Final answer:
Smaller channels that feed the main river channels are called tributaries, and they play a key role in the river's drainage basin by providing additional water and potentially carrying contaminants.
Step-by-step explanation:
The smaller subsidiary channels that feed the main channels of rivers are known as tributaries. Tributaries contribute to the river discharge by supplying additional water from rainfall, melted snow, or springs. They are vital components of the river's drainage basin, which is the geographic area drained by a river and its tributaries, including all surface runoff and groundwater seepage.
Understanding the complex network of tributaries is essential when studying environmental biology or geography, especially when investigating issues such as chemical waste entering river systems. Challenges faced during the study might include difficulty in tracing the exact source of pollution within a river system given the numerous tributaries that could be potential conduits for contaminants.