Afon Eden

children waving
Children in the classroom
Afon Eden Habitat
Afon Eden Catchment

The Site

The Afon Eden – Cors in Meirionydd, Gwynedd is designated as an Site of Special Scientific Interest (SSSI) and Special Area of Conservation (SAC). It’s home to several important ecological features such as floating water-plantain, bog-rosemary, water sedge, rare lichen assemblage, atlantic salmon, large heath butterfly, otter, and the most important being the colony of Margaritifera margaritifera Freshwater Pearl Mussels. The Afon Eden is a relatively unmodified river of approximately 12km of length. The river is mainly upland in character and it’s very diverse. The upper reaches are constituted of mainly raised bogs, transition mires and quaking bogs. The land use is dominated by low intensity agriculture.

Pearls in Peril in Wales

The Afon Eden’s substantial FWPM colony was discovered in 1997 by Killeen & Oliver (1997) and it supported ~1300 mussels. By 2011 it supports ~550, less than half what it did in 1997. These are just estimates as many mussels can be completely buried in the river gravel and therefore cannot be counted. This declining trend is seen across Europe.  The PIP project aims to mitigate against this through undertaking in-situ works and creating a more viable habitat for recruitment.

Improving woodland habitat

Natural Resourcse Wales's (NRW) Brynteg forestry block is part of the action to improve the riparian habitat (habitat that borders watercourses). The site is known to affect the natural hydrology of the catchment and is a source of acidification and run-off of nutrients and sediment into the river. It is also a key source of anthropogenic phosphate, which is released from the exposed peat soils after intensive tree felling operations.  To address the issues PIP is restoring the intensive conifer forestry into an area of native broadleaf, wet woodland and raised bog plant communities.  We will also be ditch blocking and creating settlement ponds on the tributaries of the Eden thus trapping silt and improving the water quality.

Implement in-stream restoration works

In some parts of the Afon Eden human intervention and management has changed the natural movement of river sediments. Historically, boulders and gravel were dredged from sections of the river leaving unsuitable habitat for pearl mussels. Part of the in-situ work will include introducing local clean gravel as well as large boulders and woody debris. This will re-create diversity in flow patterns, so that areas of natural gravel deposition will develop. In turn these will provide suitable habitat for juvenile pearl mussels and spawning salmonids.

Reduce nutrient and sediment input

Good water quality is crucial in maintaining the features of interest on the Eden. Diffuse pollution is known to have a direct significant negative effect on the FWPM in the Afon Eden due to it reducing water quality. The incidence of diffuse pollution in rivers is causing many of the tributary streams to fail to meet good ecological status under the water framework directive. Some land management activities and drainage from the A470 trunk road contribute to the diffuse pollution issues affecting the Afon Eden. The trunk road supplies concentration of heavy metals, particulates and NaCl from road gritting in icy conditions. In partnership with the North and Mid Wales Trunk Road Authority (NMWTRA) PIP will be working with landowners, farmers and other stakeholders  to create filtration systems that will reduce diffuse pollution flowing into the Eden.


An important part of the PIP project is to implement a structured programme of monitoring surveys. These will allow us to establish the existing conditions at the start of the project and indicate the successes of the project’s actions. We will identify the abundance of host fish at specific sites and relate this to the presence of the adult pearl mussels on the river. This analysis of the relationship between the abundance of fish and the presence of FWPM will make the data more useful for targeting the conservation actions.

Water quality monitoring is a key part of the project. Sonde data loggers will be used on many locations within the catchment and will provide invaluable data on the variation in water quality parameters along the river. The data will be very important in identifying sources of diffuse pollution and excess nutrients which can then be followed up with targeted remediation measures. Salmonid encystment levels (the number of juvenile FWPM attached to the gills of salmon and trout) will also be monitored and this data will help us understand the functioning of the FWPM colony.  Combined all of the monitoring data will contribute to the restoration of habitat and inform conservation actions to secure the populations long-term future.

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