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Live levels from 2 monitoring stations across Hertfordshire, updated every 15 minutes.
| Station | Level now | Status | ||
|---|---|---|---|---|
| Mimmshall Brook | 0.31 mASD 1h ago | Normal | ||
| 1.3 km by river | ||||
| North Mymms | 0.40 mASD 1h ago | Normal | ||
Stations are listed from source to mouth as the water flows, traced on the river map (OS Open Rivers) and the Environment Agency’s own links between its gauges. A distance “by river” is along the river, and is shown only where the map traces the path.
Contains OS data © Crown copyright and database rights 2026.
Streams and rivers with a gauge whose water joins Mimmshall Brook above one of its gauges, from the source down. Named streams without a gauge that have a page here are listed at the end.
No gauged tributary joins Mimmshall Brook above North Mymms on the river map. Streams without a gauge are not counted here.
Below North Mymms, its water next reaches London Colney, a gauge on River Colne, 8.2 km by river downstream.
Tributaries and where they join are traced on the river map (OS Open Rivers, edition 2026-04) and the Environment Agency’s links between its gauges. The map holds distances between gauges, not the junctions themselves, so a junction is placed between two gauges, and no further up than the tributary’s own lowest gauge. Contains OS data © Crown copyright and database rights 2026.
1 storm overflow is traced upstream of North Mymms, Mimmshall Brook’s lowest gauge on the river map: on Mimmshall Brook itself, on its gauged tributaries, and on smaller streams that join above there.
Official figures for 2025, for the overflow traced upstream of North Mymms: 9 spills over 25 hours.
Each overflow is listed in the stretch above the first gauge its water reaches, from the source down, as on the Gauges tab. A stretch of Mimmshall Brook includes the smaller streams that join it there.
A status is the water company’s own monitor: it says whether an overflow is discharging, never how much or why, and a spill does not by itself explain a change in a river level. A status older than two hours is shown as unknown. Annual figures are the official returns: 12/24-hour spills for England and Wales, overflow events for Scotland (Scottish Water counts differently, so the two cannot be compared). “No data” means the monitor produced no usable data that year, not zero. Overflows are placed on OS Open Rivers (edition 2026-04), which leaves out many small channels: an overflow the map cannot place may still drain here. Contains OS data © Crown copyright and database rights 2026. Sewage in Britain’s rivers →
Live status: © Thames Water storm overflow activity data, licensed under CC BY 4.0. Annual figures for England: Environment Agency Event Duration Monitoring return, © Environment Agency copyright and/or database right, Open Government Licence v3.0.
Dated samples taken by the Environment Agency, not live readings. Results are published days to months after the sample is taken, and many points are sampled only a few times a year, or have not been sampled for years.
3 sampling points on the Mimmshall Brook and the streams that join it above North Mymms, and below it. 1 has a sample from the last 12 months; 1 has any sample banked. Listed from the source down, grouped by the next gauge each one's water reaches.
| Sampling point | Watercourse (map) | Above the gauge | Last sample | Latest results (river water) |
|---|---|---|---|---|
| Catharine Bourne at Rabley Park, Shenley | Catharine Bourne | 3.0 km | No sample published 1 Oct 2023 – 7 Oct 2026 |
| Sampling point | Watercourse (map) | Above the gauge | Last sample | Latest results (river water) |
|---|---|---|---|---|
| Potters Bar Brook above Mimmshall Brook | Potters Bar Brook | 1.4 km | No sample published 1 Oct 2023 – 7 Oct 2026 |
| Sampling point | Watercourse (map) | Below the gauge | Last sample | Latest results (river water) |
|---|---|---|---|---|
| Mimmshall Brook at Water End | Mimmshall Brook | 0.2 km | 4 Sep 2026 · 34 days ago | Oxygen 79.6%Temp 17.7 °CpH 7.85Ammonia below 0.03 mg/l |
Distances are along the river to the gauge named. England's samples only.
FloodRadar does not grade these results: there is no overall score, and official river classifications are a separate dataset. A sample taken downstream of an overflow does not by itself show where anything in it came from. River connections and distances are traced on OS Open Rivers (edition 2026-04), which leaves out many small channels.
Water samples: Environment Agency Water Quality Archive, © Environment Agency and/or database right 2026, Open Government Licence v3.0.
Contains OS data © Crown copyright and database rights 2026.
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