Trends in Scottish river water quality

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Under the Harmonised Monitoring Scheme (a sub-set of our monitoring network), SEPA monitors and reports on water quality in 56 Scottish rivers. SEPA and its predecessor organisations have monitored these rivers since the mid 1970s, producing data that provides an invaluable record of environmental status and change in Scotland.

Our analysis of trends in these records supports two main conclusions:

  • Climate change is already having an effect on Scottish rivers, with water temperature increasing in rivers across Scotland and river flow increasing in some places.
  • Stewardship of Scotland’s water environment is working and improvements in water quality have been delivered through environmental regulation, cleaner technologies, improved sewage treatment and changes in agricultural practice.

The maps presented on this website are part of a joint SEPA/Macaulay Land Use Research Institute initiative to produce a dossier on water quality trends for Scottish rivers. This website presents highlights from the full Trends in Scottish river water quality pdf link (12.6mb) report.


Scotland’s rivers are actively monitored by SEPA for a whole range of regulatory and environmental management purposes. The latest status and classification of Scotland’s rivers, which is based on a three year dataset and is reported under the Water Framework Directive, can be found at

The Harmonised Monitoring Scheme (HMS) is an important subset of SEPA’s river monitoring activities. The former Scottish River Purification Boards started collecting HMS data in the mid 1970s. SEPA has continued to conduct this work since its inception in 1996. These records, and the on-going sampling, provide an invaluable record of environmental change in Scotland. Each of the 56 rivers monitored under the HMS is sampled at a location as close as possible to the tidal limit. Samples are collected monthly and water temperature and flow are recorded at the time of sampling. Our scientists analyse these samples for a range of natural and synthetic substances. Daily river flows are also recorded and used to estimate fluxes of materials from the rivers to the sea.

Changing flows can make trends in concentration more difficult to interpret. However, by assessing data on concentrations and fluxes (loads) together, we can obtain a better understanding of environmental change. For example, if river flows increase and the mass of a pollutant entering a river remains the same then the concentration of that pollutant in the river will inevitably decline.


We analysed the data for monthly, seasonal and annual trends. Experience has shown that it is important to look for monthly and seasonal trends because annual figures might not capture all environmental shifts, particularly more subtle shifts. For example, some rivers show increasing spring flows and decreasing summer flows but no trend in annual average flows.

A simple statistical tool known as the Mann-Kendall test was applied to each annual and season time series, with each of the seasons being treated as a distinct dataset for the seasonal analysis (ie summer, spring, winter and autumn data spanning the 30 year monitoring period were each assessed in isolation). The Mann-Kendall test looks for overall increases or decreases over time. While the test is robust, it is not very sophisticated and it does not provide any information about complex time-related patterns in the data, but rather identifies broad trends.

We looked for trends in both physical and chemical parameters. Physical parameters such as river flow, water temperature and suspended sediments are affected by a changing climate. Suspended sediment is also affected by changing agricultural practice and urban sewage treatment processes. For chemical parameters, we looked at Biochemical Oxygen Demand (BOD) as a broad measure of water quality because high levels of BOD are often associated with industrial and sewage pollution. Other chemical parameters we looked at were ammoniacal nitrogen (a form of nitrogen which can come from fertilisers, livestock and sewage) and total phosphorus concentrations. Phosphorus chemistry is quite complicated and high levels of the element can indicate that a river is affected by sewage, industrial activity or agricultural runoff.


Average flow
Water temperature
Suspended solids
Biochemical Oxygen Demand (BOD)
Ammoniacal nitrogen                                                                                                                                                Total phosphorus

Average flow                                                                                                                                                         River flows are changing across Scotland. Annual mean river flows are increasing in many rivers and there are no rivers showing statistically significant declining trends in annual average flow. This suggests that climate change is making Scotland a wetter place. The trends towards increasing spring flows are mostly the result of changing temperatures; warmer springs result in earlier, more rapid snowmelt. In some rivers, the effect is amplified by higher rainfall.

