Nez Perce tribal members in Idaho were mobilizing Monday to block an Oregon company’s super-sized shipment bound for Canada’s tar sands through reservation land.

The move came as a U.S. Forest Service manager warned Hillsboro’s Omega Morgan, a heavy-haul company, that the agency does not authorize the planned shipment Monday night of the so-called megaload through the Nez Perce-Clearwater National Forests.

The showdown is reminiscent of 2011 opposition that shut down megaloads, which Exxon Mobil’s Imperial Oil aimed to truck on the same route from the Port of Lewiston, Idaho, to the Alberta oil complex. Imperial ended up downsizing its oil-refinery modules and sending them instead via interstates.

Once again the conflict pits local residents and environmentalists against companies supplying and operating the oil sands, a vast complex north of Calgary where giant shovel machines extract a tar-like substance for refining into petroleum products.

This time Hillsboro heavy-haul company Omega Morgan has an Idaho Transportation Department permit to bring a 644,000-ton evaporator east on U.S. 12, a Wild and Scenic River corridor. But Rick Brazell, Nez Perce-Clearwater National Forests supervisor, faulted the department Monday for issuing the permit prematurely.

Tribal leaders cite a federal court ruling that requires approval from the tribe and the U.S. Forest Service. Members planned to assemble and block the first of two shipments as soon as Monday night.

“The tribe is shocked by Omega Morgan’s audacity,” Nez Perce Tribal Chairman Silas Whitman said in a news release Monday. “The company has apparently decided it does not need to wait for Forest Service review or tribal consultation to move General Electric’s loads.”

Olga Haley, an Omega Morgan spokeswoman, said Monday that company managers planned to start the four-day shipment that night.