The River Ehen is located at Wath Brow. Locally, it is known as Hen Beck. The River Ehen supports the largest freshwater pearl mussel population in England. This may be the last sustainable breeding population.
Exceptionally high densities (greater than 100 m2) are found at some locations, with population estimates for the entire river exceeding 100,000. The conservation importance of the site is further enhanced by the presence of juvenile pearl mussels.
The freshwater pearl mussel (Margaritifera margaritifera) is an extremely long-lived species of mollusc (a 134 year old mussel was found in Estonia in 1993), found in fast flowing rivers and streams across Europe. The pearl mussel produces small, beautiful pearls inside its thick shell which is anchored to the riverbed. However, freshwater pearl mussels are subject to increasing pressure, and their populations across Europe are listed as threatened by the IUCN due to habitat loss, declining water quality and illegal harvesting to provide pearls for jewellery.
The river Ehen was designated a Special Area of Conservation in 2005. The section of the coast into which it flows was designated as the Cumbria Coast Marine Conservation Zone in 2013.