U.S. regulators are investigating the rupture of Line 6B, part of the Enbridge-operated Lakehead pipeline system. The rupture released about 20,000 barrels of oil into southern Michigan waters starting July 26, 2010.
The U.S. Environmental Protection Agency in June warned residents that sheen may still be present in parts of the Kalamazoo River and nearby waterways.
The EPA said it notified Enbridge that more work was needed in some of the waterways affected by the pipeline spill in 2010.
"The submerged oil and/or oil-contaminated sediment in these areas generates oil sheen and/or oil globules, sometimes spontaneously, and also when disturbed by actions such as poling or motor boat engines," the EPA's report states.
Enbridge has 10 days to ask for a conference with the EPA to discuss the remaining work.
The EPA said oil-response workers have collected almost 200,000 cubic yards of contaminated sediment and debris from area waters.
Line 6B was carrying so-called tar sands at the time of the spill. The nature of that crude oil causes it to sink and mix with river sediment.