The source is the place where the river starts. The tributary is a secondary river which dies into a principal one.
The basin is an area crossed by a river and its tributaries.
The amount of water that the river carries is called flow. It is low at the beginning of the river, and far greater in the end.
The flow can oscilates by the seasons. Some rivers are dry most of the time, for exemple in Mediterranean or desert areas. Other rivers always carry full flow, for exemple the Amazon, in South America.