Cleaner Pipes, yet Struggling Rivers – What 25 Years of Data Shows about Welsh rivers

by | Nov 6, 2026 | News

Key pollutants are falling in many Welsh rivers, and effluent discharges, mostly from sewage treatment works, are cleaner than in the early 2000s. But one important measure is heading the wrong way, and river monitoring has fallen sharply.

Nutrient pollution, particularly phosphorus and nitrate from sources such as sewage and farming, is one of the biggest concerns raised for Welsh rivers. But what does the long-term data actually show?

River samples vs effluent samples: what’s the difference?

Effluent samples measure the treated effluent itself, as discharged, mostly from sewage treatment works but also from some industrial and minewater outfalls. River samples are taken from the river itself, and reflect everything upstream: treated effluent, storm overflows, runoff from farmland and roads, and the river’s natural condition. So effluent and river results can tell different stories.

We analysed 25 years of NRW’s water quality records, from 2000 to 2025, to find out what has improved, what hasn’t, and how well our rivers are still being monitored. To make fair comparisons, we only compared sites that were monitored in both the early 2000s and the early 2020s. Results are given as a share of the catchments where there was enough consistent monitoring to test for a trend.

What's Happening in Our Rivers?

Over the 25-year period, river water quality shows improvement across Wales.  Levels of several key nutrients in rivers have fallen in many catchments. Ammonia fell in 8 of the 14 catchments where a trend could be verified and rose in none. Orthophosphate (reactive phosphorus) fell in 7 of 14, again with none rising. Nitrate, measured as total oxidised nitrogen, fell in 6 of 14 and rose in only one, the Dee.  

Biochemical oxygen demand (BOD), a measure of organic pollution, is getting worse in the rivers of five of the seven catchments where a trend could be verified: the Cleddau, Dee, Teifi, Usk and Wye. None are improving. These include some of our most protected rivers, designated as Special Areas of Conservation.

Other measures have changed less. Suspended solids in rivers are largely unchanged, improving in one of ten verified catchments and worsening in one.

Improvement is clearest where pollution is regulated and measured at its source such as at the outflow from sewage treatment works. Rivers reflect everything happening upstream, and they are now sampled far less often than they once were, which makes their condition harder to track.

Effluent Discharges

Effluent discharges, mostly from sewage treatment works but also including industrial discharges, are measurably cleaner than in the early 2000s, although they still affect the rivers they flow into. Suspended solids fell in 13 of 14 verified catchments, and ammonia in 9 of 14.

But, the picture is less clear for other measures. BOD in effluent improved in four catchments but worsened in three. For phosphorus and nitrate, there is too little consistently monitored data in NRW’s records for us to test for trends robustly.

Comparing recent river samples (2021 to 2025) from above and below the same sewage works shows this local effect. Ammonia and orthophosphate were higher downstream at all eight works with enough data, typically one and a half to two and a half times higher. Nitrate was higher downstream at seven of the eight. At none of them was BOD sampled on both sides.

Overall, this shows that water company investment is making a measurable improvement to water quality as standards tighten for wastewater treatment.  

However, as the first joint report from the water regulators, published in October 2026, shows Dwr Cymru Welsh Water still has further significant improvements to make particularly on serious pollution incidents.

River Monitoring has Fallen Sharply

The reductions have not fallen evenly across either water quality parameter or geographical area. Even in designated catchments, significant declines in monitoring have occurred (as an example, for BOD, the Dee lost 89% of its sites between 2006–10 and 2021–25 and the Teifi 75%).

The map shows where river water was sampled for BOD and for ammonia between 2021 and 2025: 288 sites for BOD, compared with 1,025 for ammonia.

The falls coincide with NRW’s formation in 2013 and the pandemic in 2020. NRW’s funding from Welsh Government has fallen by around 30% in real terms since it was created, and NRW says its monitoring resources have reduced, reflecting public sector spending reductions.

Can we really understand what the pressures are, or measure improvement, with monitoring at this level?

Looking Ahead

The Welsh Government’s Programme for Government sets out an ambition for every water body in Wales to be on a clear trajectory towards good status. Showing that progress will depend on monitoring that can measure it.

So, whilst water company performance remains poor, what is clear is that significant investment has made a difference to overall river water quality. The 25-year data shows without doubt that focus on the water industry alone will not, however, fix our rivers.

Water regulation in Wales is under review following the Independent Water Commission, and the Welsh Government has begun work on how water quality improvement will be planned in future. As that develops, we think a few principles matter most:

  • The Water Framework Directive has not worked – we will have more on that this month – and  Wales really needs to set a new path if we are to deliver change.
  • Strengthening river monitoring, including better coverage of pressures from land.
  • Shared accountability for reducing pollutants across all the bodies responsible for impacts on our rivers.
  • Integrated catchment management to ensure that funding and outcomes are prioritised.

Rivers Trusts are Part of the Solution

Wales’s rivers trusts work on the ground across Wales, delivering river restoration and gathering evidence alongside communities and citizen scientists. As the next framework takes shape, we want to help make sure it is built on good evidence and delivers real change for Welsh rivers.

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