Some activists have moved to higher ground, and on Wednesday authorities arrested 74 of them who had decamped to land owned by the pipeline's developer, Energy Transfer Partners.
Acting assistant secretary of Indian affairs Michael S. Black said the agency had sent "enforcement support and will assist" the tribe "in closing the protest camps within the Standing Rock Reservation boundary."
"North Dakota Governor [Doug] Burgum, the Standing Rock Sioux Tribe leadership, local law enforcement, and local landowners have all warned the public and those still camped of the dangerous spring flooding expected due to the heavy amount of snowfall the state received this winter," Black added. "The closing of the camps is a matter of public health and safety and working together at this time will allow for the safe removal of waste and debris that will impact the local environment and protection of those camped."
The Standing Rock Sioux Tribe had already passed a tribal resolution asking protesters to leave, and asking for federal aid in closing the camp.
"In these past few weeks at camp, I see no reflection of our earlier unity, and without unity we lose," the tribe's chairman, David Archambault II, said in a statement.