The river's source is at the west end of Ennerdale Water: it runs west through Ennerdale Bridge where it is joined by Croasdale Beck (flowing from Banna Fell). Ennerdale Water itself is fed by the River Liza.
The Ehen continues past Cleator Moor and Cleator, where it is fed by the River Keekle, moving southwards through Egremont and eventually running parallel to the Irish Sea which it eventually joins at Sellafield at the same point as the River Calder.
The River Ehen supports the largest freshwater pearl mussel population in England. It is the only river in England to be part of the project Pearls in Peril. The project aims to conserve the species and is funded by the European Union's LIFE + NATURE Programme. It includes 21 sites in the UK. The freshwater pearl mussel relies for part of its life on the presence of salmonids, a group of fish including salmon and trout. The river is a breeding ground for Atlantic salmon. There is widespread poaching along the length of the river.
Sellafield has previously used water from the River Ehen for cooling on the site, but ceased this operation in the 1990s. There is a weir on Ennerdale Water, and the valley continues to be an important source of drinking water for customers of United Utilities. The Environment Agency has confirmed that for environmental reasons it will rescind the abstraction licence for Ennerdale. This means United Utilities must stop using it as a source of water by 2025 at the latest. UU has had to find one or more alternative sources of water to supply consumers in West Cumbria.
- "River Ehen". West Cumbria Rivers Trust. Retrieved 2013-10-27.
- < "Natura 2000 Standard Data Form" (PDF). Jncc.defra.gov.uk. Retrieved 2013-10-27.
- "River Ehen - Special Area of Conservation - SAC - Habitats Directive". Jncc.gov.uk. 2010-03-25. Retrieved 2012-07-15.
- "Pearls in Peril". The freshwater blog. BioFresh. 2014. Retrieved 31 January 2015.
- "Vertebrate species: fish". Jncc.gov.uk. Retrieved 2013-10-27.
- "River Ehen Poaching Prosecution". Anglersnet.co.uk. Retrieved 2012-07-15.
- Robinson, Sarah (May 2013). "Plans made to stop using Ennerdale as water source". The Whitehaven News. Retrieved 3 July 2013.
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