Charles River/Inner Harbor CSO avoid-contact warning ended July 9, 2026

A combined sewer overflow (CSO) tied to MWRA’s Prison Point outfall (MWR203) triggered a public health warning for the mouth of the Charles River and the Boston Inner Harbor area. The City’s avoid-contact guidance for that July 7 event expired Thursday, July 9, 2026 at 7:12 PM.

Quick facts: the July 7 CSO and when the warning ended

What happened: CSO notification for MWR203 at Prison Point (Charles River/Inner Harbor area).

When it started: Tuesday, July 7, 2026 at 7:12 PM.

When it ended: MWRA lists a stop time of 9:39 PM, while the Boston Public Health Commission lists the overflow ended at 9:41 PM.

When this health alert expired: Thursday, July 9, 2026 at 7:12 PM (the City describes this as 48 hours after the overflow has ended).

What the warning means for water-contact decisions

Both the City and MWRA describe the same core resident rule: during rainstorms and for the 48 hours after the overflow/discharge stops, residents should avoid contact with affected water bodies because sewage mixed with stormwater can raise health risks (including from bacteria or other pollutants).

In practical terms, if you see this kind of CSO health alert for the Charles River/Inner Harbor area, treat it as your cue to skip swimming and wading (direct contact) during the active window—then resume only after the advisory’s listed end/expiry time.

Note: MWRA also cautions that CSO information and discharge details are preliminary and subject to change, so it’s worth re-checking before heading back into the water.

Which waters this July 7 warning covered

This advisory was tied to MWR203 at Prison Point, which the City describes as impacting the waters at the mouth of the Charles River and the Inner Harbor. The responsible agencies emphasize that CSO notifications are outfall-specific, so residents shouldn’t assume one Charles River/Inner Harbor alert automatically applies to every Boston water location.

What to do now (and what to check during the next rain)

This particular warning window ended July 9, 2026 at 7:12 PM. But CSO conditions can restart with new storms.

• Before any water contact: check for new CSO health alerts for the Charles River/Inner Harbor sites.
• During/after storms: use the most recent start/stop/expiry times shown on the applicable CSO notification page—because the “48 hours after it ends” guidance depends on the specific event’s stop time.

Where to check for CSO updates

For notifications and alerts for sites on or around the Charles River, start with MWRA’s CSO notification page (which includes the Prison Point/MWR203 event details). For additional local context on how Boston-area CSO notifications are posted and sent, the Boston Water and Sewer Commission (BWSC) CSO map/public notification page can also help—especially since Boston and MWRA responsibilities can differ by outfall.

Sources

• Boston Public Health Commission — Combined Sewer Outflow Health Alerts (Charles River/Inner Harbor; MWR203 at Prison Point; advisory expiry and timing)
• MWRA — Combined Sewer Overflow (CSO) Notifications (MWR203 Prison Point; swimming safety warning; preliminary-data note; subscribe link)
• BWSC — Combined Sewer Overflow Map and Public Notification (48-hour avoid-contact guidance; how notifications are sent)