The Bosphorus - Strolling Through Istanbul: The Classic Guide to the City - page 436

three to five kilometres per hour from the Black Sea to the Marmara,
but, because of the sinuosity of the channel, eddies producing strong
reverse currents occupy most of the indentations of the shore. A very
strong wind may reverse the main surface current and make it flow
north, in which case the counter eddies also change their direction.
At a depth of about 40 metres there is a subsurface current, called
kanal in Turkish, which flows from the Marmara north towards
the Black Sea. Its waters, however, are for the most part prevented
from entering the Black Sea by a threshold just beyond the mouth
of the Bosphorus; these lower waters, denser and more saline than
the upper, are turned back by the threshold, mingle with the upper
waters, and are driven back towards the Marmara with the surface
current. The lower current is strong enough so that under certain
conditions, if fishing nets are lowered into it, it may pull the boats
northward against the southerly surface current.
The casual visitor to Istanbul, especially if one comes in summer,
might find it difficult to believe that the Bosphorus can be a perverse
and dangerous body of water. Seen from the hills along its shore as it
curves and widens and narrows, it often looks like a great lake or series
of lakes; while its rapid flow from the Black Sea to the Marmara gives
it something of the character of a river. Yet anyone who has observed
its erratic currents and counter-currents, the various winds that
encourage or hinder navigation, the impenetrable fogs that envelop it,
even occasionally the icebergs that choke it, will realize that it is indeed
a part of the ungovernable sea. Here Belisarius fought the invincible
whale Poryphyry, that Moby Dick that wrecked all shipping; here
Gyllius observed the largest shark he had ever seen; while even now
one still sees schools of dolphins sporting in its waves. Since it is an
international waterway, the Bosphorus is busy day and night with
a traffic of cargo ships, oil-tankers and ocean-liners, as well as with
the local and more colourful ferries and fishing boats. The frequent
sharp and unexpected bends in the straits, the tricky currents and
occasional storms and dense fogs can make the passage quite difficult
at times. Nearly every year large ships collide with one another on the
Bosphorus or run aground on its banks, smashing into the houses
along its shores. Old Bosphorus-dwellers will regale you with tales of