Annual average flow pdf link (372kb)
Winter average flow pdf link (369kb)

Water temperature                                                                                                                                             Annual average temperatures are increasing in many Scottish rivers. This may have important implications for river ecology and salmon behaviour. While some of the trend towards warmer water might be the result of increased urbanization, most is thought to be due to warmer air temperatures. This effect is most pronounced in the winter, when widespread warming has been observed. A cooling trend was observed in the River Wick in the far north of the country, and we are working on understanding what might be causing that.

Annual temperature pdf link (372kb)
Winter temperature pdf link (369kb)

Suspended solids                                                                                                                                                      The amount of suspended solids in water is an important measure of water quality. High levels of suspended solids are associated with soil erosion and sewage effluent. Changes in soil erosion can be caused by changing rainfall, agricultural practice or increased construction activity. Improvements in sewage treatment often lead to reductions in suspended solids concentrations in rivers which run through cities.

Two trends can be seen in the annual concentration of suspended solids in Scotland: concentrations are declining in the centre and south of the country, probably resulting from improvements in sewage treatment and industrial processes, but are typically increasing in the north east, possibly related to increased erosion. This pattern is more obvious when looking at trends in spring concentrations where there are increasing trends in many rural and agricultural catchments.

Annual suspended solids concentration pdf link (372kb)
Spring suspended solids concentration pdf link (372kb)

Biochemical Oxygen Demand (BOD)
Biochemical oxygen demand (BOD) is a measure of the oxygen consumption of a water sample. Typically, the test is run over five days and results are reported as milligrams of oxygen consumed per litre of water. High levels of BOD are an indication of poor water quality and are often associated with urban or industrial pollution.

In Scotland, BOD has declined in almost all rivers. However, this study identified possible increasing trends in BOD concentrations in the Water of Leith and the River Tyne, the underlying causes of which will be investigated further. A similarly positive picture is apparent when looking at trends in BOD flux. The absolute amount of BOD is declining in most Scottish rivers but there are some exceptions, most likely due to changes both in flow and in BOD concentration.

Annual BOD concentration pdf link (372kb)
Annual BOD load pdf link (370kb)

Ammoniacal nitrogen
Ammoniacal nitrogen is associated with sewage and agricultural fertiliser. Concentrations of ammoniacal nitrogen are declining in rivers across Scotland. The Don was the only river identified as having an increasing trend and is the subject of ongoing investigations.

A slightly different picture appears when looking at fluxes: the trends toward increasing flows in many rivers offset some of the reductions in concentration and thus fewer rivers show statistically significant declines in fluxes than is seen for concentrations.

Annual ammonia concentration pdf link (372kb)
Annual ammonia load pdf link (369kb)

Total phosphorus
Phosphorus is an essential macro-nutrient and so is applied as a major component in fertiliser. When phosphorus levels are high, the resulting excessive growth of aquatic plants (phytoplankton and macrophytes) in rivers and lakes can lead to water quality problems such as a large daily variation in dissolved oxygen. Total phosphorus measures the combined concentration of dissolved and particulate-bound phosphorus (e.g. phosphorus bound to suspended sediments).

Total phosphorus concentrations are very low in the north of the country but are higher in agricultural and urban areas. High total phosphorus concentrations in summer can be linked to sewage effluent while high concentrations in the winter and spring are often associated with sediment losses from agricultural catchments. This investigation revealed that concentrations of total phosphorus are declining in many urbanised rivers, an improvement in water quality which appears to have been achieved by improved sewage treatment and reduced use of phosphate-containing detergents. However, the total phosphorus concentration in some northern rivers appears to have increased.

Increases in summer total phosphorus flux were noted in some catchments. The reasons for the increased phosphorus concentrations and flux could be linked to historical patterns of fertiliser use, to increases in the total volume of sewage discharges in northern areas or to climate change.

Annual total phosphorus concentration pdf link (372kb)
Summer total phosphorus load pdf link (373kb)

Next Steps 

This assessment demonstrates the great value of long-term environmental monitoring in identifying and characterising the state and changing state of the environment. It is crucial that we maintain and develop such monitoring and that we do so in conjunction with other science and research providers (such as the Macaulay Institute) and other stakeholders as part of a shared approach to environmental stewardship.

Web Links

Harmonised Monitoring Scheme (HMS) external link
Trends in Scottish river water quality pdf link (12.6mb)