Both SSSI citations provide detail of the geological and biological interest and of particular note is the international importance for wintering and wading birds of passage, and of estuarine habits of outstanding ornithological significance. It is stated that the estuary supports over 10% of the British wintering population and is the single most important wintering ground for dunlin, and for significant numbers of Bewick's swans, European white-fronted geese and wigeon. Nationally important wintering populations are supported such as gadwall, shoveller and pochard.