Nathan Phillips marches with other protesters out of the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 22, 2017. Picture taken February 22, 2017. REUTERS/Terray Sylvester

Nathan Phillips (C) marches with other protesters out of the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 22, 2017. Picture taken February 22, 2017. REUTERS/Terray Sylvester

Nathan Phillips marches with other protesters out of the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 22, 2017. Picture taken February 22, 2017. REUTERS/Terray Sylvester

Nathan Phillips (C) prays with other protesters near the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 22, 2017. Picture taken February 22, 2017. REUTERS/Terray Sylvester

Nathan Phillips (C) marches with other protesters out of the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 22, 2017. Picture taken February 22, 2017. REUTERS/Terray Sylvester

Nathan Phillips (C,right) prays with other protesters near the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 22, 2017. Picture taken February 22, 2017. REUTERS/Terray Sylvester

Nathan Phillips prays with other protesters near the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 22, 2017. Picture taken February 22, 2017. REUTERS/Terray Sylvester

A man warms up by a fire in Sacred Stone camp, one of the few remaining camps protesting the Dakota Access Pipeline in Cannon Ball, North Dakota, U.S., February 24, 2017. REUTERS/Stephen Yang

American Horse, of the Standing Rock Tribe displays an American Indian Movement shirt in Sacred Stone camp, one of the few remaining camps protesting the Dakota Access Pipeline in Cannon Ball, North Dakota, U.S., February 24, 2017. REUTERS/Stephen Yang

Authorities clear the Oceti Sakowin camp in Cannon Ball, North Dakota, U.S., February 24, 2017. REUTERS/Stephen Yang

A makeshift doll is seen at the gate separating Rosebud camp and Sacred Stone, one of the few remaining camps protesting the Dakota Access Pipeline in Cannon Ball, North Dakota, U.S., February 24, 2017. REUTERS/Stephen Yang

An animal carcass and an abandoned tent is seen in Rosebud camp in Cannon Ball, North Dakota, U.S., February 24, 2017. REUTERS/Stephen Yang

Adrian Greyeyes, a Navajo from Arizona watches a police line on Rosebud camp the day after the main camp, Oceti Sakowin, was cleared in Cannon Ball, North Dakota, U.S., February 24, 2017. REUTERS/Stephen Yang

A man stands next to homemade shields in Sacred Stone camp, one of the few remaining camps protesting the Dakota Access Pipeline in Cannon Ball, North Dakota, U.S., February 24, 2017. REUTERS/Stephen Yang TPX IMAGES OF THE DAY

Raven Blackbull, a 21-year-old from Rosebud, South Dakota, looks through a scope to see construction clearing Oceti Sakowin camp from Sacred Stone, one of the few remaining camps protesting the Dakota Access Pipeline and where Blackbull is volunteering as security, in Cannon Ball, North Dakota, U.S., February 24, 2017. REUTERS/Stephen Yang

An abandoned car covered in snow in Sacred Stone camp, one of the few remaining camps protesting the Dakota Access Pipeline in Cannon Ball, North Dakota, U.S., February 24, 2017. REUTERS/Stephen Yang

Authorities occupy part of the newly cleared Rosebud camp in Cannon Ball, North Dakota, U.S., February 24, 2017. REUTERS/Stephen Yang

A woman walks towards Sacred Stone camp, one of the few remaining camps protesting the Dakota Access Pipeline in Cannon Ball, North Dakota, U.S., February 24, 2017. REUTERS/Stephen Yang

An abandoned truck reading "#NODAPL" inside Sacred Stone, one of the few remaining camps protesting the Dakota Access Pipeline, in Cannon Ball, North Dakota, U.S., February 24, 2017. REUTERS/Stephen Yang

Raven Blackbull, 21, from Rosebud, South Dakota volunteers as security in Sacred Stone, one of the few remaining camps protesting the Dakota Access Pipeline in Cannon Ball, North Dakota, U.S., February 24, 2017. REUTERS/Stephen Yang

Law enforcement officers advance into the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester TPX IMAGES OF THE DAY

Jasper Spillman, 30, of Lawrence, Kansas, watches law enforcement officers advance into the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester TPX IMAGES OF THE DAY

Protesters confront police from the surface of the frozen Cannon Ball River after being driven from the main opposition camp against the Dakota Access oil pipeline near Cannon Ball,, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Police advance into the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Law enforcement officers cut open a shelter in the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Law enforcement officers advance into the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Jasper Spillman, 30, of Lawrence, Kansas, watches law enforcement officers advance into the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Law enforcement officers advance into the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Susanna Travis (left), from Grass Valley, California, embraces Timothy Powers, also from California, before evacuating the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester TPX IMAGES OF THE DAY

Law enforcement officers move into the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Law enforcement officers advance into the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Law enforcement officers advance into the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Protesters cross the frozen Cannon Ball River as they evacuate the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Protesters evacuate the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Timothy Powers (C, R) from Southern California is detained with another protester in the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Terrence Hubbard, 28, of Cleveland, Ohio, evacuates as law enforcement officers move into the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Police detain Ricardo Salazar, 25, of San Bernardino, California, in the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Protesters embrace while evacuating the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Police detain a protester in the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Travis Harden of the Cheyenne River Sioux Tribe sings on the outskirts of the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

A protester watches a law enforcement helicopter circle the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Protesters confront police on the outskirts of the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Protesters confront police on the outskirts of the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Police guard the south edge of the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

A flyer advertising evacuation assistance hangs in the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

A protester washes mud from his truck before evacuating the opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Law enforcement officers advance into the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

A burned car rests on its roof in the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

A protester records a video of police officers after evacuating the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Protesters talk by a campfire in the morning before evacuating the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Protesters watch law enforcement officers guard the edge of the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Protesters watch law enforcement officers sweep through the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Police detain a protester in the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Police detain Ricardo Salazar, 25, of San Bernardino, California, in the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Police detain a protester in the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Police detain a protester in the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Police detain Ed Higgins, 39, of Lowell, Massachusetts, in the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Police detain a protester in the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

Protesters hold a banner after evacuating the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester

A protester watches law enforcement officers advance into the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 23, 2017. REUTERS/Terray Sylvester