Public information for residents
Merrimack River CSO Alerts
Receive notifications when Combined Sewer Overflow events may impact the Merrimack River and local waterways.
A CSO alert was issued within the past 24-48 hours.
Last reported event: June 7, 2026 at 12:00 PM ET
Resident guidance: Continue to use caution around affected waterways. People and pets should avoid primary contact for 48 hours after a discharge.
Alert Signup
Sign up to receive Merrimack River CSO alert emails from Project Merrimack when discharge notices are issued.
Alerts are intended as accessible public information for residents. For official public health and water quality guidance, follow state and local agency notices.
About Combined Sewer Overflows
Combined Sewer Overflows (CSOs) occur when heavy rain or snowmelt overwhelms older sewer systems. When this happens, excess wastewater can be discharged into rivers and local waterways.
These events are not everyday occurrences, but they can temporarily affect water quality. After a discharge, residents should use caution around affected waterways and follow guidance from local officials.
CSO alerts help residents make informed decisions about activities such as boating, fishing, paddling, swimming, and pet access near local waterways.
Learn More About CSOsHistorical Alerts
A public alert history is planned for a future update. For now, this page shows the current CSO status based on the most recent alert.