Bradford Beck is the river that few people have ever seen.
Most of the old watercourse has been culverted and runs underground beneath the buildings of the city centre - including the magnificent City Hall itself.
Yet it once had a prominent, if somewhat detrimental, role in the public life of the growing Victorian city.
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The pollution in the Beck during Bradford's industrial heyday was so noxious that gas could be seen rising from the surface and it was a major talking point among civic leaders determined to clean up the river.
Now, it's almost forgotten about, cared for by a dedicated band of volunteers who fear its condition is deteriorating once again.
The Beck is the Ice Age river that drains the dale that Bradford was built on. There are several tributaries that feed into the Beck, and the name was traditionally used once the river entered the city at Crossley Hall. It eventually joins the River Aire just downstream of Baildon Weir.
As Bradford first began to industrialise, the unculverted small rivers were important as a power source for mills, and the first weirs were built to alter the water flow.
Flooding of the Beck was always a risk, with major floods in 1946, 1947, 1968 and 1982. In the early 1990s, an alleviation scheme was built, with a 1.37km-long tunnel dug 60 metres below ground to carry water from the university area to Queen's Road.
There is little doubt that Bradford Beck was once one of the dirtiest rivers in Britain. In 1867, two million gallons of sewage water were being pumped into the Beck every day.
The nickname 'Mucky Beck' is still used today - by 1826, when industrialisation was in full swing, it was described as a 'sink of filth'. There were no public sewers, and the Beck had been diverted to run into a stagnant canal basin where effluent gathered and festered.
In hot summers, bubbles of gas caused by decaying animal offal would rise to the surface, and the stench was terrible.
In the 1860s, when London's River Thames was becoming notoriously polluted, government officials were also highlighting the state of Bradford's waterways.
They reported that children often managed to set the canal on fire for amusement, so easy was it to ignite the gases emitted.
In 1899 it was decried as 'the most filthy stream in the West Riding'. As recently as 1990, the Beck has received the lowest level of water quality classification.
Much of the pollution was caused by industrial waste. The processing of animal hides and wool creates excess products such as faeces and lanolin, which were often dumped in the river. The animal fats suck oxygen from the water, meaning that no wildlife can live there. In 1899, 18 factories and mills in Bradford were found to be discharging waste straight into one of the becks.
Domestic waste also contributed to the issues - before 1862, there were no sewers in Bradford, although much of the waste collected from privies was recycled as fertiliser or for industrial use.
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By 1870, when the installation of sewers had begun, the thirty miles of pipes discharged directly into the Beck at Frizinghall. Even attempts to treat the sewage proved problematic, as it was difficult to process the industrial by-products. Liquid effluent was not treated, and still made its way into the Aire.
Major treatment works at Esholt were finally completed in 1912, and sewage was re-routed from Frizinghall via an underground pipeline.
The Friends of Bradford's Becks are a volunteer group that formed in 2012 to protect these historic waterways. They monitor the Beck for pollution breaches and have found over 30 dangerous discharges, including waste from a food factory, ochre-tinged pollution from an old mineworking and sewage from a surface drain. The water is often discoloured.
Many of the discharges take place underground, and are difficult to detect. Some of them are attributed to misconnected pipes rather than deliberate acts.
A secret river
Hardly any of the streams and becks can be seen today. They've either been culverted and built over, or 'canalised' between high walls with little public access.
Many Bradford landowners built over the rivers in pursuit of profit, and enclosure was also seen as a convenient way of preventing smells from being released at street level.
The major 19th-century culverting drive has resulted in a maintenance programme that is still ongoing today, at the expense of Bradford Council.
A culvert beneath the Broadway shopping centre was rebuilt in the 1950s in brick and again in concrete 20 years later. The stage of the Odeon cinema was built over the culvert, leading to stories of rats emerging during showings. The Beck had to be re-culverted in 1986 when the Grattan warehouse was built, and in 2015 the Canal Road widening scheme required another 50 metres of water to be covered.
The Beck has entered urban legend - stories abound of foul smells emerging that are caused by rotting animal carcasses dumped in the river during Victorian times.
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Barney Lerner from the Friends group is quick to quash the myth.
"You can frequently smell the Beck under the city - often the smell is bad at Foster Park shopping centre. But it's nothing to do with things being dumped in it 200 years ago, it's all been rebuilt several times since then.
"We've still got a way to go before the Beck is clean again, and we are encouraging people to report incidents of pollution."
A world beneath their feet
The Friends are working hard to raise awareness of the Beck's existence and the role it has played in Bradford's history. They have recently marked the course of the culverted river from the Odeon to Lower Kirkgate and erected plaques at pavement level. They also hope to install a listening point to allow passers-by to hear the water running below.
They are also recruiting 'Beck Buddies' - people who live and work near the river who can help to monitor the health of this 'abused, misused stream'.
Email email@example.com if you are interested in becoming involved with the scheme.