08/25/2026
Let's talk about the acronym behind most Merrimack water quality alerts: CSO.
In the late 1800s, cities like Lowell, Lawrence, and Manchester built sewers that carry sewage and stormwater in the same pipe. This was way before the EPA and the Clean Water Act, with no modern standards for environmental impact or treating water. In fact, this is the beginning of water treatment – the standards for modern water treatment were developed in Lawrence in the 1880s.
Most days, in cities with combined systems, everything flows through a water treatment plant before entering the river, like household sewage and storm drains. But in heavy rain, more water enters the system than any plant can treat. Combined sewer systems are built with relief points that release the excess — a mix of stormwater and sewage — into the river. That's a combined sewer overflow.
Cities along the river are actively upgrading away from this system, but the watershed wide cost is estimated to be over $1B.
Is it gross? Yes. Is it illegal dumping by cities that don't care? No, and that distinction matters. CSOs are permitted and regulated by the EPA, and every discharge must now be publicly reported in Massachusetts. The alternative in the moment is sewage backing up into basements and streets.
The permanent fix is separating the pipes or building storage to hold flows until after the storm. And that's exactly what the consent decrees require.
In the meantime: CSOs happen during and after heavy rain. That's the pattern to remember. Rain means waiting before you swim, fish or interact with the river.
The cities want a clean river. You want it. We want it. Waste water treatment plants are working on it. Updates are happening — our job is to help advocate for continued progress, help you see, measure it, and keep the progress in view.
Want to be a part of the process? Learn more about CSOs at merrimack.org.
