Thornhill Syphon CSO
Storm overflow run by Yorkshire Water, discharging into River Calder.
"Not discharging" means the company's monitor reports no discharge right now — not that the water is clean. The feed publishes status only; no discharge volume is measured.
Year by year
The Environment Agency's official annual returns, counted the EA's way (12/24-hour spills). 2026 is our own running count, the same way, from Yorkshire Water's monitor log and the live feed. The official figure is published next spring.
A spill is the EA's 12/24-hour count: the first 12 hours of a discharge count as one, each further 24 hours with discharge as one more. "Monitor" is the share of that year the monitor was working, so a 0 beside a working monitor is a real zero; "no data" means the monitor produced no usable data that year — not zero. Years before 2024 are matched to this site by name and location, because the EA's site IDs changed that year; a monitor figure beside one of them means the outlet's permit confirms the match.
Twin outlet
Yorkshire Water runs another overflow within 300 m that also discharges into River Calder. They share the sewer and the storm, so one outlet's figures are only part of what reaches the water.
This overflow (Thornhill Dewsbury/2 CSO): 16 spills in 2025. Distances are between the published outlet points. The names in brackets are Yorkshire Water's own, from its return.
Downstream of this overflow
The next gauge downstream that the river map places is Horbury (River Calder), 5.2 km along the river.
Water-sampling points downstream
The nearest Environment Agency sampling points the river map places below Horbury. Points between this overflow and that gauge cannot be put in order with it on the map, so they are not listed.
- Calder at Kirkgate Wakefield Bridge
14 km along the river · Last sampled 4 Mar 2020, before the imported period - Calder at Stanley Ferry
20 km along the river · Last sampled 14 Aug 2026 · 2 months ago - Calder at Methley Bridge
32 km along the river · Last sampled 7 Sep 2026 · 47 days ago
A sample is dated: it is not the river now, and its results are on its own page. A sample taken after a discharge does not show that the discharge caused what was measured. Samples: Environment Agency Water Quality Archive, © Environment Agency and/or database right 2026, Open Government Licence v3.0.
A river gauge measures the level of the river; it does not measure this overflow's discharge, and a spill does not by itself explain a change there. Traced on OS Open Rivers (edition 2026-04), which leaves out many small channels.
Contains OS data © Crown copyright and database rights 2026.
What the 2025 return says
From Yorkshire Water's annual return to the Environment Agency for this overflow.
"High spill threshold not met" means this overflow did not spill often enough to trigger the EA's formal high-spill review for that year. It is not a statement that the spills were harmless.
Discharges we have recorded
15 discharge periods since 21 Dec 2025. 12 of them come from Yorkshire Water's own event-duration-monitoring records (published on Stream, Jan 2026 – Aug 2026); the live feed adds anything after that. The official annual count is the figure to quote.
-
44 min
-
1 h 59 min
-
2 h 30 min
-
3 h 29 min
-
8 h 14 min
-
2 h 0 min
-
1 h 45 min
-
13 h 30 min
-
30 min
-
1 d 6 h
Show 5 earlier
| Started | Ended | Discharge time |
|---|---|---|
| 1 h 45 min | ||
| 1 d 19 h | ||
| 9 h 0 min | ||
| 17 h 58 min | ||
| 3 h 14 min |
The year figures above use the EA's 12/24-hour counting method, so they are comparable with the official figures; the monitor log's 33 raw blocks were merged into 15 continuous discharge periods (gaps under 15 min joined).
What the feed showed
FloodRadar's own record of every change of state Yorkshire Water's live feed has shown for this overflow, kept since : 3 states. The feed publishes its status now and its latest discharge, nothing older; the discharges above are read from this. It is here so the figures can be checked.
Show the last 3 states
| Seen | Status | Since | Latest discharge |
|---|---|---|---|
| Not discharging | → | ||
| Discharging | → no end yet | ||
| Not discharging | → |
Rain nearby last 30 days
Nearest live rain gauge: Rainfall gauge nr Heckmondwike, 4.1 km away. Brown shading = when this overflow was discharging; ▼ = a discharge start.
Overflow shading is a timing match, not proof of cause — heavy rain and spills share a trigger. Times come from the water company’s event-duration monitor (site status only; no discharge volume is published).
Where it is
Tell me when this overflow discharges
One message when the company's monitor reports a new discharge here. At most one every 6 hours.
Setting this alert needs JavaScript. The alerts you already have are on your alerts page.
The alert repeats the monitor's status: whether the overflow is discharging, not how much or why. It can only tell you about a discharge that starts after you turn it on.
Concerned about this overflow?
Report pollution you can see or smell to the regulator straight away; the company handles complaints about the overflow itself.
Write to the MP
The Environment Agency's return places this overflow in the Dewsbury and Batley constituency. An MP can ask the company and the regulator questions on a constituent's behalf. These words carry the official figures; change them as you like.
The link opens Parliament's own "Find your MP", which gives the MP's name and how to write to them. FloodRadar sends nothing and keeps nothing you write. Check the constituency against your own address: an MP acts only for people who live in it.
Live status: © Yorkshire Water storm overflow activity data, licensed under CC BY 4.0. Annual spill figures: Environment Agency Event Duration Monitoring return, © Environment Agency copyright and/or database right, Open Government Licence (OGL) v3.0. Storm-overflow spill events: © the respective water company, published on Stream (data.streamwaterdata.co.uk) under CC BY 4.0. Discharges marked "via Top of the Poops", come from Top of the Poops' record of the same company feed, used under CC BY-SA 4.0: the feed's own start and end times as its pages print them, not their derived hours. Times are shown in UK local time. FloodRadar republishes the monitor's status and the EA's figures; it does not measure discharges itself. Constituency and council area from the EA's return: contains OS data © Crown copyright and database right 2026.