Thames Water storm overflows in 2025
What the official annual return shows for Thames Water’s storm overflows in England, compared with 2024 and with the other water companies.
Summary
The 2025 annual return lists 581 storm overflows run by Thames Water in England. 579 have a usable spill count. Together they recorded 9,189 spills and 107,822 hours of discharge. The median outlet recorded 6 spills and 20.9 hours.
For a fair comparison with 2024, we use only the 518 outlets with a count in both years and a monitor that worked at least 90% of each year. Their spills went from 21,404 to 8,427, a fall of 61%. 435 recorded fewer spills and 26 recorded more.
135 outlets (23.3%) recorded no spills in 2025. 29 (5%) recorded 60 or more.
Thames Water’s median of 6 spills per outlet is the 10th highest of the 10 companies in the return. The England median is 11.
32 outlets had a monitor that worked for less than 90% of the year, and 1 gave no operation figure. Counts from those outlets may be low.
- 581
- Outlets listed
- 9,189
- Spills in 2025
- 6
- Median per outlet
- 107,822
- Recorded hours
How many spills each outlet recorded
Outlets with a usable 2025 count, grouped by the number of spills. The longest bar is the largest group.
- 0
- 135 23.3%
- 1 to 9
- 199 34.4%
- 10 to 19
- 76 13.1%
- 20 to 39
- 101 17.4%
- 40 to 59
- 39 6.7%
- 60 or more
- 29 5%
Spills over time
The median outlet run by Thames Water recorded 6 spills in 2025, against 11 across England.
518 outlets have a comparable count for both 2024 and 2025, with the monitor working at least 90% of each year. 26 recorded more spills, 435 fewer and 57 the same. Together they went from 21,404 to 8,427 spills, a fall of 61%.
Largest rises from 2024 to 2025
- East Shefford STW35 → 102
- Stag Lane CSO13 → 37
- Streatham storm relief B1 → 18
| Year | Outlets with a count | Total spills | Median per outlet | England median |
|---|---|---|---|---|
| 2025 | 579 | 9,189 | 6 | 11 |
| 2024 | 575 | 22,903 | 20 | 19 |
| 2023 | 422 | 13,699 | 16 | 20 |
| 2022 | 88 | 2,332 | 20.5 | 13 |
| 2021 | 86 | 4,122 | 42.5 | 19 |
Earlier returns list fewer of these outlets, so totals are not comparable between years. The median per outlet and the like-for-like change are fairer comparisons. Counts are events, not volume.
Largest falls from 2024 to 2025
Only outlets with a count in both years and at least 90% monitor operation in each. Counts depend on rainfall as well as on the sewer network, so a fall does not show on its own that an overflow improved.
- Amersham balancing tanks/amersham vale stk210 → 13
- Boddington STW235 → 74
- Clanfield STW172 → 14
- Shutford STW190 → 32
- Cottered STW190 → 36
- Appleton STW160 → 17
- Chalgrove STW147 → 8
- Chesham STW141 → 7
- Cuddington STW135 → 4
- Hanwell STW152 → 27
Most spills in 2025
- 1Markyate STW155spills
River Ver
- 2Stewkley STW143spills
Hardwick Brook
- 3South Moreton STW137spills
Mill Brook
- 4Cirencester STW125spills
Shorncote Ditch
- 5Winterbourne STW116spills
Winterbourne Stream
- 6Fairford STW107spills
River Coln
- 7East Shefford STW102spills
River Lambourn
- 8Sevenhampton STW99spills
Tributary of the River Cole
- 9Standlake STW98spills
Windrush
- 10Marlborough STW96spills
Kennet
Most hours in 2025
- 1Markyate STW3,531hours
River Ver
- 2Stewkley STW3,069hours
Hardwick Brook
- 3South Moreton STW2,721hours
Mill Brook
- 4Winterbourne STW2,635hours
Winterbourne Stream
- 5Fairford STW2,235hours
River Coln
- 6Standlake STW2,214hours
Windrush
- 7East Shefford STW2,207hours
River Lambourn
- 8Cirencester STW2,067hours
Shorncote Ditch
- 9Sevenhampton STW2,055hours
Tributary of the River Cole
- 10Gerrards Cross STW1,605hours
Misbourne
Waterbodies with the most spills
Total 2025 spills from outlets the return links to each waterbody. A waterbody with more outlets will usually have a higher total.
| Waterbody | Outlets | Spills | Spills per outlet |
|---|---|---|---|
| Lower Roding (Loughton to Thames) | 32 | 315 | 9.8 |
| Thames Middle | 54 | 226 | 4.2 |
| Mole - Horley to Dorking | 7 | 213 | 30.4 |
| Wandle (Croydon to Wandsworth) and the Graveney | 28 | 159 | 5.7 |
| Thame (Scotsgrove Brook to Thames) | 3 | 155 | 51.7 |
| Ver | 1 | 155 | 155 |
| Pang | 5 | 153 | 30.6 |
| Windrush and tributaries (Little Rissington to Thames) | 3 | 147 | 49 |
| Upper Roding (to Cripsey Brook) | 5 | 145 | 29 |
| Hardwick Brook (Source to Thame) | 1 | 143 | 143 |
How to read this report
- A spill is a counted event under the 12/24-hour method. It is not a volume, and it does not measure harm or water quality.
- Missing counts are left out, not treated as zero. Low monitor operation can make a count too low.
- Rankings compare counts only. They do not show whether an overflow broke its permit or the law.
- Earlier years list fewer outlets, so compare medians or like-for-like groups, not totals.
Data: Environment Agency event duration monitoring annual returns, Open Government Licence v3.0. How spills are counted · How the data works · Sources