In the name of saving the environment, thousands of green activists fighting to stop the Dakota Access pipeline are making a huge mess.

Those familiar with the camps near Cannon Ball, North Dakota, increasingly are distressed over the pits of human waste and garbage pockmarking the formerly pristine prairie revered by the Standing Rock Sioux as sacred ancestral land.

Rob Keller, spokesman for the Morton County Sheriff’s Department, said the protesters are “saying one thing and doing another” when it comes to safeguarding the environment.

“We’ve seen pictures of trenches and the garbage thrown in there. So that’s protecting the land?” Mr. Keller said. “And then the snow came in, and I’m sure it’s just a muddy mess now, because that’s river-bottom water, which is silt. It will be a mess.”

Even Standing Rock Sioux Chairman David Archambault II, who has urged protesters to come “stand with Standing Rock” against the pipeline, is disgusted with how the environmental activists living in the camps have treated the federal property.

“Before this entire movement started, that was some of the most beautiful land around,” Mr. Archambault told the news website Vice. “There was a place down there where eagles, over 100 eagles would come and land. There were game down there — deer, pheasants, elk, geese. Now, it’s occupied by people. And when masses of people come to one place, we don’t take care of it.”

What’s especially alarming is that the camps are located in a flood plain, meaning that the waste and garbage will be carried into the Cannonball River and the water supply as the snow melts and submerges the area.

Mr. Archambault compared the environmental damage inflicted by the protesters to that of fossil fuel companies.

“We’re no different than the oil company, if we’re fighting for water,” said Mr. Archambault. “What’s going to happen when people leave? Who has to clean it up? Who has to refurbish it? It’s going to be us, the people who live here.”

National environmental groups backing the protest, including Earthjustice, the Sierra Club, 350.org and the Indigenous Environmental Network, did not respond to requests asking for comment, but Greenpeace did.

Greenpeace spokesman Perry Wheeler said the blame for any damage lies with those behind the $3.8 billion, 1,172-mile project, which Energy Transfer Partners is building almost entirely on private land in order to transport oil from the Bakken field in North Dakota to Illinois.

“Any environmental concerns sit at the feet of the pipeline decision-makers,” Mr. Wheeler said in an email.

After issuing an easement for a 1,100-foot stretch of federal land in North Dakota, the U.S. Army Corps of Engineers is stalling the project as it reviews the tribe’s concerns. The four-state pipeline is about 90 percent complete.

“The best way to ensure the Standing Rock Sioux Tribe and our earth are treated the right way is for the Administration to stop what should have never started,” said Mr. Wheeler.

State and local officials say they are worried about the environmental damage to the area, but there’s only so much they can do, given that the camps are on federal land.

Scott A. Radig, director of the state division of waste management, said he sent a letter with photos of protesters dumping and burning waste in pits to the U.S. Army Corps of Engineers, which has jurisdiction over the area, but that he has heard nothing back. That was in September.

“They did not respond to us,” said Mr. Radig. “It is federal land, but even though it’s federal land, they still have to follow state laws on state management practices.”

The Army Corps, Interior Department and Environmental Protection Agency did not respond immediately to a request for comment.

Mr. Radig said he has been in contact with Alison Two Bears, the tribe’s environmental director.

“She said that when the camp was closed that they would send us their plan for making sure the site is cleaned up and restored to its original conditions,” he said.

Despite its hard line on other environmental transgressors, the Obama administration has given the protesters a pass on camps north of the Cannonball River, allowing them to remain illegally for months and insisting the activists will not be removed forcibly if they defy a Monday deadline to leave.

“They’re on [what] I’ll call a federal refuge because the Army Corps and the Obama administration have refused to demand that they leave that federal land,” North Dakota Lt. Gov. Drew Wrigley said in a Thursday interview with WDAY-AM’s Rob Port.

“We’ve had no authority to go in and remove them,” said Mr. Wrigley, a Republican. “But now the Army Corps is saying they have to leave by the fifth. We’ll see.”

This week’s snowstorm and subfreezing temperatures have done what the administration has not by motivating many activists to leave their tents, teepees and campers and return home, or at least check into the reservation hotel and casino.

Even some activists are fed up with the sanitation of the camps, criticizing outsiders who have treated the protest as a hippie festival instead of helping keep the area clean.

“When Chairman Archambault talks about the destruction of the land with pitching of tents, digging pits in Mother Earth, the garbage and human waste, he is correct,” Yvette Hatchere wrote on the Red Warrior Camp’s Facebook page.

“How would some of you feel if we camped in your backyard & left garbage behind and left holes in the ground,” she said. “Well, he feels the same way. Pick up your garbage and find ways to get rid of it.”