A more thorough explanation:

Definition: The equidistant point in the main channel of the river between the well-defined banks on either shore; the middle thread of a river's current. It is also known as mid-channel or middle line of main channel.

• When swimming in the river, it is important to stay in the middle of the river to avoid getting too close to the banks.
• The boat captain steered the vessel towards the middle of the river to avoid hitting any rocks or obstacles.

These examples illustrate that the middle of the river is the area between the two banks where the water flows in a straight line. It is important to stay in the middle of the river to avoid getting too close to the banks where there may be hazards such as rocks or strong currents.