Why is the Congo river full of water all year round?

The Congo Basin is located on either side of the equator and has many tributaries to the north and south of it. The main power source is rainwater. The river is full of water throughout the year due to tributaries, which, depending on the season, provide water alternately. In the summer, the river is replenished by the tributaries of the northern ones, in the winter at the expense of the southern ones.