A lost river has returned to the Somerset countryside for the first time in 70 years, and with it a new habitat for several species of rare and threatened wildlife.

The lost reach of the River Chew from the Mendip Hills down to the River Avon was severed by a dam in 1956, when the valley was flooded to form the Chew Valley Lake reservoir that supplies Bristol and Bath.

Downstream, the river ecosystem withered into a dry ditch and died. An important ecological corridor was lost for aquatic creatures that had been journeying along the route since the last ice age. Instead, an occasional spill from the reservoir was diverted into a concrete spillway, bypassing the old riverbed and destroying the central linking piece of the route’s ecological jigsaw.

The water has now been redirected as part of a restoration project by Bristol Water with Bristol Avon Rivers Trust (Bart) and other partners. The river’s revival has included dramatic re-engineering of reservoir overflows, redirecting water along its former path.

The restored route meanders through an abandoned 1950s poplar and alder plantation, once used for matchstick making. Dense thickets of bramble and shady canopy have been opened up to encourage the river wildlife, bringing in light and encouraging aquatic plants, and the channel has been reshaped with berms, riffles and pools so undesirable silt buildup is scoured out by the natural energy of a faster current, opening up fish and invertebrate spawning areas.

The stream is one of three connected tributaries that join the River Avon further down the valley. Vulnerable populations of water vole, eels and white clawed crayfish should be able to weave their way upstream and recolonise the area.

Fish likely to return include salmon, trout, chub, perch, bream, carp, pike, minnow and stickleback.

Otters, kingfishers, barn owls and rare bats already hunt close by. Spring forest floor flowers have reappeared on the river banks. Chattering green woodpeckers and orange-tip butterflies flash through fresh sunlight among primroses, wild garlic and bluebells.

Matthew Pitts, the catchment strategy manager for Bristol Water, hopes to see the stream billowing with clouds of mayflies, damselflies and dragonflies. A fly monitoring project staffed by volunteers will help scrutinise the river’s health.

Facebook Twitter Pinterest It is hoped fish and vulnerable wildlife will recolonise the river. Photograph: Alex Morss

Pitts said: “It is the first time the river has been permanently rewetted since the 1950s and will offer a considerable environmental benefit. We were able to get an immediately visible quick win by moving the river compensation discharge, which means allowing water to return to its former riverbed. We’ve devised a new, more natural flow regime, to benefit the river ecology and instantly created half a kilometre of river. We should see a healthier river and healthier fish populations.”

The river’s return was not initially welcomed by all. Legend has it that the River Chew earned its name from a Celtic word meaning “rushing water” – which served as an ominous warning. During a storm in 1968, the waters rose so high that it was feared the reservoir dam wall would burst and release a torrent into the villages of Chew Magna and Pensford. Police evacuated the valley. Although the dam held fast, torrential rains running off the hills demolished bridges on the Chew, submerged homes and washed away cars.

Pitts said: “We’ve consulted with local people – they still remember the devastating flood in 1968, so there has been apprehension, but we have been able to reassure them, and done modelling to prove we are not going to increase flood risk.”

He added: “The old abstraction licences were based on historical priorities that included providing more water in summer and the needs of lots of mills in the Chew Valley – we had cotton, flour and gunpowder mills and lots of industry. Nowadays we would expect the ecology of the river to be first and foremost.”

• This article was amended on 26 April 2019 to refer to the “lost reach” of the River Chew. A previous version called it “an unnamed tributary”. That description also appeared on the map, now relabelled to refer to the river’s lost path.