South West Water, first of 10 companies in the Environment Agency's 2025 return for England measures 299.7 hours per monitored overflow of spill duration, which is the figure this table is ordered on and is a floor rather than a total wherever the monitors behind it were not working for the whole year, per Environment Agency — Event Duration Monitoring: Storm Overflows Annual Return, 2025 (31 December 2025, read 20 September 2026).
Source: Environment Agency — Event Duration Monitoring: Storm Overflows Annual Return, 2025 — dated 31 December 2025, read 20 September 2026
What these figures cover, and what they do not
The return behind this page covers England, and holds 10 water and sewerage companies.
This return covers England. Wales and Scotland have their own environmental regulators, which publish separately and are not in this dataset, and Northern Ireland has no equivalent return here at all. Nothing on this page is a figure about the United Kingdom.
Dŵr Cymru appears here for assets inside England rather than for its whole business. The publisher labels that column “Dwr Cymru Welsh Water (DC/WW) (in England)”.
Source: Environment Agency — Event Duration Monitoring: Storm Overflows Annual Return, 2025 — dated 31 December 2025, read 20 September 2026
How these companies are compared
The figure everything here is ordered on is spill hours per monitored overflow.
The total duration of a company's monitored spill events in one return year, divided by the count of storm overflows that company had a commissioned monitor on in that year. Both quantities are the Environment Agency's own, taken from its summary sheet for that return, and neither is recomputed here.
A company with fewer monitors reports fewer spills, so ordering on the raw number of spills would rank these companies by size and reward the ones monitoring least. The two orderings disagree on this return: Thames Water sits at 2 here and would sit at 9 in a raw-count table.
The count each return publishes to divide by
| Return year | Published as |
|---|---|
| 2025 | Total number of storm overflows with EDM commissioned |
| 2024 | Total no. storm overflows with EDM commissioned |
| 2023 | Total no. storm overflows with EDM commissioned 2 |
| 2022 | Total no. storm overflows with EDM commissioned |
| 2021 | Total no. storm overflows with EDM commissioned |
| 2020 | Overflows with numeric data |
Every company in the return, ordered on that figure
| # | Company | Spill hours per monitored overflow | Monitored overflows | Spill hours | Spill events | Monitors below the availability threshold | Is the figure a total? |
|---|---|---|---|---|---|---|---|
| 1 | South West Water | 299.7 | 1,358 | 407,005.8 | 46,164 | 3.3% | No — a floor |
| 2 | Thames Water | 185.9 | 580 | 107,822 | 9,189 | 5.5% | No — a floor |
| 3 | United Utilities | 144.3 | 2,270 | 327,452.5 | 60,843 | 8.5% | No — a floor |
| 4 | Wessex Water | 141.7 | 1,346 | 190,666.4 | 24,442 | 2.5% | No — a floor |
| 5 | Yorkshire Water | 130.9 | 2,184 | 285,930.5 | 51,431 | 9.3% | No — a floor |
| 6 | Dŵr Cymru | 119.1 | 122 | 14,525 | 2,843 | 4.1% | No — a floor |
| 7 | Southern Water | 117.6 | 977 | 114,868 | 15,554 | 11.8% | No — a floor |
| 8 | Severn Trent Water | 82.4 | 2,432 | 200,361.4 | 36,471 | 3.7% | No — a floor |
| 9 | Northumbrian Water | 79.3 | 1,561 | 123,803 | 27,860 | 12.9% | No — a floor |
| 10 | Anglian Water | 73.4 | 1,442 | 105,805.6 | 17,322 | 8.3% | No — a floor |
Every row is one company’s own return year, and the column headed “spill events” is the raw count this table is not ordered on.
Every figure above: Environment Agency — Event Duration Monitoring: Storm Overflows Annual Return, 2025 — dated 31 December 2025, read 20 September 2026
Download this table
The 2025 table as a CSV file — one row per company, in the order of the table above, with the same figures.
© Environment Agency copyright and/or database right 2025. All rights reserved.
Where the publisher’s own tables disagree
These are the publisher’s own figures contradicting each other, not a disagreement of ours with it. Both numbers are below.
