English storm overflows discharged into rivers and the sea 290,127 times in 2025 — an average of 20.4 spills per outfall. In Wales the rate was more than double, at 43.2 spills per outfall. Here is what the 2025 data shows.
Storm overflows are release valves that let the sewerage network discharge diluted, untreated sewage into rivers and the sea during heavy rain, to stop it backing up into homes. They are supposed to be used only in exceptional conditions. The 2025 monitoring data shows they are used far more routinely than that.
The 2025 numbers
| England | Wales | |
|---|---|---|
| Monitored outfalls | 14,245 | 2,193 |
| Total spills | 290,127 | 94,738 |
| Average spills per outfall | 20.4 | 43.2 |
| Average hours per outfall | 131 | 354 |
| Outfalls that spilled at least once | 83.9% | 91.1% |
In England, storm overflows spilled 290,127 times across 14,245 monitored outfalls in 2025 — an average of 20.4 spills each. In Wales the picture is worse per outfall: 43.2 spills on average, more than double the English rate, with the typical Welsh outfall discharging for around 354 hours over the year.
Why is the Welsh rate so much higher?
Wales has higher rainfall and a large number of small, older combined-sewer networks serving rural and valley communities, which fill and overflow more readily. A higher share of Welsh outfalls — 91% — spilled at least once in 2025, compared with 84% in England. The figures are not directly comparable as a measure of company performance, because geography and network design differ so much.
What this means for tap water
Storm overflows affect rivers and bathing waters, not the tap. Drinking water is abstracted upstream, treated and tested separately, and remains safe. But sewage spills are the clearest measure of the strain on the wider water system, and they feed directly into the investment driving up bills.
Check the spills near you
Our sewage spill checker lets you see storm-overflow activity near any postcode, and the live incidents map tracks current events. For context on how this fits the wider picture, see our 2026 annual report.
Source: MyTapWater storm-overflow dataset (2025 data year), compiled from Environment Agency and Natural Resources Wales Event Duration Monitoring returns.