It is the second largest estuary in Cumbria, after the Solway Firth, and contains a wide variety of habitats, protected by a variety of directives to ensure wildlife continues to flourish.

An unusual barrier island, Walney, bounds the south east corner of the estuary, while to the north Hodbarrow sea wall forms an enclosed sea lagoon. Some of this area is owned and managed as a bird reserve by the RSPB.

The estuary contains internationally important numbers of breeding birds, a breeding colony of Sandwich Terns, migratory bird populations of international importance such as Pintail, Knot and Redshank, and a regular gathering of over 20,000 wintering waterfowl.

During severe winters numbers can sometimes reach 70,000 and include Shellduck, Red-breasted Merganser, Oystercatcher, Ringed Plover, Dunlin and Curlew.

It is also a significant place for the rare Natterjack Toad. This amphibian can only be found at 50 sites across the UK, 5 of which are at Duddon and 20% of the national population can be found here.

The estuary is also botanically rich, with a nationally rare shingle vegetation community at Haverigg Haws and North Walney.