Northumbrian Water, 2025: the average duration of a monitored spill event the return publishes implies a count of 27,839.4, while the same sheet publishes 27,860.
This page's figure for the 2025 return does not divide by that count, so the disagreement does not move it. It is recorded here because it is the publisher's own, in a table this page reads.
Source: Environment Agency — Event Duration Monitoring: Storm Overflows Annual Return, 2025 — dated 31 December 2025, read 20 September 2026
Northumbrian Water, 2025: the average number of spills per overflow the return publishes implies a count of 1,563.7, while the same sheet publishes 1,559.
This page's figure for the 2025 return does not divide by that count, so the disagreement does not move it. It is recorded here because it is the publisher's own, in a table this page reads.
Source: Environment Agency — Event Duration Monitoring: Storm Overflows Annual Return, 2025 — dated 31 December 2025, read 20 September 2026
What this monitoring can and cannot tell you
A monitor only counts what it is watching
Where monitors were out of service for part of the return year, the spill figures beside them count only the part of the year that was watched. They are floors rather than totals, and a table ignoring this would put the companies monitoring least at the bottom of it.
Duration is not volume, and neither is a count of events
The return records how many times an overflow discharged and for how long. It does not record how much was discharged, what was in it, or what happened to the water that received it.
A spill is not by itself a breach
Storm overflows discharge under permits that allow them to. A high figure on these pages is a measurement of how much a network is discharging, not a finding that any permit condition was broken; that finding belongs to the environmental regulator.
These pages are per company, not per outfall
The return publishes a row for every monitored overflow, with the water it discharges into and how often. What this site holds of those per-overflow rows is a sample rather than the whole return, so a table of any one company's heaviest-spilling outfalls built from it would rank the sample and not the network. The complete per-overflow rows are in the published return itself, which is linked from this page.
When each return had to be published
| Return year | Published by | Under |
|---|---|---|
| 2025 | 31 March 2026 | section 141D of the Water Industry Act 1991, inserted by the Environment Act 2021 |
| 2024 | 31 March 2025 | section 141D of the Water Industry Act 1991, inserted by the Environment Act 2021 |
| 2023 | 31 March 2024 | section 141D of the Water Industry Act 1991, inserted by the Environment Act 2021 |
| 2022 | 31 March 2023 | section 141D of the Water Industry Act 1991, inserted by the Environment Act 2021 |
| 2021 | 31 March 2022 | section 141D of the Water Industry Act 1991, inserted by the Environment Act 2021 |
| 2020 | No statutory deadline | Published before the duty began |
Those are the dates each return under the duty had to be published by, not dates on which anybody here watched one appear.
“The EA has a duty under section 141D of the WIA 1991 (inserted by the Environment Act 2021) to publish an annual summary of measured EDM data by the end of March each year for the preceding calendar year.”
The duty: Defra and the Environment Agency — Storm overflows: policy and guidance — read 21 September 2026
The 2020 return was published before this duty applied, so it had no statutory deadline. The data is the Environment Agency’s either way.
“Reports under this section are to relate to successive calendar years, starting with 2021.”
The range: Water Industry Act 1991, section 141D(3) — read 21 September 2026
Using these figures somewhere else
© Environment Agency copyright and/or database right 2025. All rights reserved.
The Environment Agency's own catalogue record for this dataset declares no licence at all — its licence id, title and URL are each empty — so the notice above is the only permission statement its publisher makes. A government domain is not a licence, and nothing on this page should be read as granting one.
Get the data yourself: Environment Agency — Event Duration Monitoring: Storm Overflows Annual Return, 2025 — dated 31 December 2025, read 20 September 2026
One company at a time
- South West Water: its own storm overflow record
- Thames Water: its own storm overflow record
- United Utilities: its own storm overflow record
- Wessex Water: its own storm overflow record
- Yorkshire Water: its own storm overflow record
- Dŵr Cymru: its own storm overflow record
- Southern Water: its own storm overflow record
- Severn Trent Water: its own storm overflow record
- Northumbrian Water: its own storm overflow record
- Anglian Water: its own storm overflow record