Increasing congestion in the Bosporus, which is the main shipping route for Black Sea states, is the motivating factor behind the canal scheme. In the 1930s, around 3,000 ships, generally 50-m-long, would transit the Bosporus annually. Last year, 43,000 vessels seven times longer on average made the trip, according to government data. Annual transits are forecast to reach 78,000 by 2050 and 86,000 in 2070.

The planned canal would be excavated roughly 30 km west of the Bosporus. Its southern entrance would be in Küçükçekmece Lake exiting into the Black Sea at Karaburun west of Istanbul's third airport by way of the Sazlıdere reservoir.

The plan calls for more than 1 billion cu m of excavation in an environmentally sensitive area that also provides Istanbul with a major water source. It has stirred significant opposition. Because of the Bosporus’s unique position, the canal project also poses potentially serious geopolitical challenges, according to some critics.

Ship movements through the Bosporus are governed by an international treaty called the Montreux Straits Convention, signed by Turkey and other powers in 1936. Essentially, the convention ensures the free movement of commercial ships through the waterway while setting limits on warships of non-Black Sea states.

Since the government asserts that the canal would not be covered by the convention, “what would be Turkey's response if the United States decided to send military vessels to the Black Sea above the limits set by the Montreux Convention and asks for Ankara to agree to send them through the [canal]?” asked former minister Yaşar Yakış, a founding member of Erdoğan's Justice and Development party (AKP).

The Montreux convention also poses potential economic threat to the canal, according to Sinan Ülgen, executive chairman of Istanbul-based think tank EDAM. Since the convention stops Turkey from charging for merchant ship access to the Bosporus, "the new canal can only generate revenues if shipping companies use it—and why would they, while the Bosporus passage remains free?" he asks.

To charge for Bosporus transits, Turkey would need to amend the convention. To do that it, would need approval from at least seven of the 10 states that are party to the convention, says Ülgen.

Despite these doubts, the government seems to be already banking on future canal revenues. Transportation Minister Cahit Turhan recently told state-run Anadolu Agency that “an estimated total of $1 billion per year will be collected from ships passing through Canal Istanbul.” Revenues would rise to $5 billion when canal traffic rises to 50,000 vessels a year in 2035, he projected.

President Erdoğan launched his canal project nine years ago, setting off a series of studies and route evaluations. Of five potential paths for the canal, the government selected the 45-km Küçükçekmece-Sazlıdere-Durusu corridor. With a base width of 275 m, the 20.75-m-deep canal would generate 1.17 billion cu m of excavated material, of which 80% could be used as fill, estimates the government.

Detailed design and construction would probably be under a public-private partnership, take seven years and involve 10,000 workers. The canal could open in 2026, according to the government. But in an early step, it is procuring the small bridge relocation contract that so angered Istanbul's mayor.