An estimated 46,000 gallons of sewage spilled into the Jones Falls when a sewer overflowed Friday, the Department of Public Works said.
The overflow was stopped about 5 p.m. Friday.
It happened underground in East Baltimore, so the sewage did not run onto any streets, officials said.
The spill was a structured overflow designed in Baltimore's sewer system a century ago. Such overflows are intended to release rain and wastewater during downpours. But these structures around the city are being replaced, part of a $1 billion sewer renovation effort.
City officials are urging residents to avoid contact with waterways after heavy rains.