Hamstead Marshall STW
Storm overflow operated by Thames Water. The source names Hamstead Stream as the receiving water.
- 42
- Spills in 2025
- 565
- Recorded hours
- 100%
- Monitor operation
- 5
- Years listed
The reported spill count was 44 lower than in 2024. This compares event counts, not discharge volume.
Checking the latest monitor report…
How it compares in 2025
Hamstead Marshall STW recorded 42 spills in 2025. That is more than 83% of the 14,239 overflows in England with a reported figure, and ranks 2,238th nationally. Among Thames Water overflows it ranks 59th of 579; the median Thames Water overflow recorded 6 spills.
| Group | Rank by spills | Median spills | Median hours |
|---|---|---|---|
| This outlet | – | 42 | 565 |
| England | 2,238th of 14,239 | 11 | 22.3 |
| Thames Water | 59th of 579 | 6 | 20.9 |
Rank 1 has the most spills. Equal counts share a rank. Only overflows with a reported count are ranked. Counts do not measure volume, so a higher rank does not mean more sewage.
Annual spill history
| Year | Spills | Duration | Monitor operation | Source |
|---|---|---|---|---|
| 2025 | 42 | 565 hours | 100% | Official data |
| 2024 | 86 | 1,009 hours | 100% | Official data |
| 2023 | 93 | 1,021.5 hours | 97.9% | Official data |
| 2022 | 57 | 899.1 hours | 99.8% | Official data |
| 2021 | 57 | 631.9 hours | 97.3% | Official data |
A missing figure means the source did not provide a usable value. A zero is shown as zero. Monitor operation below 90% can make comparison less reliable.
Outlet details
- Water company
- Thames Water
- Outlet type
- Storm tank at a wastewater treatment works
- Permit
- CSSC.1381
- Activity reference
- A2
- Grid reference
- SU4172065210
- Waterbody ID
- GB106039023174Official waterbody record ↗
- Location source
- Official annual grid reference
- Location precision
- About 1 metres
- Location note
- Unknown
About the name and type
The annual return lists this asset as “Storm tank at WwTW”: storm tank at a wastewater treatment works. The overflow comes from storm storage at a treatment works.
The waterbody link opens the Environment Agency record for the exact ID in the return. It is a source lookup; we do not import its classification.
Where it is
The recorded position is about 2.1 km north of the Ball Hill reference point in the ONS built-up areas data.
Area and water pages
Check the official record
Public access to this outlet is unknown. Do not enter restricted land or use this page as a route guide.
Open official source ↗Other outlet records nearby
- Knights Lane (Ball Hill) SPSCurrent status unknown
Thames Water · Enbourn
- Kintbury STWCurrent status unknown
Thames Water · Kennet & Avon Canal
- Blind Mans Gate (Highclere) SPSCurrent status unknown
Thames Water · Penwood Stream
- Wash Water STWCurrent status unknown
Thames Water · Pound Street Brook
Questions about Hamstead Marshall STW
- Who operates Hamstead Marshall STW?
- Thames Water is the operator named in the annual record. The permit reference is CSSC.1381.
- Where does Hamstead Marshall STW discharge?
- The source names Hamstead Stream as the receiving water. This does not show how far a discharge travelled.
- How many times did Hamstead Marshall STW spill in 2025?
- The 2025 return records 42 counted spill events and 565 hours of recorded discharge. Spills are counted with the 12/24-hour method, so this is not a count of every start signal.
- Can I trust the 2025 figures for this outlet?
- The monitor worked for 100% of the reporting period, so the annual figures cover most of the year. They still do not measure discharge volume.
- Is Hamstead Marshall STW discharging right now?
- This record is linked to a company live feed. The panel above shows the latest report we hold, with its date. That report can be old, and the monitor can be offline, so check the date before relying on it. A report that no discharge is taking place is not a water-quality result.
- How accurate is the map marker?
- The marker comes from the grid reference in the official annual return, which is accurate to about 1 metres. It shows the recorded outlet position, not the extent of any discharge.
- What kind of outlet is Hamstead Marshall STW?
- The annual return lists the asset type as “Storm tank at WwTW”, which means storm tank at a wastewater treatment works. The overflow comes from storm storage at a treatment works.
- How does Hamstead Marshall STW compare with other overflows?
- Hamstead Marshall STW recorded 42 spills in 2025. That is more than 83% of the 14,239 overflows in England with a reported figure, and ranks 2,238th nationally. Among Thames Water overflows it ranks 59th of 579; the median Thames Water overflow recorded 6 spills. Ranks compare counted spill events only. They do not compare volume or harm.
- Does zero spills mean the water was safe?
- No. A zero means the annual return recorded zero counted spill events for this outlet and year. It is not a water-quality test, and pollution can come from other sources.