"This decision puts our people's sacred sites at risk for continued ruining and desecrating what's important to us," Sioux chairman Dave Archambault said.
The $3.8 billion pipeline is set to pump oil along a 1,100-mile stretch between North Dakota and Illinois. The company said it hasn't damaged any historical sites during construction.
But the Sioux say opposing the pipeline isn't just to preserve their past. They're also concerned about what it could mean for their future.
Tribal leaders are worried that the pipeline was constructed too quickly and could rupture, contaminating their drinking water, which comes from the Missouri River.
"Everything that oil is used for, there's an alternative for, there's a renewable alternative for that oil. There is no alternative for water," a Sioux tribe member told Newsy.
A resolution could be coming soon. In July, the tribe filed a lawsuit that alleges the Army Corps of Engineers didn't follow federal laws when it gave the company the go-ahead to drill under the river.
The case could prompt the judge to issue a permanent injunction against the pipeline. That ruling is expected by the end of the week.