Overflow history upstream of River Itchen, Compton Lock
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 | 5 | 4 | 6 | 0 |
| 2022 | 5 | 8 | 12 | 1 |
| 2023 | 5 | 20 | 153 | 55 |
| 2024 | 6 | 134 | 2,320 | 1,007 |
| 2025 | 6 | 13 | 52 | 17 |
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 6, 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 |
|---|---|---|---|---|---|---|---|---|---|
| Harestock storm sewage overflow Harestock SSO - 115789 · SWS00399 | Southern Water | 6.9 | 4 | 49% | 10 | 98 | 4 | 32 | 100% |
| Garnier Road Winchester pumping station overflow Garnier Road Winchester CEO - 108433 · SWS00344 | Southern Water | 2.9 | 2 | 16% | 0 | 2 | 2 | 7 | 99% |
| Spring Gardens Alresford pumping station overflow Spring Gardens Alresford CEO - 108484 · SWS00804 | Southern Water | 18.1 | 10 | 6% | 0 | 2 | 0 | 0 | 100% |
| Morestead Road Winchester Pumped storm sewage overflow Morestead Road Winchester Pumped SSO - 116368 · SWS00983 | Southern Water | 2.8 | 2 | 4% | – | 28 | 7 | 14 | 100% |
| Morestead Road Winchester storm sewage overflow Morestead Road Winchester SSO - 115740 · SWS00585 | Southern Water | 2.8 | 2 | 4% | 8 | 0 | 0 | 0 | 100% |
| New Alresford storm sewage overflow NEW Alresford SSO - 115779 · SWS00593 | Southern Water | 20.9 | 12 | 2% | 2 | 4 | 0 | 0 | 98% |
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.