Overflow history upstream of River Thames, Clifton Hampden

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	1,374	18,487	128
2022	34	736	8,781	28
2023	39	2,428	34,570	189
2024	40	3,161	47,580	328
2025	40	1,160	15,543	95

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 40, 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
Heyford Hill Lane (Littlemore) pumping station overflow Heyford Hill Lane (Littlemore) SPS · TWL00273	Thames Water	20.3	11	28%	5	26	1	6	100%
Cassington sewage works overflow Cassington STW · TWL00075	Thames Water	32.6	18	9%	2	22	0	0	100%
Islip sewage works overflow Islip STW · TWL00244	Thames Water	35.8	20	4%	70	61	34	697	100%
Charlton-on-Otmoor sewage works overflow Charlton-on-Otmoor STW · TWL00085	Thames Water	40.7	23	2%	77	120	54	814	100%
Drayton sewage works overflow Drayton STW · TWL00162	Thames Water	9.9	6	2%	116	128	47	639	100%
Didcot sewage works overflow Didcot STW · TWL00155	Thames Water	5.3	3	1%	61	94	26	339	100%
Stanford in the Vale sewage works overflow Stanford in the Vale STW · TWL00387	Thames Water	30.4	17	1%	2	0	3	6	100%
Standlake sewage works overflow Standlake STW · TWL00385	Thames Water	48.4	27	1%	131	180	98	2,214	100%
Oxford sewage works overflow Oxford STW · TWL00323	Thames Water	20.3	11	1%	133	162	43	590	100%
Woodstock sewage works overflow Woodstock STW · TWL00462	Thames Water	46.7	26	1%	59	41	21	89	99%
Culham sewage works overflow Culham STW · TWL00147	Thames Water	0.7	under 1	1%	32	78	38	378	100%
The willows, north hinksey lane TWL00411	Thames Water	25.7	14	0%	11	8	7	48	100%
West way, botley (syphon) TWL00439	Thames Water	26.5	15	0%	26	18	10	40	96%
Appleton sewage works overflow Appleton STW · TWL00013	Thames Water	22.2	12	<1%	121	160	17	159	100%
Wantage sewage works overflow Wantage STW · TWL00425	Thames Water	23.0	13	<1%	78	96	30	170	100%
Abingdon sewage works overflow Abingdon STW · TWL00004	Thames Water	9.4	5	<1%	38	85	13	112	100%
Combe sewage works overflow Combe STW · TWL00110	Thames Water	47.6	26	<1%	111	130	30	356	100%
Bicester sewage works overflow Bicester STW · TWL00038	Thames Water	47.6	26	<1%	66	89	34	450	100%
Tubney XX pumping station overflow Tubney SPS XX · TWL00806	Thames Water	21.9	12	<1%	0	7	1	1	100%
Bampton sewage works overflow Bampton STW · TWL00024	Thames Water	56.3	31	<1%	135	160	82	926	99%
Uffington sewage works overflow Uffington STW · TWL00419	Thames Water	37.9	21	<1%	86	101	48	654	100%
Kingston Bagpuize sewage works overflow Kingston Bagpuize STW · TWL00257	Thames Water	24.5	14	<1%	25	143	32	389	100%
Weston-on-the-Green sewage works overflow Weston-on-the-Green STW · TWL00443	Thames Water	40.0	22	<1%	63	86	42	696	100%
Weston-on-the-Green pumping station overflow Weston-on-the-Green SPS · TWL00442	Thames Water	40.0	22	<1%	0	3	2	21	100%
Stanton Harcourt sewage works overflow Stanton Harcourt STW · TWL00389	Thames Water	39.3	22	<1%	91	123	42	709	100%
Finstock sewage works overflow Finstock STW · TWL00184	Thames Water	55.6	31	<1%	29	38	13	144	100%
Church Hanborough sewage works overflow Church Hanborough STW · TWL00098	Thames Water	37.8	21	<1%	120	158	88	1,236	100%
South Leigh sewage works overflow South Leigh STW · TWL00377	Thames Water	40.7	23	<1%	96	71	49	927	100%
Bletchingdon sewage works overflow Bletchingdon STW · TWL00043	Thames Water	50.0	28	<1%	120	120	32	302	95%
Marsh Gibbon sewage works overflow Marsh Gibbon STW · TWL00294	Thames Water	53.6	30	<1%	99	69	22	208	99%
Middle Barton sewage works overflow Middle Barton STW · TWL00298	Thames Water	58.4	32	<1%	17	44	15	152	100%
Grendon Underwood sewage works overflow Grendon Underwood STW · TWL00204	Thames Water	57.7	32	<1%	76	68	31	306	100%
Charlbury sewage works overflow Charlbury STW · TWL00083	Thames Water	59.1	33	<1%	11	22	7	52	100%
Upper Heyford sewage works overflow Upper Heyford STW · TWL00223	Thames Water	59.0	33	<1%	44	108	33	302	95%
Ludgershall sewage works overflow Ludgershall STW · TWL00283	Thames Water	55.2	31	<1%	98	99	52	616	100%
Witney sewage works overflow Witney STW · TWL00457	Thames Water	58.7	33	<1%	119	141	37	576	100%
Beckley sewage works overflow Beckley STW · TWL00113	Thames Water	41.2	23	<1%	22	10	4	24	100%
Middleton Stoney sewage works overflow Middleton Stoney STW · TWL00300	Thames Water	53.0	29	<1%	37	55	18	145	100%
Tackley & Storm pumping station overflow Tackley SPS & Storm SPS · TWL00403	Thames Water	49.6	28	<1%	1	30	4	47	100%
XX (Bucknell) pumping station overflow XX (Bucknell) SPS · TWL00498	Thames Water	53.8	30	<1%	–	7	0	0	94%

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.