Final answer:
Smaller subsidiary channels that feed the main channels of rivers are known as tributaries. These tributaries are part of a river's watershed or drainage basin and play a key role in contributing water and sometimes pollutants to the main river.
Step-by-step explanation:
The smaller subsidiary channels that feed the main channels of rivers are called tributaries. Tributaries are streams or rivers that flow into a larger stream or main river, contributing additional water, sediment, and sometimes pollutants to the main river. They are an important component of a river's drainage basin or watershed, with each tributary having its own smaller drainage basin that ultimately feeds into the larger basin of the main river.
Tributaries play a crucial role in the hydrological cycle by helping to move water from the source, often in highland areas, to a river's mouth at a lake or ocean. Their presence can also affect the biodiversity and health of the overarching river system. In cases where environmental concerns such as chemical waste are noted in the river, it is possible that such pollutants are entering through the tributaries within the same drainage basin.