Overflow history upstream of Ham and Kingston, River Thames
The spot's forecast · For event organisers
This page lists the monitored storm overflows upstream of this spot and how often each spilled in each year. It is not a water test, and it does not say whether the water is fit to swim in. It does not include farm runoff, misconnected drains, treated sewage effluent, wildlife or overflows without a monitor. A year's count depends on how long the monitor worked, shown as monitor uptime.
Each year, all overflows upstream
| Year | Overflows with a return | Spills | Spill hours | Spill hours, weighted by reach |
|---|---|---|---|---|
| 2021 | 34 | 768 | 7,647 | 186 |
| 2022 | 34 | 515 | 5,757 | 170 |
| 2023 | 43 | 987 | 11,833 | 321 |
| 2024 | 40 | 1,393 | 18,070 | 399 |
| 2025 | 39 | 604 | 7,884 | 122 |
Reach is the share of a spill's bacteria SwimSignal's model expects to arrive here, after die-off on the way and dilution by the river. Weighting by it counts an hour of spilling just upstream for far more than one 40 km away. Spills are counted by the Environment Agency's 12/24 method: any discharge in the first 12 hours counts as one spill, then one more for each 24 hours with any discharge. How reach is worked out.
The overflows, by reach
All 43, the one whose spills matter most here first. Hours away is how long sewage takes to arrive at the model's river speed.
| Overflow and id | Company | km upstream | Hours away | Reach | Spills 2023 | Spills 2024 | Spills 2025 | Spill hours 2025 | Monitor uptime 2025 |
|---|---|---|---|---|---|---|---|---|---|
| Kingston Main pumping station overflow Kingston Main SPS · TWL00258 | Thames Water | 1.7 | under 1 | 92% | 1 | 0 | 0 | 0 | 100% |
| Portsmouth road, uxbridge road TWL00140 | Thames Water | 3.3 | 2 | 83% | 5 | 6 | 2 | 3 | 100% |
| Church Street XX TWL00522 | Thames Water | 18.6 | 10 | 17% | 0 | – | – | – | – |
| Weybridge sewage works overflow Weybridge STW · TWL00445 | Thames Water | 21.7 | 12 | 11% | 81 | 81 | 28 | 426 | 100% |
| Esher sewage works overflow Esher STW · TWL00174 | Thames Water | 10.2 | 6 | 9% | 87 | 78 | 19 | 238 | 100% |
| Cobham TWL00107 | Thames Water | 19.5 | 11 | 5% | 40 | 53 | 24 | 268 | 100% |
| Stoke road, cobham TWL00744 | Thames Water | 26.7 | 15 | 4% | 2 | 14 | 6 | 45 | 96% |
| Windsor sewage works overflow Windsor STW · TWL00452 | Thames Water | 35.7 | 20 | 3% | 87 | 133 | 34 | 333 | 95% |
| St james's road TWL00142 | Thames Water | 2.5 | 1 | 3% | 4 | 5 | 2 | 2 | 100% |
| Hogsmill sewage works overflow Hogsmill STW · TWL00229 | Thames Water | 3.9 | 2 | 3% | 33 | 24 | 14 | 138 | 100% |
| Guildford sewage works overflow Guildford STW · TWL00206 | Thames Water | 42.2 | 23 | 2% | 15 | 46 | 9 | 133 | 100% |
| River Lane (Leatherhead) pumping station overflow River Lane (Leatherhead) SPS · TWL00349 | Thames Water | 34.4 | 19 | 2% | 4 | 6 | 2 | 5 | 100% |
| Markenfield road, guildford TWL00135 | Thames Water | 45.3 | 25 | 2% | 1 | 1 | 0 | 0 | 100% |
| Chertsey sewage works overflow Chertsey STW · TWL00088 | Thames Water | 25.6 | 14 | 2% | 67 | 104 | 27 | 472 | 97% |
| Trumps Green Road (Virginia Water) pumping station overflow Trumps Green Road (Virginia Water) SPS · TWL00418 | Thames Water | 27.9 | 16 | 1% | 0 | 5 | 0 | 0 | 100% |
| Bridle Close, Ewell TWL00492 | Thames Water | 9.4 | 5 | 1% | 0 | – | – | – | – |
| Dorking sewage works overflow Dorking STW · TWL00159 | Thames Water | 45.6 | 25 | 1% | 71 | 69 | 23 | 296 | 99% |
| Stonebridge sso, brockham TWL00106 | Thames Water | 48.4 | 27 | 1% | 19 | 11 | 26 | 218 | 98% |
