The spill has gotten bigger each day since it was discovered last week. The oil is believed to be industrial-grade waste oil.
"From Wednesday until now, there's been more than 10,000 gallons spilled, and we expect that number to increase," said Adam Wine, chief petty officer with the U.S. Coast Guard.
The river, which flows eastward into the Detroit River, was closed for a second straight day on Sunday as cleanup work continued, creating potential problems for industries that rely on the waterway to transport goods.
"There will be a need to move ships up and down to Ford and Rouge Steel to provide raw materials - iron ore, coal and limestone," said Rep. John Dingell.
The Coast Guard hoped the river would be reopened Monday.
The cleanup could last three to six weeks and cost more than $2 million, officials estimated.
The spill's environmental impact is unknown, although about 70 birds have been found with oil on their feathers, according to Dan Sheill, special agent with the U.S Department of Fish and Wildlife.
The spill is the largest on a Great Lakes waterway in more than a decade. Wine said the last major spill that affected Great Lakes waters was a 1991 gasoline spill in the Bay City area. The source of that spill was an explosion on the tanker Jupiter.