What Are Combined Sewer Overflows (CSOs)?

Combined Sewer Overflows, or CSOs, are one of the most important — and least understood — factors affecting water quality at beaches.

BeachSafeCheck combines:
• Official water quality advisories
• Rainfall patterns and runoff impact
• Combined sewer overflow (CSO) data

to give you a clearer, real-time picture of beach conditions — even before official results are updated.

What is a CSO?

In some cities, stormwater and wastewater share the same pipes. These are called combined sewer systems.

During heavy rain, the system can become overwhelmed.

When that happens:

Untreated or partially treated wastewater is released into nearby rivers or lakes.

This release is called a combined sewer overflow (CSO).

Why do CSOs happen?

CSOs are not accidents — they are part of how older infrastructure was designed.

They occur when:

Rainfall exceeds system capacity
Treatment plants cannot keep up
Overflow points are activated to prevent backups into homes

Why does this matter for beaches?

CSOs can introduce:

Bacteria (including E. coli)
Viruses
Nutrients
Other contaminants

These can travel into:

rivers
nearshore lake areas
public beaches

When are CSOs most likely?

CSOs are most common:

During or after heavy rain
In older urban areas
Near river mouths and lakefront cities

This is why water quality often drops after storms.

How long do effects last?

Typically:

24–48 hours after a rainfall event
Longer if multiple storms occur

How BeachSafeCheck helps

Beach Safe tracks CSO risk by combining:

real overflow reports
rainfall patterns
proximity to affected waterways

This helps identify when contamination may impact nearby beaches — even before official test results are available.

What should you do?

If there has been recent rain or a known overflow:

Avoid swimming for at least 24–48 hours
Be cautious near river outlets
Follow posted advisories

CSOs are a normal part of many city systems — but understanding them helps you stay safer in the water.