Texas-based Energy Transfer Partners is behind the 1,885-kilometre Dakota Access pipeline, which, if completed, will cross four American states, carrying Bakken oil from North Dakota to Illinois. The $3.8-billion US pipeline is almost finished, save for a section that would tunnel under a reservoir of the Missouri River.

The largest of the anti-pipeline protest camps is the Oceti Sakowin Camp, located just north of the Cannonball River. (Tyson Koschik/CBC)

Pipeline opponents, who call themselves "water protectors," see the project as a grave threat to the region's drinking water and sacred sites. Others say the pipeline is safer than moving oil by other modes of transportation, such as rail.

For months, thousands of people have descended upon a handful of camps in the area to voice opposition to the pipeline. The largest is the Oceti Sakowin Camp, north of the Cannonball River.

But as weather conditions change, authorities are ramping up the pressure, saying it's time for people to leave the area — and soon.

In the past week, the U.S. Army Corps of Engineers, which manages the federal land on which the Oceti Sakowin Camp sits, and North Dakota Gov. Jack Dalrymple have both issued evacuation orders.

They point to the onslaught of a harsh winter, the need to protect the public from violent confrontations between law enforcement and protesters, and a lack of ability to effectively provide proper emergency services in the area.