Pollen after an intense rain discolors Alewife Brook in June 2014. (Photo: ocschwar via Flickr)

People and pets should avoid contact with the Alewife Brook and parts of the Mystic River through 5 a.m. Saturday due to the release of stormwater mixed with sewage into the brook, the Cambridge Public Health Department said Thursday.

The release of mixed stormwater and sewage – known as a combined sewer overflow – is needed to prevent contaminated water from backing up into homes, businesses and city streets. This CSO release occurred in Alewife Brook Reservation in North Cambridge between 2:05 and 4:15 a.m. Thursday, said Suzy Feinberg, the city’s public information officer for health, in an 11:49 a.m. email.

Affected areas may include the Alewife Brook and Little River in Cambridge, Somerville and Arlington, as well as the Mystic River from the intersection of Alewife Brook Parkway and Mystic Valley Parkway in Somerville to the Fellsway/Route 28 bridge in Medford, near the Mystic River State Reservation, health officials said. Contact with the affected areas has “increased health risks due to bacteria or other pollutants carried by the stormwater, such as fertilizers or pesticides.”

Contact with the water can make you and your pets sick, health officials said, calling for people to avoid paddle boarding, boating, fishing or other activities that could lead to contact.

Major work is underway to prevent the overflows; residents from more than a half-dozen communities learned a new timeline for the work at a Dec. 15 meeting organized by Cambridge and Somerville governments and the Massachusetts Water Resources Authority. A draft plan due by June 2023 and a final plan sue by December 2023 have been delayed by three years for more public input and analysis, health officials said.

This post took significant amounts of material from a press release.