South Thames Estuary and Marshes is a 5,289-hectare (13,070-acre) biological Site of Special Scientific Interest which stretches between Gravesend and the mouth of the River Medway in Kent.

[4] It is part of the Thames Estuary and Marshes Ramsar internationally important wetland site[5] and Special Protection Area under the European Union Directive on the Conservation of Wild Birds.

[6] The site consists of a range of habitats including mudflats, saltmarsh, grazing marsh and stretches of shingle.

Over 20,000 migratory waterfowl use this site in the winter, and some species such as greater white-fronted goose, shelduck, gadwall, teal, northern pintail, shoveler, grey plover, curlew and black-tailed godwit are present in internationally important numbers.

There are also a number of breeding birds including garganey, avocet, northern pintail, bearded reedling, hen harrier, short-eared owl, ruff, common tern and European golden plover.