Overflow history upstream of River Cam, Grantchester Meadows
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 | 13 | 348 | 2,668 | 360 |
| 2022 | 16 | 271 | 865 | 113 |
| 2023 | 22 | 547 | 5,110 | 1,106 |
| 2024 | 22 | 926 | 11,180 | 1,885 |
| 2025 | 22 | 357 | 2,552 | 506 |
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 22, 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 |
|---|---|---|---|---|---|---|---|---|---|
| Haslingfield sewage works overflow Haslingfield-STW · AWS00569 | Anglian Water | 4.2 | 2 | 33% | 172 | 128 | 30 | 383 | 100% |
| Foxton (Cambs) sewage works overflow Foxton WRC (Cambs) · AWS01557 | Anglian Water | 10.9 | 6 | 22% | 14 | 188 | 102 | 1,644 | 87% |
| Barrington-Slid La storm overflow 2 Barrington-Slid LA #2 OV · AWS00056 | Anglian Water | 11.4 | 6 | 19% | 85 | 60 | 14 | 16 | 100% |
| Great Chesterford sewage works overflow Great Chesterford STW · AWS00489 | Anglian Water | 22.1 | 12 | 8% | 1 | 0 | 1 | 0 | 100% |
| Arrington sewage works overflow Arrington Water Recycling Centre · AWS00022 | Anglian Water | 20.7 | 12 | 6% | 17 | 20 | 6 | 6 | 90% |
| Lt Chesterford pumping station overflow LT Chesterford-TPS · AWS00747 | Anglian Water | 25.9 | 14 | 6% | 3 | 16 | 0 | 0 | 100% |
| Linton sewage works overflow Linton STW · AWS00739 | Anglian Water | 21.8 | 12 | 4% | 8 | 13 | 9 | 20 | 99% |
| Linton-Recreation Ground pumping station overflow Linton-Recreation Ground TPS · AWS00738 | Anglian Water | 23.3 | 13 | 3% | 2 | 0 | 1 | 0 | 100% |
| Orwell-Meadowcroft Way pumping station overflow Orwell-Meadowcroft WAY SP · AWS00830 | Anglian Water | 17.7 | 10 | 1% | 37 | 55 | 8 | 101 | 90% |
| Wimpole Road pumping station overflow Wimpole Road Pumping Station · AWS01507 | Anglian Water | 6.7 | 4 | 1% | 36 | 49 | 18 | 190 | 100% |
| Saffron Walden sewage works overflow Saffron Walden STW · AWS01087 | Anglian Water | 30.2 | 17 | 1% | 26 | 27 | 9 | 4 | 99% |
| Saffron Walden sewage works overflow Saffron Walden STW · AWS01086 | Anglian Water | 30.2 | 17 | 1% | 1 | 10 | 2 | 4 | 100% |
| Sawston sewage works overflow Sawston STW · AWS01096 | Anglian Water | 11.6 | 6 | 1% | 0 | 24 | 1 | 0 | 100% |
| Melbourn sewage works overflow Melbourn STW · AWS00833 | Anglian Water | 17.4 | 10 | 1% | 52 | 119 | 37 | 59 | 99% |
| Barley sewage works overflow Barley STW · AWS00050 | Anglian Water | 25.5 | 14 | 1% | 34 | 30 | 29 | 37 | 100% |
| Castle Street/High Street storm overflow Castle Street/High Street Combined Sewer Overflow · AWS00207 | Anglian Water | 31.1 | 17 | 1% | 17 | 55 | 35 | 18 | 100% |
| Royston sewage works overflow Royston STW · AWS01072 | Anglian Water | 18.2 | 10 | 0% | 5 | 2 | 0 | 0 | 100% |
| Guilden Morden sewage works overflow Guilden Morden STW · AWS00538 | Anglian Water | 28.4 | 16 | <1% | 5 | 33 | 19 | 6 | 100% |
| Saffron Walden-George Abbey storm overflow Saffron Walden-George Abbey OV · AWS01088 | Anglian Water | 31.1 | 17 | <1% | 4 | 24 | 18 | 10 | 100% |
| Saffron Walden-Victoria Thx storm overflow Saffron Walden-Victoria THX OV · AWS01355 | Anglian Water | 32.0 | 18 | <1% | 2 | 7 | 0 | 0 | 100% |
| Ashwell sewage works overflow Ashwell STW · AWS00030 | Anglian Water | 33.6 | 19 | <1% | 24 | 45 | 10 | 6 | 100% |
| Wicken Bonhunt pumping station overflow Wicken Bonhunt-SP · AWS01494 | Anglian Water | 37.4 | 21 | <1% | 2 | 21 | 8 | 47 | 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.