Sunday, November 20. Morton County Sheriff's Department/AP

Tonya Stands recovers after being pepper-sprayed by police on Wednesday, November 2. Stands was pepper-sprayed after swimming across a creek with other protesters hoping to build a new camp to block construction of the Dakota Access Pipeline. John L. Mone/AP

Dozens of protesters wade in cold creek waters as they confront local police on November 2. John L. Mone/AP

Tribe members make their way back to their camp on Saturday, October 29. Sara Sidner/CNN

Cousins Jessica and Michelle Decoteau take part in a protest outside the North Dakota state capitol in Bismarck on October 29. John L. Mone/AP

The burned hulks of heavy trucks sit on Highway 1806 on Friday, October 28, near a spot where Dakota Access Pipeline protesters were evicted a day earlier. James MacPherson/AP

Pipeline protesters sit in a prayer circle as a line of law enforcement officers make their way across the camp to relocate the protesters a few miles south on Thursday, October 27. Protesters had camped on private property. Mike McCleary/The Bismarck Tribune/AP

A protester is arrested as law enforcement surrounds the camp on October 27. Mike McCleary/The Bismarck Tribune/AP

Tires burn as armed soldiers and law enforcement officers stand in formation to force Dakota Access Pipeline protesters off the private land in Morton County. Mike McCleary/The Bismarck Tribune/AP

A protester shows where he was hit by a bean-bag round fired by officers trying to force protesters off the private land. James MacPherson/AP

JR American Horse leads a march to the pipeline site on Friday, September 9. Will Kincaid/The Bismarck Tribune/AP

Native Americans head to a rally at the state capitol in Denver on Thursday, September 8. They were showing their support for members of the Standing Rock Sioux tribe in North Dakota opposting the Dakota Access Pipeline. David Zalubowski/AP

People hang a sign near what they say was sacred burial ground disturbed by bulldozers in Cannon Ball. ROBYN BECK/AFP/Getty Images

Marlo Langdeau of the Lower Brule Sioux Tribe joins hundreds of Native Americans for a march near Cannon Ball on Sunday, September 4. ROBYN BECK/AFP/Getty Images

Protesters march on September 4 in opposition to the Dakota Access Pipeline. ROBYN BECK/AFP/Getty Images

Native Americans march to the site of a sacred burial ground on September 4. ROBYN BECK/AFP/Getty Images

Native Americans ride with raised fists to the sacred burial ground on September 4 to protest the Dakota Access Pipeline. ROBYN BECK/AFP/Getty Images

The Missouri River is seen beyond an encampment near Cannon Ball, where hundreds of people gathered to join the protest on September 4. ROBYN BECK/AFP/Getty Images

Flags of Native American tribes from across the United States and Canada line the entrance to a protest encampment on Saturday, September 3. ROBYN BECK/AFP/Getty Images