Istanbul city is divided by the Bosphorus, not a river that is, it is the sea.
Up up up the Bosphorus, to the north, there you have the Black Sea.
Down Down Down the Bosphorus, to the south, there you have the Marmara Sea.
In Greek mythology, as the legend says, Hera (the goddess of women and marriage) the wife of Zeus (the god of gods) fell in love with Io (who was the princess of Argos). But Hera learnt the affair and since Zeus scared of his wife’s malice, he turned Io to a cow, to protect her from Hera. But Hera learnt this and sent flies to attack Io, and while she was struggling with the flies, Io crossed the Bosphorus, which the word means ‘cow passage’ in Greek. And no we do not have cows in the sea.