Thursday marks the second anniversary of the rupture of Line 6B in southern Michigan. Part of the Lakehead oil pipeline system, the rupture spilled about 20,000 barrels of so-called tar sands oil into the Kalamazoo River in Marshall, Mich.
A preliminary investigation into the incident by the Pipeline and Hazardous Materials Safety Administration suggested Enbridge operators misinterpreted alarms that indicated a leak occurred in Marshall.
An independent report on Enbridge by the National Wildlife Federation finds that Enbridge has a "steady history" of major oil spills in the United States and Canada.
NWF Vice President Jeremy Symons said hundreds of thousands of barrels of oil was spilled by the company.
"Enbridge's long history of pipeline spills can't be explained by mistakes or bad luck," he said in a statement.
Enbridge in May announced a multibillion-dollar plan to overhaul and expand its pipeline systems in Canada and the United States.
The company, in response to the NTSB report, said "safety has always been core to our operations."