Sheffield residents got a sudden reminder of the existence of hidden rivers under the city centre when a hole appeared in Decathlon's car park earlier this year.
The roof of a culvert covering the River Porter had collapsed, leaving the waterway exposed.
It was an unexpected but timely look at part of the network of streams and rivers that runs under many of Sheffield's landmarks.
Sheffield Council is carrying out a programme of 'renaturalisation' of the city's hidden rivers in order to create attractive new features in the city.
Head of city regeneration and chairman of the Sheffield Waterways Strategy Group Simon Ogden said the discovery in Decathlon's car park pointed a way forward for Sheffield's 'mysterious, fascinating but often hidden waterways'.
He said: "The hole had actually appeared due to the collapse of a concrete slab culvert over the River Porter, probably built to support a factory.
"As we’ve seen elsewhere in the city, culverts created for such purposes only have a certain shelf-life (this one seems to have been built in the 1940s) and many are now reaching their structural sell-by date.
"But, stare a little bit deeper into this now de-culverted section of the Porter, and you can see Sheffield’s history up close, but also, get a tantalising idea of its future. This is the same river which forms the focus of the much loved Endcliffe, Bingham and Forge Dam parks.
"How many people have stood on Platform 1 at Sheffield railway station, not realising that Sheffield’s Porter Brook joins with the flow of the RiverSheaf in the darkness below, elevated on massive Victorian stone tunnels?"
Both the Sheaf and Porter rivers flow in culverts all the way to the River Don at Castlegate, apart from a brief section next to the Digital Campus in Sheaf Street.
They also pass through the Megatron – a culvert so large and impressive, it has featured on a list if the most famous caves in the UK.
Mr Ogden said: "Culverts were constructed at a time when most of our urban rivers were dirty, noxious and devoid of life. At the time this was seen as an improvement.
"Along the 10km of the Porter Brook, there are about 1km of culverts, mostly in the city centre where the latest sinkhole has appeared. This means, incredibly to some, that almost 10 per cent of the river is hidden."
The Porter runs through Sheffield's Cultural Industries Quarter, an area that was once at the heart of Sheffield’s steel and industrial beginnings.
It once boasted flour mills, cutlery, steel and lead factories but is now being reinvented as a place of education, innovation and city living.
Waterways appear in short sections, and Mr Ogden said they were 'acquiring a new significance' as a restored wildlife habitat - 'an important element of preparation for climate change and as an attraction for investment'.
He added: "So whilst it is understood that Decathlon’s first priority will be to get their store and car park back in safe operation, members of the Waterways Strategy Group, which I chair, would be keen to talk with them about options for repair which make more of this ‘hidden river’ for people and wildlife and also improve flood resilience.
"This could include elements of re-naturalising, access for fishing or kayaking, and light wells for fish passage."
Evidence of the council's work so far can be seen at a pocket park around the corner from Decathlon, off Sidney Street. Working with the Environment Agency and the Wild Trout Trust, the authority has created a relaxing space down by the water.
Mr Ogden said: "People can paddle in the Porter, and trout have been frequently spotted in its waters. Students at the new Shoreham St University Technical College are among its most frequent users.
"This followed the removal of a similar collapsing culvert over the Porter, which has shown how the river bank can be restored to become a pleasant park and eventually a green corridor with reduced flood risk.
"It’s been a remarkable transformation, and an award-winning one too - last year, the Porter Brook park won the ‘contribution to the built environment’ prize at the 2016 Living Waterways Awards, organised by the Canal and River Trust - and which will eventually form part of a city centre walk which extends from the station to St Mary’s.
"But it has also added significant value to the site next to it ."
The park, and a similar one along the banks of the River Don in Nursery Street, was created through the council's city centre breathing spaces programme, using money contributed under planning rules by developers as well as from the Environment Agency..
Further downstream, the council and Environment Agency plan to restore more of the Porter near the station and open up the River Sheaf in Castlegate.
Mr Ogden said: "The proposal - called ‘Putting the Sheaf back into Sheffield and recently featured on BBC’s One Show - involves taking the roof off an underground culvert and bringing the waterway back into the open, surrounded by grass, flowers, trees and seating.
"Beyond that the progressive reclamation of the River Don is also continuing and the return of the North Atlantic Salmon to spawn in the Don and its tributaries is now only a few years away thanks to the Living Don Project.
"The long-standing ambition of the Waterways Strategy Group of reconnecting the city centre with its waterways – the canal, and Rivers Don, Porter, and Sheaf, is now achievable.
"Every unexpected opportunity should be seized and made most of."
Simon Ogden’s top three hidden Sheffield rivers
1. The Lower Sheaf – 'glorious in Millhouses Park it largely disappears from view from Granville Square'.
2. The lower Porter Brook – 'the jewel of the Porter Valley Parks becomes an invisible drain below Frog Walk but has been revealed again at Matilda Street'.
3. The Car Brook – 'flows from Manor Top to the Don at Brightside Weir through the Woodthorpe and Carbrook Ravines and Bowden Housesteads Wood before disappearing under Darnall and Attercliffe'.
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