The emptying of the site ended a remarkable mobilization of activists to the remote patch of North Dakota prairie near the Cannonball and Missouri Rivers. The protest lasted months, drew international attention and was widely credited with leading to a temporary halt in construction of the Dakota Access Pipeline. Smaller numbers of protesters remain camped at other sites.

The Standing Rock Sioux Tribe and protesters say the oil pipeline, which crosses the Missouri River nearby, will pose a grave threat to drinking water on the reservation and farther downstream if it ever leaks. Construction on the pipeline, which will carry oil from the Bakken fields in western North Dakota, resumed this month with support from President Trump, and the project could be finished this spring.

The North Dakota authorities said the closing of the main protest camp, which sits on Army Corps of Engineers land, was necessary to prevent pollution during imminent spring floods. The protest site had developed into a make-do city, with semipermanent buildings, medical tents and abandoned cars. If that washed into the Missouri River, the authorities said, the results could be damaging to the environment.