It was a perfect storm—or, more accurately, a combination of heavy snow melt, intense spring rains, and late-season snowpack in the mountains. Together, they caused the Missouri River in North Dakota to rise aggressively over the course of several months in 2011.

Double Ditch State Historic Site, a major Mandan village along the river, felt the wrath of the rising waters. Cracks started forming as the land began to erode, and 18 ancient burial sites were exposed. Eventually, 1,000 linear feet of the site, home to North Dakota’s first agriculturalists from the late 1400s to the 1700s, became unstable. The situation threatened to endanger the 24-acre village, which became a state historic site in 1936 and possesses tremendous cultural significance.

“It should be on the level of a UNESCO site; that’s how important it is to our history,” says Calvin Grinnell, historian for the Mandan, Hidatsa and Arikara Nation (MHA) and State Historical Board member.

The State Historical Society of North Dakota (SHSND) felt the same way. “We were very concerned about losing that site,” says Fern Swenson, director of the society’s Archaeology & Historic Preservation Division. “If we didn’t stop the erosion, half the site would slump into the river.”

So the MHA and the SHSND came together to save it. “[We] and MHA worked together to secure state funding, select the engineering company, and plan together to ensure the project was completed in a respectful way,” says Swenson. They engaged a team of firms (Atwell Engineers, Water Resources Solutions, Wildhorse Riverworks, and Braun Intertec), and a plan was drawn up to stabilize a total of 2,200 linear feet along the riverbank. The 2015 and 2017 North Dakota Legislative Assemblies pledged $3.5 million for the project, including State Disaster Relief Funds and General Fund dollars. Work began in 2012 and concluded late last year.

The plan required the redistribution of weight from the top of the bank to the bottom, with pipe piles threaded below to stabilize the land, but moving things around meant exposing ancient burial sites. There was no way around it: 216 of them had to be relocated, which has been happening under the supervision of the tribal leadership of the MHA and archaeologists from SHSND. All remains and burial goods will be repatriated to the MHA and interred in private, traditional Mandan ceremonies.

So far, 18 have been reinterred, with each body covered in red cloth and asked forgiveness for the disruption. “We buried them in the immediate area just outside the village,” says Grinnell. “They’re still close to home.”