Authorities in Cherry Hill say the incident happened around the area near the Cuthbert Boulevard Bridge.
Investigators say someone pulled a 75-millimeter shrapnel round from World War I out of the Cooper River while they were magnet fishing.
Magnet fishing involves using a strong magnet to retrieve metal objects from the water.
When the device was found, police were called, and the area around the bridge was shut down while officers investigated.
The device was later cleared. The bridge was reopened at approximately 4:30 p.m. Sunday in both directions.