e-Digest Statistics about: Inland Water Quality and Use
River Water Quality data for regional and local authority areas in England and Wales
Information about the data
This data provides information about chemical and biological river water quality and nutrient (nitrate and phosphate) concentrations of monitored rivers in England and Wales for Government Office Regions and Local Authorities (covering approximately 40,000km of rivers monitored by the Environment Agency).
- chemical quality: 1990 and 1993-2006
- biological quality: 1990, 1995, 2000, 2002- 2006
- nitrate concentrations: 1990, 1995, 2000-2006
- phosphate concentrations: 1990, 1995, 2000-2006
These data are best estimates of monitored river water quality for regions and local authorities within England and Wales. When looking at the figures, we recommend using the percentage distribution rather than lengths of rivers, as these are the most robust indicators of the quality of monitored rivers within an area. Further details are provided on how the data were calculated below.
It should be borne in mind that river courses and catchment areas often cross local authority boundaries and therefore the quality of some river waters within a local authority area may be affected by factors outside the borders of the authority. The same will be true to a lesser extent for Government Office Regions.
How the data were calculated
The Environment Agency's national database contains chemical and biological river water quality measurements for around 7,000 monitoring sites across England and Wales. Each site represents a stretch of river and information on river stretches such as quality and location (given by National Grid references) is stored in the database. These locations were linked to stretches on a map of the river network that was digitised in 1995. This was then overlaid onto a digitised map of all Local Authority and Regional boundaries. This enabled identification of the intersections between boundaries and river stretches. River lengths on either side of the boundary were assigned to the respective neighbouring authority/region. Results on water quality for each local authority/region were then calculated by aggregating the lengths of individual stretches for each quality category within the area, using the quality information held on the national database.
The percentage figures are a more robust measure of water quality for local areas than river lengths. There are two main reasons for this: incomplete coverage of stretches arising from using the 1995 map, and changes between years in the total length of rivers monitored by the Environment Agency.
The 1995 digitised map provides consistently smaller total river lengths for local areas than the equivalent EA figures. There are two reasons for this. The first is that stretch lengths on the national database were estimated by each EA region 'on the ground', whereas stretch lengths on the map were estimated by the digitising process. The second reason was that it was not possible to link all stretch codes to the map, so some stretches were excluded.
In 1995, the scale of these differences was around 2 per cent of the network. By 1999, this difference had grown (but was still less than 4 per cent) for two reasons. First, the reported river network on the EA's database had increased. Secondly, some changes in the network resulted in losing the key between the stretch on the map and on the database. This meant that some river stretches which were originally included were lost in later years. It should be noted that in earlier years nitrate and phosphate monitoring did not comprehensively cover the network.
It should be noted that the monitoring network only covers stretches of water the Environment Agency are required to monitor, that is rivers and streams with a flow greater than 1m³/second. On this basis 40,000km of river network are monitored in England and Wales out of an estimated total river length of 150,000km.
For further information about the classification systems used in England and Wales see the Inland Water Topic or use the link below:
- e-Digest: Chemical river quality surveys, Biological river quality surveys, Nutrients
- Environment Agency, River Water Quality pages
- Data provided by Government Office Region in Excel workbook
- Summary of all regions [Excel 180kb]
- East Midlands [Excel 630kb]
- East of England [Excel 700kb]
- London [Excel 430kb]
- North East [Excel 350kb]
- North West [Excel 670kb]
- South East [Excel 910kb]
- South West [Excel 670kb]
- Wales [Excel 340kb]
- West Midlands [Excel 540kb]
- Yorkshire and The Humber [Excel 350kb]
Your questions and comments about information presented on this page are welcome. Contact information and Email. Copyright of data and/or information presented or attached in this document may not reside solely with this Department. Please see guidance on Copyright.
Page last modified: 28 August 2007
Page published: 28 August 2007