The unnamed tributary of the River Chew will contain water for the first time since 1956, when a dam was installed and the valley was flooded to form the Chew Valley Lake reservoir.
The river ecosystem eventually dried up and died, taking with it an ecological corridor used by aquatic creatures since the last ice age.
But the half-a-kilometre stretch of river has now been restored and will provide habitats for wildlife including herons, otters and fish.
The work comes as part of a restoration project by Bristol Water and Bristol Avon Rivers Trust (BART).
Matthew Pitts, catchment strategy manager at Bristol Water, said: “It is the first time the river has been permanently re-wetted since the 1950s and will offer a considerable environmental benefit for the downstream river.”
He added that he hoped to see the stream teeming with mayflies, damselflies and dragonflies.
Some residents living in the area have been apprehensive about the river extension following a devastating flood in 1968 which killed seven people.
During a storm which saw five inches of rain fall within just 24 hours, the waters rose so high that it was feared the dam wall would burst.
Bridges were washed away as torrents of water poured down the Chew Valley, flooding buildings and sweeping away cars.
However Mr Pitts said his team had carried out modelling to reassure people the project would not increase the flood risk.
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