Data note

What the English river water quality record can and cannot show

Hydrionis holds annual chemistry for thousands of English river monitoring points, computed from millions of laboratory results. It does not publish a national trend line from them, and this note sets out why: the archive begins after the improvement most people want to see, sampling frequency has fallen unevenly, and a large share of results for some determinands sit below the laboratory detection limit.

2026-08-24Version 1.0

5,214

Monitoring points held

758,883

Annual values

2000–2025

Period

61%

Fall in points sampled

The network has shrunk

English river monitoring points contributing an annual value, by year

Counted from the values Hydrionis holds, each of which required the sample floor set for its determinand — eight samples in the year for the monthly-sampled nutrients, four for the quarterly-sampled metals.

View the data behind this chart

Underlying data
Period	Monitoring points with a published annual value (points)
2000	4,814
2001	4,308
2002	4,832
2003	4,872
2004	4,894
2005	4,806
2006	4,808
2007	4,091
2008	4,020
2009	3,877
2010	3,522
2011	3,764
2012	3,859
2013	3,904
2014	3,357
2015	2,920
2016	2,810
2017	1,939
2018	1,845
2019	2,792
2020	202
2021	1,560
2022	1,455
2023	1,904
2024	1,893
2025	1,877

Source: Environment Agency Water Quality Archive, aggregated by Hydrionis.

In 2000 Hydrionis holds annual values for 4,814 monitoring points. In 2025 it holds 1,877 — a fall of about 61%. The lowest year in the record is 2020 at 202 points, when routine sampling was suspended.

Why there is no national trend line here

Three reasons, each sufficient on its own

The archive starts too late. The public interface begins in 2000. The large improvement in English river chemistry — the collapse in ammonia and biochemical oxygen demand that followed the Urban Waste Water Treatment Directive — largely predates that, so the thing most people want to see is outside the window.
The panel is not constant. With the number of points sampled falling by roughly 61%, a national average computed year by year would partly measure which points were still being visited rather than what was in the rivers. Restricting to points with a complete record leaves too few to be representative.
Much of the signal is below the detection limit. For ammoniacal nitrogen, a large share of results are reported as less than the laboratory limit. Those are not missing and they are not the limit either, and a mean taken over the detected minority reads as pollution that is not there.

What the data does support is per-point analysis: how one river reach has changed, against its own record, with the sampling density visible. That is what the monitoring index publishes.

Coverage by determinand

Determinand	Points	Annual values
`ph`	4,637	71,936
`water-temperature`	4,633	71,560
`dissolved-oxygen-saturation`	4,470	68,150
`total-oxidised-nitrogen`	4,502	67,902
`dissolved-oxygen`	4,244	63,250
`orthophosphate-as-p`	3,877	57,362
`nitrate-as-n`	3,923	57,328
`nitrite-as-n`	3,576	54,595
`ammoniacal-nitrogen`	3,003	44,369
`copper-dissolved`	2,234	31,536
`alkalinity`	2,347	30,121
`suspended-solids`	2,075	28,520
`chloride-river`	1,353	21,050
`conductivity`	1,586	20,681
`bod-5-day`	1,291	17,185
`iron-dissolved`	565	9,277
`total-phosphorus-as-p`	342	6,503
`turbidity`	373	5,986
`dissolved-organic-carbon`	359	5,424
`zinc-dissolved`	332	5,160
`nickel-dissolved`	335	4,744
`manganese-dissolved`	128	2,524
`lead-dissolved`	153	2,088
`arsenic-dissolved`	100	1,724
`sodium-dissolved`	136	1,634
`cadmium-dissolved`	125	1,627
`potassium-dissolved`	135	1,619
`sulphate-river`	107	1,278
`calcium-dissolved`	92	1,249
`chromium-dissolved`	80	1,179
`magnesium-dissolved`	70	988
`aluminium-dissolved`	22	307
`mercury-dissolved`	2	27

Data behind this

England River Water Quality Dataset v1.0.0 · 748,160 observations

Every figure above is computed from these datasets rather than written into the text. If the underlying record changes, this page changes with it.

Citation

Hydrionis (2026), What the English river water quality record can and cannot show, version 1.0. Retrieved from https://hydrionis.com/research/what-english-river-quality-data-can-show/