Nearly 300,000 gallons of sewage overflowed from a manhole into the University of Michigan's Nichols Arboretum this weekend, according to the city of Ann Arbor.
Some of the sewage also ended up in the Huron River, but in a statement the city said it doesn't know how much.
Ann Arbor is blaming the blockage on an uninflated sewer plug and 3,000 feet of rope.
The rope was left by a contractor who recently inspected the sewer.
Parts of the Arboretum are closed and marked with signs until the cleanup is done.
River trips were canceled Sunday and Monday.
The city has notified the Michigan Department of Environmental Quality.