In a statement, Standing Rock tribal chairman Dave Archambault II said: 'Today, the US Army Corps of engineers announced that it will not be granting the easement to cross Lake Oahe for the proposed Dakota Access Pipeline.
'Instead, the Corps will be undertaking an environmental impact statement to look at possible alternate routes.
'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 do the right thing.
'The Standing Rock Sioux Tribe will forever be grateful to the Obama administration for this historic decision.'
He went on to thank all of those who had given moral, physical and financial support from around the globe, and in particular thanked those who had joined the protesters at Standing Rock.
'We hope that [Energy Transfer Partner CEO] Kelcy Warren, [North Dakota Governor Jack] Dalrymple and the incoming Trump administration respect this decision and understand the complex process that led us to this point,' he added.
'Treaties are paramount law and must be respected, and we welcome dialogue on how to continue to honor that moving forward,' Archambault said.
'We are not opposed to energy independence, economic development, or national security concerns but we must ensure that these decisions are made with the considerations of our Indigenous peoples.'
He also said he hoped to 'heal' the tribe's relationship with local law enforcement and that he looked 'forward to a future that reflects more mutual understanding and respect'.
In an official statement, the Army Darcy had decided not to approve the easement, would have allowed the proposed Dakota Access Pipeline to cross under Lake Oahe, based on 'a need to explore alternate routes for the Dakota Access Pipeline 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 the statement.
'The best way to complete that work responsibly and expeditiously is to explore alternate routes for the pipeline crossing.'
That pipeline would have covered 1,172 miles under the existing plans, connecting the Bakken and Three Forks oil production areas in North Dakota with an existing crude oil terminal near Pakota, Illinois.
The 30-inch-diameter pipe was designed to transport 470,000 barrels of oil per day - putting Lake Oahe at risk of serious contamination in the case of a breach, the protesters said.
Darcy said the new routes would be explored using an Environmental Impact Statement, 'with full public input and analysis'.
The news came after thousands of veterans descended on Standing Rock Sioux reservation this weekend in a show of solidarity with the Dakota Access pipeline protests.
An estimated 2,000 servicemen and women poured into the Oceti Sakowin Camp, located on U.S. Army Corps of Engineers land near Cannon Ball, North Dakota.
The Veterans Stand for Standing Rock group intends to form a human barrier in front of police to assist thousands of activists, who prefer to be known as water protectors.
They have spent months braving freezing temperatures and snowstorms in a stand against the $3.8billion pipeline which is planned to tunnel beneath a lake half a mile from the reservation.
Activists say it threatens water resources and has destroyed sacred Native American burial sites.
The Oceti Sakowin campsite, the original name of the Sioux, meaning Seven Council Fires, has swelled in numbers since Friday.
Long lines of traffic pass through checkpoints to enter the camp staffed by volunteers.
Green military tents have sprung up as temporary barracks for veterans, many of whom are wearing camouflage fatigues and baseball caps of their regiments.
The weekend temperatures have hovered around 30F with bright sunshine but brutal weather is due to set in next week.
More snow is expected and a temperature drop to well below freezing.
On Sunday morning, veterans at the camp awoke to the sound of a lone trumpeter, Tony Lughan, playing Reveille and Amazing Grace on the hill.
Chief Joseph River Wind, of the Northern Arawak Tribal Nation on White Earth Reservation, Minnesota, said he felt a calling to come to Standing Rock and believes 'there is no expiration date on my oath to serve and protect'.
Chief River Wind served from 1991-2000 as a military police officer in the U.S. Army's 4th Infantry.
'I've been on the other side of the shield but I have not seen civil rights violations against Americans like this,' he said.
He said the veterans' operation orders were for peaceful and prayerful protests along with protection.
'We are here as a show of peaceful support for the people of the water protectors and the Lakota and Dakota people. There is no weapons, no ammunition.
'We just have our prayers and presence to let the indigenous people and the world know that there are many military veterans who are here to protect and defend them in a peaceful way.'
He said that the veterans would stand up to any group trying to incite violence.
'Veterans are looking out for each other. Camp security even came and raided veterans' tents while I was sleeping last night. We don't want anything to happen that way.'
Donna Williams, a member of the Sioux Nation and a U.S. Navy Gulf War veteran, told DailyMail.com: 'I am here because our tribe also gets its water from the Missouri River and it is being affected by a utility company.
'I have come here to stand with our brothers and sisters of Standing Rock and to take a stand that we need to have clean water and have rights respected.'
The 1,170-mile pipeline project, owned by Energy Transfer Partners, is mostly complete, except for a segment planned to run under Lake Oahe, a reservoir formed by a dam on the Missouri River.
Protesters have been winterizing shelters and plan to bed in for the harsh winter months as they await the Army Corps decision on whether to allow the company to tunnel under the river. That decision has been delayed twice by the Army Corps.
The Veterans Stand for Standing Rock (VSSR) movement is led by retired U.S. Army General Wesley Clark Jr and Marine veteran and former Baltimore police officer, Michael Wood Jr.
Congresswoman and veteran Tulsi Gabbard, a Democrat from Hawaii who has served two tours in the Middle East, will speak on behalf of the veterans on Sunday.
A message on the group's Facebook page warned that no drugs, alcohol or weapons are allowed at the campsite but asks that veterans bring 'body armor, has masks and earplugs'.
