When we connect in a river the points where the water particles have the greatest speed, we get a line that we call the current wire.
In a definition:
The current wire of a river is the imaginary line that connects the points of that river where the flow velocity of the water particles is greatest.
In the drawing you can see the current wire from the left bank to the right bank crossing. In the hollow bend, which is the outer bend, the current wire approaches the bank closest. It will therefore not surprise you that ships sailing downstream would like to follow the current wire.