The site was designated in 1991 for its biodiversity and varying habitats that support many wader and wildfowl species, as well as the special interest of its marine biology.

[9] The SSSI, due to its habitats, is of international importance for nature conservation, in particular as a wintering site for wildfowl and wader birds.

[10] Mudflats form the lower reaches of the estuary system and are bordered by salt marsh, inundation grassland and rocky shoreline habitats.

The upstream part of the system supports freshwater marsh, fen, rush pasture and reedmarsh habitats, along with wooded valleys in places.

Species include the common reed and the only known population in the British Isles of triangular club-rush, part of the genus Schoenoplectus.