Water levels at Chasewater are set to rise after two years of repairs.
The 200-year-old reservoir was drained in early 2010 after concerns over the safety of the dam, which dates back to 1797.
The £5.5million project was started by Lichfield District Council before being taken over by Staffordshire County Council.
“This has been a major engineering project that has been carried through to a swift and successful conclusion by the county council,” said Cllr Mark Winnington. “I am now delighted to be able to confirm that the plug is back in and the reservoir will start to refill naturally with water.”
The original role of the reservoir was to regulate water in the Midlands canal network, but fears had grown that the earth dam was no longer safe to withstand major floods, was leaking, and could pose a danger to nearby homes.
Cllr Winnington added: “We now have a much better understanding of the dam. Overflow precautions have been improved to safeguard nearby residents in the event of severe flooding, and we will be able to monitor the condition of the dam much more closely in the future.
“The drawdown culvert – the equivalent of the plughole in a bath – was located and inspected for the first time in over 200 years. Vital improvements have now been made. A mystery brick-built chamber that does not appear on any plans was also discovered inside the dam, and will now be used as part of our monitoring procedures.”
It is hoped the reservoir should have refilled fully by summer 2013.