In the case of the Sea of Marmara and the Black Sea, the dense bottom Mediterranean Sea waters operated the same way they did through Gibraltar; however, the deep channel leading from the Bosporus between the Sea of Marmara and Black Sea did not extend all the way. Instead it extended about half way from the coastline of Turkey into the Black Sea across the continental shelf, then it took a 90 degree turn to the north-northeast and grew shallower until ending abruptly a great distance from the deep waters of the Black Sea. This meant a Soviet submarine hugging the bottom to stay silent and rift with the current was at considerable risk of collision with the sharp bend in the course of the canyon. Even if they successfully managed to negotiate the turn , they would have to use some power maneuver to do so safely giving away their location to any listening NATO sonar personnel. Even worse, because the canyon swallowed at the turn and kept doing so, they would be forced up into the mixed water layer well short of the deep Black Sea basin and again be forced to use engine power to maneuver.
Therefore Stalin concluded the only solution was to extend the channel from the point of the turn to continue in the same direction all the way to the edge of the continental shelf, deep enough for his submarines to maintain a bottom hugging depth in the strong current flow all the way to the deep Black Sea basin. There existed a 70 meter depth sill, a high spot, in the Dardanelles passage and even worse near the beginning of the Bosporus there was an even shallower 40 meter sill. However , neither was blocking the USSR submarines, when skillfully operated, from sneaking through the passages themselves hugging the bottom. This was a risky maneuver particularly in the Bosporus because it had several sharp curves as it passed from the Sea of Marmara to the underwater canyon in the Black Sea continental shelf. Even so the best submarine handlers had managed to prove it was possible right up to the sharp turn in the canyon already mentioned. Therefore the initial goal of the mega project was to create a passage from the sharp bend in the canyon to the nearest edge of the continental shelf and the deep Black Sea basin.
To carry out the project without being too obvious to NATO observers, a half a dozen Soviet fishing trawlers were fitted with modified drag nets. The new 'nets' were designed so that the ship could deploy them, dragging over the sea floor with thousands of tiny scoops that would loosed and drag soil and rocks, even moderate size boulders. Each night the trawlers would head out, and, once the ship had completed its passage across the continental shelf, large debris like boulders and rocks as small as a baked potato would simply tumble over the edge into the deeper basin. After they had passed several miles into the deep basin , the trawler would slow to steerage speed. This would cause the 'net' to change angle from nearly horizontal to nearly vertical and the water flow over the small 'scoops' would wash them free of the muck and small particles they had dragged along in their shallow paddle shapes. The trawler would then pull in its 'net' and go to a nearby soviet port where it would pretend to unload its catch of fish. The next evening, it would go back to sea deploying its 'net' at the bend in the underwater canyon and repeat the process. With six ships conducting the work and the vagaries of the navigational skills of day to day life at sea, the drag path was not exactly a precise location. In fact, the trawlers paths were mostly overlapping but in a swath the better part of a kilometer wide. As a result each passage by all six ships in a night took a month to start carving a new channel just a meter deep.
If nothing else, however, the USSR was persistent. Once the plan was devised and the orders were given the process would continue until the orders changed, or the funding ran out completely. Over time the dredging 'nets' were improved. It was fortunate for the project that the bedrock in this portion of the continental shelf was friable shale and not granite or basalt. This meant the 'nets' were able to gouge and scrape the shale away small layer by small layer. Eventually, nearly five years after the plan was enacted Comrade General Secretary Stalin died. Whispered rumor was he had been poisoned, but his successors were not about to permit a full autopsy with published results to take place. The scraping and dredging had gone on month after month gradually creating a wide shallow passage 47 meters deep by the time the successor cancelled the project. Turkey and Greece had joined NATO two years earlier, and the Turks were grumbling about Soviet fishermen working so close to the Bosporus passage. Rather than have their secret discovered, the new General Secretary declared the Red Navy would have to live with the dredged channel the way it was instead of the way they wished it could be. Starting in December 1954 when the dredging was cancelled, the Soviet submarines started practicing using the artificial channel. Previously use had been forbidden as there was concern a sub might become disabled and interrupt the ability of the trawlers to proceed dredging. The new nuclear submarine in design at the time would be 12 meters from the top of the sail to the bottom of the hull, but the periscopes and snorkels would add two or three meters of additional height on top of that. Hugging the bottom of the channel it was hoped the water flowing to the deep basin of the Black Sea would be strong enough to pull them along silent as a hole in the ocean, as the American navy liked to boast about their own silent running techniques. The concern was the passage was so wide that the water flow might be too weak, but a new diesel submarine (NATO designated WHISKEY class) successfully pulled off the 'silent passage' within a week of the dredging being halted.
Whatever else people might say about General Secretary Stalin, he never thought small and when he decided to have something done he put resources into doing it. Often for projects that meant much slave labor working itself to death but in the case of the "Stalin Channel" as the Red Navy called this project few gulag slaves had been involved because the work had been simple dredging, once the 'nets' were built. Certainly they had been involved in building the first six 'nets' and in the replacements as those wore out quickly, but compared to a project like the White Canal the effort was trivial in terms of labor.
