Port installations, rubbish dumps and industry have left much of the estuary an eyesore and during the postwar years many of its traditional, wildlife-rich grazing marshes have been ploughed up for cereals.
But, says the society, it remains Britain's most important estuary for birds. Nearly 300,000 wading birds and wildfowl spend the winter on its mudflats and marshes and many more stop there to feed and rest during their migrations between Arctic breeding grounds and warmer climes.
The report urges Michael Howard, Secretary of State for the Environment, to bring together local and central government to draw up a strategy for its future, including riverside councils, the National Rivers Authority and English Nature, the Government's nature conservation arm.
The RSPB and other environmental organisations have been worried by the East Thames Corridor initiative - plans for large-scale housing, leisure and industrial development along both banks of the estuary promoted enthusiastically by Michael Heseltine, Mr Howard's predecessor at Environment.
They fear that once economic growth resumes, a building boom east of London Docklands will destroy large areas of remaining natural habitat, along with large transport developments like a new motorway crossing - under consideration by the Department of Transport - and the Channel tunnel rail link.
Rising sea levels also threaten the estuary's saltmarshes. The eastern coast is sinking slowly and while in the past the marshes could have moved gradually inland, sea walls now prevent this.
The conservationists won a victory in July when Mr Howard decided to reject plans for a large development on the Isle of Sheppey on the Kent side of the estuary. The plans by Lionhope (Kent) were for housing and leisure facilities in Minster and Queenborough, a business park and extension of the docks in Sheerness and a business and industrial park nearby at Ridham.
The developer has now gone into receivership and the receiver is challenging Mr Howard's decision, made after a public inquiry, in the courts.
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