The Environment Agency's 2025 storm overflow return for England measures 1,878,240.4 hours of monitored spill duration across 10 water and sewerage companies, and a floor rather than a total for 10 of them, because their monitors were not all working for the whole return 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 a storm overflow is, and why it is allowed to spill
A combined sewer carries foul water from homes and rainwater from roads in the same pipe. When heavy rain fills that pipe faster than the treatment works downstream can accept it, a storm overflow lets the excess out into a river, a stream or the sea instead of backing it up into streets and houses.
These discharges are permitted ones. Each overflow operates under a permit from the environmental regulator, which sets the circumstances in which it may spill, and a discharge inside those circumstances is lawful. What the record on these pages counts is how often and for how long each overflow discharged — not whether any particular discharge broke the terms of a permit, which is the regulator's judgement to make and not ours.
How the monitoring works
Event duration monitoring puts a sensor on an overflow that records when it starts discharging and when it stops. The water and sewerage companies operate the monitors; the environmental regulator collects what they report and publishes it as a statutory annual return. Every figure on these pages is one of those returns, read as published and divided in one stated way.
A monitor only counts what it is watching. An overflow with no commissioned monitor is not in the return at all, and an overflow whose monitor was out of service for part of the year is in the return carrying only the part of the year it saw. That is why a spill figure here is described as a floor wherever the monitors behind it were not working for the whole year.
What this record does not tell you
It counts duration and events, not volume, and it says nothing about what was in the water. A short discharge into a small stream may matter more than a long one into a tidal estuary, and the return distinguishes neither.
It is also a record of overflows rather than of companies. Comparing companies means choosing a way to divide one quantity by another, and the choice changes the order — which is why the method used here is set out on the page rather than left to be inferred from the table.
Scotland: no figure is held here
No storm overflow figure for Scotland is held on this site. Scottish Water reports its overflow events to the Scottish Environment Protection Agency and publishes that record on its own website, and that website refused this site's identified reader when it was read.
A refusal is never read around by another route, so the record is to be requested from Scottish Water under the Environmental Information (Scotland) Regulations. Scotland's figures will appear here only from what that request returns, counted the way Scottish Water counts them and never ranked against England or Wales.
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. 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
Northern Ireland publishes no monitored return like this one: NI Water's own figure comes from its predictive models, so it is modelled rather than measured, and all rights in it are reserved, which is why it is quoted here once and not reused — “Every year, around 16–20 million cubic metres of water are released from storm overflows in Northern Ireland’s public wastewater network.”
Source: NI Water — Storm Overflow performance — read 27 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 |
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
The record, company by company
- Every company, ranked on spill hours per monitored overflow
- 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