Hatfield (Mill Green)
Storm overflow run by Thames Water, discharging into Lee.
"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 Thames Water's monitor log. 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.
Downstream of this overflow
The next gauge downstream that the river map places is Waterhall (River Lee), 6.2 km along the river.
Water-sampling points downstream
The nearest Environment Agency sampling points the river map places below Waterhall. Points between this overflow and that gauge cannot be put in order with it on the map, so they are not listed.
- Lee at Hertford Castle
11 km along the river, below Waterhall · Last sampled 7 Oct 2026 · 19 days ago - Lee Nav at Kings Meads
13 km along the river, below Waterhall · Last sampled 2 Dec 2024 · 23 months ago - Lee above Ware Lock (for New Gauge Intake)
15 km along the river, below Waterhall · Last sampled 7 Aug 2026 · 3 months 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 Thames 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
59 discharge periods since 17 Nov 2022. 59 of them come from Thames Water's own event-duration-monitoring records (from its open data service, Nov 2022 – Aug 2026); this overflow has no live feed, so nothing after that log is held. The official annual count is the figure to quote.
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1 h 45 min
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30 min
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45 min
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30 min
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30 min
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2 h 30 min
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2 h 15 min
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11 h 0 min
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14 h 30 min
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1 d 8 h
Show 30 earlier (of 59)
| Started | Ended | Discharge time |
|---|---|---|
| 13 h 30 min | ||
| 16 h 15 min | ||
| 7 h 30 min | ||
| 6 h 45 min | ||
| 4 h 0 min | ||
| 6 h 0 min | ||
| 30 min | ||
| 11 h 15 min | ||
| 3 h 15 min | ||
| 5 h 30 min | ||
| 2 h 0 min | ||
| 16 h 15 min | ||
| 15 h 45 min | ||
| 8 h 15 min | ||
| 4 h 45 min | ||
| 9 h 30 min | ||
| 7 h 0 min | ||
| 7 h 0 min | ||
| 15 h 45 min | ||
| 30 min | ||
| 6 h 15 min | ||
| 4 h 45 min | ||
| 6 h 30 min | ||
| 1 h 15 min | ||
| 19 h 15 min | ||
| 3 h 15 min | ||
| 5 h 30 min | ||
| 9 h 30 min | ||
| 12 h 0 min | ||
| 45 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 64 raw blocks were merged into 59 continuous discharge periods (gaps under 15 min joined).
Rain nearby last 30 days
Nearest live rain gauge: Rainfall gauge nr Hatfield, 0.6 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
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 Welwyn Hatfield 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.
Annual spill figures: Environment Agency Event Duration Monitoring return, © Environment Agency copyright and/or database right, Open Government Licence (OGL) v3.0. Spill-event history: Thames Water open data (its discharge alerts), © Thames Water Utilities Limited, used under its open data terms (https://data.thameswater.co.uk/s/terms-of-service). Changed by FloodRadar: each Start alert is paired with the Stop that follows it to make one discharge period; monitor-offline alerts are left out. 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.