The Environmental Protection Agency issued an administrative order requiring Enbridge to dredge the Kalamazoo River in southern Michigan.
"EPA has repeatedly documented the presence of recoverable submerged oil in the sections of the river identified in the order and has determined that submerged oil in these areas can be recovered by dredging," the agency said. "The dredging required by EPA's order will prevent submerged oil from migrating to downstream areas where it will be more difficult or impossible to recover."
The rupture of Line 6B, part of the Enbridge-operated Lakehead pipeline system, released about 20,000 barrels of oil into southern Michigan waters starting July 26, 2010. The pipeline was carrying so-called tar sands oil, which sinks and mixes with river sediment.
The EPA said oil-response workers have collected almost 200,000 cubic yards of contaminated sediment and debris from area waters.
Enbridge has less than a week to respond to the EPA order and about two weeks to develop a work plan.