Threats & issues facing the rivers of Wales
Sewage
This map shows where the sewerage network discharges treated sewage and overflows of untreated sewage and storm water into our catchment in 2021.
Releasing sewage into rivers introduces a large amount of nutrients into the water. This process, known as eutrophication, can have serious consequences.
When sewage is released into rivers, it contains a lot of nitrogen and phosphorous. These can stimulate the growth of algae, such as phytoplankton in the water or mats known as biofilms in the sediments, which can lead to algal blooms.
'A large amount of algae has an impact on the rest of the ecosystem, such as blocking out the light that plants need for photosynthesis. When these plants, and the algae itself, start to die, they are eaten by bacteria which reduces oxygen in the water, killing fish and other organisms.
Insects are like a canary in a coal mine
One of the groups which is most badly affected by a lack of oxygen are freshwater insects, many of which spend large periods of their development in rivers.
'Most aquatic invertebrates in a river depend on high oxygen levels because most don't have the option to swim to the surface as they'll be swept away. When this oxygen disappears because of sewage, then the species disappear too.
'The most affected insects are stoneflies and mayflies as they don't survive well without high oxygen. In turn, this can have a knock-on impact on fish. While fish can swim away from an area of sewage contamination, they can't escape if their prey has disappeared.
Agricultural Pollution
The majority of our rivers flow through farmland and have been affected by serious agricultural pollution. This has taken the form of both what is called “spot” or “point source” pollution (where a pollution event is traceable to one source) and “diffuse pollution”.
Diffuse pollution affects the water quality of a river or a whole catchment but is not necessarily traceable to a single point. It is often more serious because it is harder to detect and prevent, and punctuated with more significant incidents. In fact, those responsible are often unaware that they are causing a problem as the impact of their activity alone does not cause any significant damage. Combined, however, it has a disastrous impact on fish and all other wildlife. Diffuse pollution from agriculture comes mostly in the form of phosphates and pesticides, usually attached to soil particles.
Point source events can also occur from agriculture that can be catastrophic for rivers and fish. Slurry entering the river is often the cause, stripping the water of oxygen, suffocating the fish and other aquatic creatures. There are less regulatory controls around slurry in Wales compared to England.
Eutrophocation
The process of eutrophication (nutrient enrichment) may have both direct and indirect effects on fish populations and the latter may be particularly dramatic when combined with the introduction of alien species.
Acidification
Acidification can come from several sources. Forest practices and felling, exposed peatlands and exposed soils washed in to rivers across our catchment. Acidifaication is also a huge issues win the upland areas.
A study from the University of Cardiff in Wales reveals that the after-effects of acid rain in the 1970s and 1980s can still be felt today.
Acid rain occurs when sulphur dioxide and nitrogen oxides are emitted into the atmosphere, for instance by burning fossil fuels in energy generation and industry. They are absorbed by water droplets in clouds. When the droplets, which display an unusually low potential of hydrogen (pH), fall to earth, this increases acidity of the soil and affects the chemical balance of lakes and streams.
In the 1970s and 1980s, acid rain was one of the world's worst pollution problems, affecting large areas of upland Britain, as well as the rest of Europe and North America. In Wales, more than 12,000 kilometres of streams and rivers have been acidified. The acidification has harmed fish, insects and river birds.
Environment Agency Wales surveys of spawning streams in the Afon Conwy catchment have declined from an annual survey down to one every five/six years and evidence pointed to a decline of fish stocks in the upper Conwy both of wild brown trout and migratory salmonids. This is despite the commissioning of the fish pass at the Conwy falls and in 2010 the Trust decided to investigate the reasons for this apparent decline in the fish population.
