A Baltimore County pumping station dumped 22 million gallons of diluted sewage into the Patapsco River Saturday night, prompting the Anne Arundel County Health Department to issue a warning against entering the river until after Jan. 4.

Starting at 5:30 p.m. Saturday, the station was allowed to overflow into the Patapsco for 9 1/2 hours, Baltimore County public works officials said.

The spill is the 18th largest in the state since 2005, according to Maryland Department of the Environment officials.

Warning signs have been posted in the area of the overflow. The Patapsco forms the border between Baltimore and Anne Arundel...