The U.S. Army Corps of Engineers issued a letter Friday announcing its plan to close land that has been the campsite for months-long protests against the North Dakota Access oil pipeline, according to Dave Archambault II, chairman of the Standing Rock Sioux Tribe.

Protesters, or “water protectors,” were notified that land north of the Cannonball River will be closed on Dec. 5, in roughly 10 days, he said.

The chairman said he was “deeply disappointed” by the Army Corps’ decision, “but our resolve to protect our water is stronger than ever.”The Oceti Sakowin camp, which rests on the banks of the river, contains a loose collective of tribal nations and out-of-state supporters opposing the 1,172-mile pipeline. The camp is about 45 minutes south of Bismarck, the state capital.

“It is both unfortunate and disrespectful that this announcement comes the day after this country celebrates Thanksgiving — a historic exchange of goodwill between Native Americans and the first immigrants from Europe,” Archambault wrote. “Although the news is saddening, it is not at all surprising given the last 500 years of the mistreatment of our people.”

The Army Corps did not immediately respond to requests for comment.

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