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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.

Paramedics were called to The Mitre pub, on Spider Island, in Allenton, at about 15:00 GMT on Sunday.
The 24-year-old man was arrested in the early hours of Monday and is in police custody. The teenager remains in a serious condition in hospital.
"We would urge anyone who might have information that could help us to come forward immediately," police said.

To give some idea of the amount of the currency that represents - there were more than 90 billion banknotes in circulation in India last March.
Most central banks destroy soiled and mutilated banknotes on a regular basis and replace them with new, crisp ones.
The Reserve Bank of India, similarly, shreds such notes and makes briquettes of them. But they are not your usual briquettes.
Briquettes - usually made of farm waste in India - are used for cooking, lighting and heating. They are cheaper than coal, have lower ash content, are less polluting, and easier to store and pack. They are mostly used as fuel in factory boilers.
But briquettes made out of shredded cash are brittle and serve no such purpose, a senior central bank official told me.
So the bank's 27 shredding and briquetting machines in 19 offices across India will now snip the expired banknotes into the smallest of pieces and the resulting briquettes will be then dumped in India's vast landfills.
Sometimes the shredded currency is also recycled to make files, calendars and paper weights and ballpoint pen shells, tea coasters, cups and small trays as souvenirs for guests.
The practice is similar in the US: counterfeit banknotes are sent to the Secret Service, while unfit notes are shredded and sent to landfills or given away as souvenirs to the public touring the Federal Reserve Bank.
Central bank officials believe shredding 20 billion banknotes will not be a huge challenge. In 2015-16 the Reserve Bank of India destroyed more than 16 billion soiled notes. More than 14 billion were removed in 2012-2013 after nearly 500,000 fake notes were found in the system.
"Destroying so much cash is not a challenge because we have enough shredding and briquetting machines with very high capacities. These are automatic machines which shred the cash into the finest of pieces," says an official.  So, India's mountain of expired currency will soon become rubbish, literally.
A stretch of land is being deliberately flooded at an Essex nature reserve to create more habitat for wildlife.
[NEXT_CONCEPT]
A man has been arrested on suspicion of attempted murder after a 16-year-old boy was stabbed outside a pub in Derby.
[NEXT_CONCEPT]
India's central bank will have to destroy, by one estimate, some 20 billion "expired" banknotes after it scrapped two high-value denominations - the 500 ($7.60) and 1,000 rupee notes - this month to crack down on "black money" or illegal cash holdings.