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30 September 2015 Last updated at 18:44 BST
Part of the sea wall at Fingringhoe Wick nature reserve on the Colne Estuary, near Colchester, has been breached to allow sea water to gush in.
The high tides will help to form a new area of salt marshes, mudflats and saline lagoons.
The reserve is already recognised nationally and internationally and is home to thousands of birds.
The 22-hectares (54 acres) of new wetlands include a nursery area for marine fish, little tern nesting areas, a bird hide and public footpaths on what was once a private part of the estuary.
The Essex Wildlife Trust and Environment Agency project will also help to protect the coast as the salt marshes and mudflats will absorb waves and floodwater.
Footage courtesy of Tim Mitchell.
A stretch of land is being deliberately flooded at an Essex nature reserve to create more habitat for wildlife.