A pipeline carrying oil from the US state of Indiana to Ontario, Canada has spilled more than 800,000 gallons (3m litres) of oil into a creek which flows into the Kalamazoo River in Michigan.
A spokesman for the firm running the pipeline, Enbridge Energy Partners, said a malfunction had caused the leak.
The spill has killed fish and endangered wildlife in the region.
Officials said the pumps which feed the pipeline had been shut down as soon as the leak was discovered.
The 30-inch (76cm) pipeline carries 8m gallons of oil per day between the towns of Griffith, in Indiana, and Sarnia, in Ontario.
The pipeline, called the Lakehead System, is one of the largest in the world.
A pungent odour now hangs over the Kalamazoo River valley, local media report.
The oil originally spilled into the Talmadge Creek in Calhoun County, Michigan, north-west of the Kalamazoo River.