Overflow history upstream of River Thames, Henley-on-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	46	1,722	22,856	422
2022	46	945	11,682	43
2023	50	2,162	30,272	195
2024	51	2,768	37,543	341
2025	51	1,075	14,723	100

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 51, 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
Friday Street (Henley) pumping station overflow Friday Street (Henley) SPS · TWL00191	Thames Water	0.0	under 1	100%	14	17	4	10	98%
Wargrave sewage works overflow Wargrave STW · TWL00428	Thames Water	7.3	4	31%	28	46	12	121	100%
Caversham (Reading) pumping station overflow Caversham (Reading) SPS · TWL00345	Thames Water	14.3	8	12%	0	0	0	0	91%
Goring sewage works overflow Goring STW · TWL00199	Thames Water	34.6	19	3%	0	0	0	0	100%
New Mill sewage works overflow New Mill STW · TWL00176	Thames Water	29.0	16	3%	36	68	27	325	99%
Reading sewage works overflow Reading STW · TWL00343	Thames Water	18.2	10	3%	24	47	14	168	99%
Hartley Wintney sewage works overflow Hartley Wintney STW · TWL00216	Thames Water	35.0	19	1%	46	39	21	300	100%
Mortimer sewage works overflow Mortimer STW · TWL00306	Thames Water	27.9	16	1%	150	63	40	689	100%
Newbury sewage works overflow Newbury STW · TWL00309	Thames Water	46.3	26	1%	61	94	28	381	100%
Sherfield-on-Loddon sewage works overflow Sherfield-on-Loddon STW · TWL00366	Thames Water	35.0	19	1%	67	60	26	405	99%
Pangbourne sewage works overflow Pangbourne STW · TWL00324	Thames Water	25.3	14	1%	5	70	14	271	100%
Arborfield sewage works overflow Arborfield STW · TWL00014	Thames Water	21.3	12	0%	52	53	28	251	100%
Fleet sewage works overflow Fleet STW · TWL00186	Thames Water	40.3	22	<1%	86	103	39	525	99%
Dorchester sewage works overflow Dorchester STW · TWL00157	Thames Water	50.0	28	<1%	4	63	15	121	100%
Crooked Billet (Hook) & Storm pumping station overflow Crooked Billet (Hook) SPS & Storm SPS · TWL00121	Thames Water	38.7	22	<1%	15	19	7	13	100%
South Moreton sewage works overflow South Moreton STW · TWL00378	Thames Water	47.0	26	<1%	162	261	137	2,721	99%
Ash Vale sewage works overflow Ash Vale STW · TWL00017	Thames Water	49.0	27	<1%	15	23	9	103	100%
Ash Ridge (Wokingham) sewage works overflow Ash Ridge (Wokingham) STW · TWL00459	Thames Water	18.3	10	<1%	20	74	11	136	100%
Burghfield sewage works overflow Burghfield STW · TWL00064	Thames Water	24.8	14	<1%	132	96	34	655	100%
Silchester sewage works overflow Silchester STW · TWL00370	Thames Water	35.8	20	<1%	112	97	42	760	100%
Cholsey sewage works overflow Cholsey STW · TWL00097	Thames Water	41.4	23	<1%	2	45	4	44	99%
Easthampstead Park sewage works overflow Easthampstead Park STW · TWL00169	Thames Water	24.4	14	<1%	30	42	16	160	98%
Waterloo crescent outfall, wokingham TWL00756	Thames Water	22.2	12	<1%	–	29	7	17	100%
Water End (Newnham) pumping station overflow Water End (Newnham) SPS · TWL00431	Thames Water	42.7	24	<1%	1	2	0	0	100%
Kingsclere sewage works overflow Kingsclere STW · TWL00256	Thames Water	47.5	26	<1%	134	119	49	852	100%
Aldershot sewage works overflow Aldershot STW · TWL00007	Thames Water	53.3	30	<1%	17	20	9	38	100%
South lane, ash TWL00743	Thames Water	53.3	30	<1%	1	36	12	119	100%
Aldermaston sewage works overflow Aldermaston STW · TWL00006	Thames Water	34.9	19	<1%	39	48	31	248	100%
Benson sewage works overflow Benson STW · TWL00034	Thames Water	43.7	24	<1%	12	101	18	361	97%
Chapel Row sewage works overflow Chapel Row STW · TWL00081	Thames Water	37.7	21	<1%	85	112	21	253	95%
Aldershot storm overflow Aldershot CSO · TWL00472	Thames Water	53.3	30	<1%	29	6	2	8	100%
Beenham sewage works overflow Beenham STW · TWL00031	Thames Water	36.0	20	<1%	62	54	14	132	100%
Blind Mans Gate (Highclere) pumping station overflow Blind Mans Gate (Highclere) SPS · TWL00044	Thames Water	58.1	32	<1%	46	45	21	183	100%
Hampstead Norreys sewage works overflow Hampstead Norreys STW · TWL00211	Thames Water	45.6	25	<1%	83	55	33	628	100%
Kintbury sewage works overflow Kintbury STW · TWL00259	Thames Water	58.7	33	<1%	39	56	11	159	100%
Knights Lane (Ball Hill) pumping station overflow Knights Lane (Ball Hill) SPS · TWL00260	Thames Water	58.8	33	<1%	50	59	30	425	100%
Sandhurst sewage works overflow Sandhurst STW · TWL00359	Thames Water	38.9	22	<1%	18	37	10	114	100%
St Stephens Hall (Little London) pumping station overflow St Stephens Hall (Little London) SPS · TWL00319	Thames Water	43.2	24	<1%	60	72	21	18	100%
Briff Lane (Bucklebury) sewage works overflow Briff Lane (Bucklebury) STW · TWL00061	Thames Water	40.2	22	<1%	85	73	23	221	100%
Wash Water sewage works overflow Wash Water STW · TWL00430	Thames Water	55.1	31	<1%	111	67	53	736	100%
Swains Road (Tadley) pumping station overflow Swains Road (Tadley) SPS · TWL00401	Thames Water	38.7	22	<1%	2	2	1	5	97%
West Heath (Charter Alley) pumping station overflow West Heath (Charter Alley) SPS · TWL00437	Thames Water	45.3	25	<1%	56	99	38	547	88%
Camberley sewage works overflow Camberley STW · TWL00070	Thames Water	42.2	24	<1%	26	25	7	69	100%
Wedmans Lane (Rotherwick) pumping station overflow Wedmans Lane (Rotherwick) SPS · TWL00354	Thames Water	36.0	20	<1%	6	11	8	74	100%
Basingstoke sewage works overflow Basingstoke STW · TWL00028	Thames Water	42.2	24	<1%	6	82	40	397	100%
Midgham sewage works overflow Midgham STW · TWL00301	Thames Water	38.5	21	<1%	3	3	1	6	99%
Culham sewage works overflow Culham STW · TWL00147	Thames Water	55.6	31	<1%	32	78	38	378	100%
Elvetham Road (Fleet) pumping station overflow Elvetham Road (Fleet) SPS · TWL00171	Thames Water	41.2	23	<1%	3	9	7	10	78%
Knightsbridge Road, Camberley TWL00627	Thames Water	42.4	24	<1%	2	2	0	0	66%
Avondale Road (Fleet) pumping station overflow Avondale Road (Fleet) SPS · TWL00021	Thames Water	43.5	24	<1%	0	0	0	0	98%
Hamstead Marshall sewage works overflow Hamstead Marshall STW · TWL00212	Thames Water	56.4	31	0%	93	86	42	565	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.