There are two different currents in the Bosphorus: the main surface current flowing from north, north-east to south, south-west which carries the excess water of the Black Sea to the Mediterranean via the Sea of Marmara and the Aegean Sea, and the counter-current below the surface flowing from south to north which brings the saline water from the Sea of Marmara to the Black Sea. The water of the undercurrents which is called canal is lukewarm in winter and cold in summer.
The main surface current which flows in the middle of the Bosphorus at the entrance on the north of the strait, increases its speed and forms the "Devil's current" between Bebek and Amavutkoy. When the traveller-author, the Rev. Robert Walsh, witnessed this current he wrote his impressions:
"The current of the Bosphorus was so very strong that we were obliged to row under the shore for some time to avail ourselves of the eddy. There are three rapids in the Bosphorus. One is called Seytan Akintisi or the Devil's current by the Turks. It is evidently caused by the- sudden and abrupt descent of the bed of the Bosphorus. The convulsion which disrupted the strait left this rugged inequality below; and when water poured down from the Black Sea, it fell over it as the waters of the great American lakes tumble down the Falls of Niagara. The stream here foams and whirls and boils with considerable violence, but is easily passed even by boats."
The average speed of the main surface current is 0.9 metres per second. In front of the Kandilli promontory where the current is named MASKARA AKINTISI its speed reaches 1.45 metres per second.
The degree of saltiness in the water flowing from the Black Sea is 0.17 while in the water flowing from the Sea of Marmara into the Bosphorus it is. 035.