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Proposed route slices through a major drinking water source and an important stop for migratory birds

To the west of Istanbul, between the Black Sea and the Marmara Sea, lie diverse landscapes of forest, farms, marshes and ancient settlements. Following a curving line from north to south would connect you through the Terkos Lake, Sazlıdere stream and reservoir and Küçükçekmece lagoon, important water sources for migratory birds – not to mention the city’s residents.

This line, however, is also the path of Kanal İstanbul, a $12.6bn(£9.7bn) mega-infrastructure plan described by President Recep Tayyip Erdoğan as his “crazy project” when he first mentioned the idea in 2011.

Kanal İstanbul, approved by the environment ministry last month, would be a 45km (28 mile) shipping canal joining the Black Sea to the Marmara, running parallel to the Bosphorus strait, which already cuts through the centre of Istanbul. The government says the canal is needed to reduce water traffic through the city.

Critics – who include the mayor of Istanbul – warn of a great environmental cost, including the potential ecological destruction of the Marmara Sea. There is a delicate ecological balance between the Black Sea and the Marmara Sea, currently connected by the Bosphorus strait.

“The salinity of Black Sea is less than Marmara Sea, and the organic content of Black Sea is much higher than that of Marmara Sea,” says Cevahir Efe Akçelik, Istanbul secretary of the Union of Chambers of Turkish Engineers and Architects.

Because the Black Sea is 50cm higher than the Marmara, both the level and salinity density would change if the two were connected by the canal.

Not only that, the flow of cellular organisms into the Marmara would eat up extra oxygen.

Facebook Twitter Pinterest The canal will run through Küçükçekmece lagoon, home to hundreds of species of birds. Photograph: Ahmet Bolat/Anadolu

The loss of oxygen could see bacteria and other organisms create a sulphurous gas, the scent of which could spread across Istanbul like rotten eggs. And there are fears it could wipe out life in the Marmara entirely.

“Some oceanographers say 30 years later there will be no oxygen left in the Marmara Sea. It’s a really harmful and dangerous project.” said Akçelik

One section also directly threatens Istanbul’s water supply. Terkos Lake and Sazlıdere reservoir provide a quarter of the city’s drinking water; the city has drawn water from the lake since the Roman period and it has always been regarded symbolically as Istanbul’s main water source.

Should those reserves be lost, Akçelik says, there is no alternative water source on the European side of Istanbul. Instead the government would have to pump water from the Sakarya River, deep on the Asian side.

That would be much more expensive. It also carries health hazards, given the river passes through Turkey’s capital, Ankara, and through polluted industrial areas to reach the Black Sea.

The canal would also destroy Küçükçekmece lagoon, which in turn could see hundreds of different species vanish, scientists warn. The lagoon is an important stop for migratory birds, particularly the Cetti’s warbler, grey heron, eastern imperial eagle and the flamingo, who all lay their eggs in Küçükçekmece.

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“If you lose that lake, you lose not only a drinking water source, but a really important ecology,” Akçelik says.

The Turkish government finalised the Environmental Impact Assessment (EIA) for Kanal Istanbul, declaring it did not foresee any issues with the project.

The ministry of environment and urbanisation opened the project to public consultation. More than 70,000 people signed a petition against it.

But in January the ministry approved approved it anyway.

Facebook Twitter Pinterest The plans have sparked protests and a petition signed by more than 70,000 people. The banner reads: ‘Either canal, or Istanbul’. Photograph: Lefteris Pitarakis/AP

The mayor, Ekrem İmamoğlu, who is affiliated with Turkey’s main opposition party, the Republican People’s Party (CHP), has been at loggerheads over the project with Erdoğan’s ruling Justice and Development Party (AKP).

İmamoğlu has started to campaign publicly against the canal, and the municipality is taking legal action to halt construction.

According to the head of the zoning and urbanisation department, Gürkan Akgün, the objections made by public institutions, non-governmental organisations and residents have been ignored.

“The north end of Istanbul is the life support system for all of us,” he says. “It all will disappear irreversibly with this project.”

He argues that the ministry’s report contains “erroneous assumptions and insufficient analytical surveys” and says the city is appealing the decision.

Greenpeace has submitted an appeal of its own.

“In many respects, Istanbul, one of the most important and unique cities in the world, is going to be thrown into disaster with this unpredictable project,” says Greenpeace’s Mediterranean program director, Deniz Bayram.

The national government and the city’s ministry for environment and urbanisation did not respond to requests for comment.

Erdoğan has said there is no alternative to the Kanal İstanbul project for limiting traffic through the Bosphorus, and claimed all appropriate environmental and technical surveys have been undertaken.

The Bosphorus strait is an internationally important shipping route. Around 48,000 ships pass through its waters annually, including oil tankers and war ships. The government claims there are risks posed by ships carrying dangerous goods.

Critics note, however, that fewer ships have been using the strait of late. “In the last 10 years, traffic decreased by 10%,” Akçelik says.

Facebook Twitter Pinterest Drilling rigs and war ships are among the 48,000 vessels that pass through the Bosphorus strait each year. Photograph: Muhammed Enes Yildirim/Anadolu

The construction of new petroleum and natural gas pipelines between countries has meant less reliance on ships and tankers to transport resources.

Accidents have been falling as well, according to municipal officials, who point to improvements in maritime transport and shipping sector technology. “For many years, we have not encountered a serious accident in the Bosphorus and I hope we will not in the future,” Akgün says.

Akçelik says he believes the true long-term aim of the canal, rather than easing shipping traffic, is to create a major infrastructure project north of the city to spur development.

“The main idea is creating a new city in northern Istanbul,” Akçelik explained.

Akgün agrees and says he believes the new works will significantly disrupt the area’s climate. “The construction areas to be formed around it will transform into an urban heat island, changing the temperature, humidity, evaporation and wind regimes in a very short time,” he says.

“Considering all these points, it becomes clearer what kind of an environmental disaster we are facing.”