At a law enforcement press conference on Saturday, National Guard Major General Alan Dohrmann said he had conversations with veterans' organizers and had reassurances the protest would remain peaceful.
'We had a good discussion and walked away with a mutual commitment to maintaining peace, showing mutual respect for one another, and ensuring adequate space between law enforcement and protestors who peacefully exercise their first amendment rights,' said Dohrmann.
'Both Law Enforcement and the VSSR leaders understand that there is no way to control a minority in the camp looking for confrontation, but we would continue dialogue to mitigate the action of people who don't protest in the spirit of the event,' he added.
'Mr. Clark did say that if the leaders of the VSSR group identified anyone getting out of line, they would remove those individuals from the protest area.'
Cass County Sheriff Paul Laney sounded a further note of warning.
'We have received very concerning intel that an element with the protest movement wants to exploited vets with PTSD and arm them and try to trigger their PTSD and turn them aggressive.
'Besides being horrible and wrong, it could be dangerous and deadly.'
He said the information is a 'credible threat' based on intelligence received from a credible source.
'We believe this is a tactic that is actively being worked. As a veteran, it is sickening for someone to do that for an activist reason to someone who has suffered the effects of war.'
He said law enforcement were committed to pulling back from Backwater Bridge, a scene of previous violent protests, at 4pm on Sunday once they were convinced protesters would stay off the bridge and in the main camp.
Sheriff Laney said that local law enforcement had no intention of removing protesters from the Oceti Sakowin camp on Monday.
State officials last week ordered activists to vacate the camp, located on U.S. Army Corps of Engineers land, citing harsh weather conditions.
'I've been asked a hundred times what's going to happen on December 5. We're not moving on that camp and nor planning to anytime in the near future,' he said.
'If you think you'll see a mass of law enforcement and National Guard rolling in there, you'll be waiting a long time, it's not happening.
He added: 'A whole lot of people are coming here expecting to see some sort of confrontation. They are going to be pretty bored. It's not happening. We are not pushing people off that land. We are not pushing people off those camps.'
Laney urged the federal government to make a decision on the easement, adding: 'It's a federal problem that's been dumped in local laps.'
Sheriff Kyle Kirchmeier emphasized that work on construction is not happening at the drill location until the easement is signed and also urged the federal government to make a decision.
Veterans were hopeful for peaceful demonstrations this weekend.
US. Army veteran, Mike Matusewic, of Hurley, Wisconsin told DailyMail.com that the veterans had come to Standing Rock with the aim of giving some 'relief and protection to the water protectors and walk alongside them.'
'I fought for this freedom and what they are doing here unprotected. I wanted to fight for their freedom also,' he said.
John Endrizzi, a U.S. Airforce veteran who served in Vietnam, had made his third trip to Standing Rock from Rome, Wisconsin.
He said: 'It's all about the water. We also have a very serious water fight in Wisconsin. Factory farms are destroying our rivers and streams. I'm a pipeline awareness advocate against Line 61 which travels through Wisconsin and creates a huge risk. They are trying to expand it to carry 1.2million barrels of oil per day.'
Both veterans said that their group was 'all about non-violence and dealing with things peacefully'.
Mr Endrizzi added: 'Those that would provoke us are sinful people.'
Art Desmarais, from New Hampshire, drove 30 straight hours to make it Standing Rock reservation in what will be his third visit. He served with the U.S. Army both at home and overseas.
'For me personally, having served for all the wrong reasons, we are going to serve for the all right reasons now. We are going to bring this country back to how it should be.'
He was optimistic that the sheriff's department and National Guard would keep their distance. 'I think they are going to leave us alone,' he said.
The veteran was with Jehann El-Bisi, from Amherst, Massachusetts, whose father had worked for the Department of Defense and whose mother, who has Lakota, Scottish and French heritage, was an environmental activist.
'My mother was an activist and I watched her protect the water and the environment. I see it as my responsibility for all people to protect the environment.
'In the Lakota language the words are, Mitakuye Oyasin, which means we are all related.'
She added: 'I think it is the most critical moment in time for all of humanity and Standing Rock is a defining moment in our history.'
Several members of the North Dakota Veterans Coordinating Council, which represents five veterans organizations in the state, held a news conference to decry the involvement of veterans in a protest that has damaged property and asked veterans not to participate in the demonstration.
'We agree that it is our constitutional right to assemble and to peacefully protest,' council President Russ Stabler said. 'However, protests over the last 100-plus days in North Dakota have been less than peaceful.
'Participating in this kind of assembly even as a peaceful bystander or participant will only mar the image of the North Dakota veterans and the veterans of our nation.'
North Dakota Governor Jack Dalrymple said on Wednesday it was 'probably not feasible' to reroute the pipeline, but he would try to rebuild a relationship with Standing Rock Sioux leaders.
Dalrymple said state officials never contemplated forcibly removing protesters and his evacuation order was mainly due to concerns about inclement weather endangering people.
The Standing Rock Sioux, in a statement on Wednesday, said that because 'the Governor of North Dakota and Sheriff of Morton County are relative newcomers' to the land, 'it is understandable they would be concerned about severe winter weather'.
They said the camp has adequate shelter to handle the cold weather, adding that the Great Sioux Nation has survived 'in this region for millennia without the concerns of state or county governments'.
