Engineers have completed a five-year flood protection project to raise a river bank and protect more than 2,000 homes and agricultural land.
Work to raise the Middle Level Barrier Bank of the Ouse Washes "flood storage reservoir" in the Fens began in 2017 and cost £40m.
It had to be carried out during the summer and early autumn to avoid disturbing breeding wildlife.
The Environment Agency said its priority was protecting communities.
When full, the flood storage reservoir holds enough water to fill Wembley Stadium 22 times.
The stretch is about 19 miles (30km) long and covers an area of about 4,695 acres (1,900 hectares).
Work to install moveable road blocks across the flood-prone A1101 Welney Wash Road, on the Norfolk-Cambridgeshire border, was completed this summer.
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The barrier will prevent water spilling from the reservoir into Welney village, Norfolk, and will replace the need to use large sandbags to create a temporary barrier, the agency said.
It will be installed when the water level in the reservoir reaches a predetermined level, and the road has been closed by Norfolk County Council.
Nicola Oldfield, project executive for the agency, said: "This has been a vital long-term investment and refurbishment programme.
"We know the impact that flooding can have, which is why protecting people and communities is our priority.
"The results of this work will help to protect thousands of homes and businesses in the area for years to come."
A further two years of maintenance on the site will be carried out, the agency added.