As the Stalin Channel had been gradually carved out after the first few months a little of the water from the Bosporus canyon started using the new channel instead of continuing down the canyon around the bend and gradually being forced up as it shallowed. By the time dredging was called to a halt, nearly all of the flow down the natural canyon had begun traveling down the new Stalin Channel because despite its shallow depth it was very broad and offered less resistance to the dense bottom water than the up slope after the bend in the natural pathway. When the Mediterranean Sea bottom water had followed the chain of natural connections all the way from the Aegean Sea through the Dardanelles, Sea of Marmara and land bound portion of the Bosporus Strait that heavy warm bottom water had stayed almost unchanged from its condition when leaving the Mediterranean Sea. Where it had struck the bend in the natural canyon of the Bosporus it had considerable inertia and a score of miles of entrained flow following it. this had given it the energy to climb the gradual slope up the last portion of the natural canyon until it was on the continental shelf itself, which has a gradual slope down to the edge of the continental plate where the drop off into the deeps takes place. As a consequence this warm dense very salty water had been forced up into turbulent contact with the brackish surface waters of the Black Sea which were only about half as salty as the bottom waters were. the resulting blended water had then fanned out over the continental shelf flowing down slope into the deep basin where over the eons since the passage opened it had displaced the fresh lake water which had originally filled the basin with the mixed water. The Mediterranean Sea bottom water at the sharp turn in the canyon still had a salinity of 35 parts per thousand (ppt) while the brackish surface waters ranged from 12 to 17 ppt. The mixture created by the turbulent flow of the deep waters swirling into the brackish waters had averaged out to 22 ppt and that concentration of water had filled the deep basin of the Black Sea over the millennia after the passage opened. In the modern era, that deep basin water was turbulently mixed with river water from the Don, Danube and others which flowed into the Black Sea and which had originally formed the fresh water lake in that basin. the resulting mixture of that river water and the 22 ppt bottom water where the rivers poured in the fresh water averaged out to 12 ppt to 17 ppt brackish surface water. It was literally halfway between the fresh water from the rivers and the deep salt water from the Mediterranean Sea in its total salt content. This was a very unusual situation because normally when a river enters a large body of salt water the lighter fresh water spreads out in a thinner and thinner layer. Once a crucial thickness threshold is passed the thin fresh water layer gets mixed with the salt water beneath through wind driven wave action, and the quantity of salt water is so vast compared to the rivers flowing in the salt content quickly averages out. Because the Black Sea basin had started out completely fresh water and the flow rate of the rivers entering it was so enormous in the modern era, even thousands of years after the passages opened it still had a much lower salinity on average than the world ocean, and this created a vast surface pool of brackish water.
Now for the first time thanks to the Stalin Channel dredged across the continental shelf the dense very salty Mediterranean Sea bottom water had a passage way to the deep Black Sea basin. The total quantity of bottom water flowing into the Black Sea had not changed, the Dardanelles and Bosporus sills were still in place at 70 and 40 meters respectively which restricted how much water could pass eastward on the bottom. However, now instead of being premixed with the brackish surface waters and having its salinity lowered to 22 ppt before it reached the deep basin the 35 ppt water had a free passage to cross the continental shelf and drop off into the abyss. The heavy high salinity water was reaching the edge of the continental shelf and dropping straight down the steep slope without having mixed with the brackish water first. Because if its density it went all the way to the bottom which forced the 22 ppt water which had occupied those depths up. On the surface where the 22 ppt waters mixed with the fresh river waters nothing obvious was changed. However 22 ppt water was no longer forming from all of the in flowing 35 ppt bottom water, about 90% of the in flowing water was falling into the abyss unchanged just as it had been during the passage all the way from the Mediterranean Sea and only 10% was mixing to form 22 ppt water on the continental shelf depth.
It might take eons from a human perspective, but the 35 ppt Mediterranean Sea water would eventually displace all that 22 ppt water sitting in the bottom of the Black Sea in the modern era. When that process was complete the basin waters would gradually rise up the slope of the Continental Shelf until they met the 10% of the bottom water flowing in and still mixing with the brackish surface waters to form the 22 pt water. When that event took place in that far distant (from a human perspective) point in time the in flowing river waters would no longer be mixing with 22 ppt partially diluted water, instead they would be mixing directly with 35 ppt Mediterranean Sea water itself. The brackish nature of the surface waters of the Black Sea would be shifted from 17 ppt much closer to 30 ppt, nearly the same as the 34 ppt world average. This would make it easy for many species of aquatic plants and fish which did not do well in the modern Black Sea to thrive in response to the altered environmental conditions. Ultimately it would also change the distribution of flow in and out of the basin through its multiple steps into the Mediterranean Sea. In the modern era about 1/3rd of the water exchange was bottom water flowing in and 2/3rds was brackish water flowing out. Once the water had moderated to approach world sea averages the depth of the brackish layer would first decline and then cease, becoming just a case of excess surface water exiting to join the Mediterranean Sea. By that time the in flow of bottom water would balance the outflow of surface water rather than exceeding it.
In the modern era, however, for the USSR the channel was a large boon as it made it possible to sneak a number of submarines from other operating areas into the Black Sea without detection and for them that was all that mattered. For their distant descendants enjoying the altered Black Sea it was unlikely they would realize how or why those changed conditions had come about, even if they were curious enough to ask.
Provine's note: Western intelligence made note of the change in fishermen's attitudes with Greek and Turkish fishermen in the Mediterranean excited by boosted catches while the Black Sea fishermen began to suffer worse and worse hauls, although theories about a Soviet channel were dismissed for decades.
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