Hydrological Study of the Upper Clun

Hydrological Investigations of Potential Re-Location Sites for the Freshwater Pearl Mussel in the River Clun (Shropshire)

Natural England commissioned Atkins consultants to develop a better understanding of the key characteristics of the Upper Clun catchment. Based on previous survey work, three potential receptor sites for the relocation of freshwater pearl mussels were identified in the upper Clun; on the River Clun, the Folly Brook and downstream of the Clun-Folly confluence near to Newcastle. This study assesses the viability of the three sites with a particular focus on the flow regime, namely to determine:

  • How ‘flashy’ are the watercourses? Particularly how vulnerable the watercourses are in supporting suitable low flows, and maintenance flows.
  • How quickly do they recover from rainfall?
  • How does land-use influence the flow regime?
  • What are the underlying issues/constraints to the viability of these sites for pearl mussels, and how could these be managed?

The objectives of the study are addressed through a multi-scale evidence based approach, including Hydro-Ecological Flow Thresholds (HEFT) assessments, Hydrogeological characterisation, Land-use risk assessment and a wet weather survey.

The study concludes that the upper Clun is towards the periphery of the ecological niche of pearl mussels. Pearl mussels at the proposed translocation sites would be particularly vulnerable to low flow pressures exerted by the upstream catchment. The characteristics of the lower Clun are broadly similar to those of the upper Clun. However the lower Clun is considered to sit closer to the centre of the ecological niche for pearl mussels because flows here are supported by a higher catchment area and baseflow, ensuring sufficient security of flow (although other adverse sediment and water quality pressures are severely affecting the current population).