| Castle Gardens (Dorking) pumping station overflow Castle Gardens (Dorking) SPS · TWL00077 | Thames Water | 47.8 | 26 | 1% | 5 | 12 | 7 | 48 | 100% |
| Manor drive stormwater tanks TWL00288 | Thames Water | 10.6 | 6 | 1% | 9 | 9 | 4 | 12 | 96% |
| Chobham sewage works overflow Chobham STW · TWL00096 | Thames Water | 34.1 | 19 | 0% | 45 | 78 | 20 | 288 | 100% |
| Lightwater sewage works overflow Lightwater STW · TWL00266 | Thames Water | 37.2 | 21 | 0% | 12 | 14 | 17 | 136 | 100% |
| Hockford sewage works overflow Hockford STW · TWL00331 | Thames Water | 44.1 | 24 | <1% | 39 | 73 | 24 | 175 | 100% |
| Ripley sewage works overflow Ripley STW · TWL00348 | Thames Water | 30.5 | 17 | <1% | 22 | 43 | 37 | 428 | 94% |
| Blacknest (Ascot) pumping station overflow Blacknest (Ascot) SPS · TWL00041 | Thames Water | 33.7 | 19 | <1% | 15 | 29 | 11 | 84 | 100% |
| Epsom storm tank overflow Epsom storm tanks · TWL00566 | Thames Water | 12.3 | 7 | <1% | 6 | 2 | 2 | 2 | 100% |
| Tonstall Road TWL00749 | Thames Water | 12.5 | 7 | <1% | 0 | – | – | – | – |
| Shamley Green sewage works overflow Shamley Green STW · TWL00365 | Thames Water | 53.8 | 30 | <1% | 0 | 0 | 0 | 0 | 100% |
| Commonside (Leatherhead) pumping station overflow Commonside (Leatherhead) SPS · TWL00111 | Thames Water | 30.7 | 17 | <1% | 2 | 13 | 2 | 5 | 100% |
| Brockham Bridge XX pumping station overflow Brockham Bridge XX SPS · TWL00058 | Thames Water | 48.9 | 27 | <1% | 8 | 15 | 8 | 72 | 100% |
| Bookham pumping station overflow Bookham SPS · TWL00483 | Thames Water | 31.6 | 18 | <1% | 0 | 0 | – | – | – |
| Maidenhead sewage works overflow Maidenhead STW · TWL00287 | Thames Water | 52.7 | 29 | <1% | 0 | 2 | 0 | 0 | 100% |
| Leatherhead sewage works overflow Leatherhead STW · TWL00262 | Thames Water | 33.4 | 18 | <1% | 61 | 82 | 49 | 787 | 100% |
| Woking sewage works overflow Woking STW · TWL00458 | Thames Water | 32.0 | 18 | <1% | 23 | 35 | 10 | 125 | 100% |
| Bramley (Guildford) pumping station overflow Bramley (Guildford) SPS · TWL00052 | Thames Water | 52.5 | 29 | <1% | 16 | 30 | 12 | 42 | 99% |
| Gerrards Cross sewage works overflow Gerrards Cross STW · TWL00196 | Thames Water | 51.1 | 28 | <1% | 120 | 144 | 88 | 1,605 | 100% |
| Godalming sewage works overflow Godalming STW · TWL00197 | Thames Water | 51.1 | 28 | <1% | 14 | 25 | 10 | 143 | 100% |
| Marsh Lane (Taplow) pumping station overflow Marsh Lane (Taplow) SPS · TWL00295 | Thames Water | 48.6 | 27 | <1% | 0 | 0 | 0 | 0 | 99% |
| Slough sewage works overflow Slough STW · TWL00397 | Thames Water | 45.7 | 25 | <1% | 32 | 70 | 14 | 246 | 99% |
| Spooks hill, north holmwood TWL00381 | Thames Water | 53.6 | 30 | <1% | 6 | 12 | 3 | 12 | 97% |
| White Waltham sewage works overflow White Waltham STW · TWL00448 | Thames Water | 55.1 | 31 | <1% | 0 | 2 | 0 | 0 | 100% |
| Bentsbrook road TWL00738 | Thames Water | 53.4 | 30 | <1% | 1 | 4 | 0 | 0 | 100% |
| Ironsbottom sewage works overflow Ironsbottom STW · TWL00242 | Thames Water | 59.7 | 33 | 0% | 34 | 63 | 70 | 1,095 | 100% |
A dash: no annual return for that overflow in that year.
Sources and credits
Storm overflow annual returns: © Environment Agency copyright and/or database right 2025, Open Government Licence v3.0. Overflow positions: the water companies via the National Storm Overflow Hub, CC BY 4.0. River network: contains OS data © Crown copyright and database right 2026 (OS Open Rivers). Traced and weighted by SwimSignal (swimsignal.co.uk), whose method is at swimsignal.co.uk/methods.html. All credits and licences.