Final answer:
The mouth of a river contains more water than the mouth of a tributary because the river collects water from multiple tributaries along its course.
Step-by-step explanation:
The mouth of a river contains more water than the mouth of one of its tributaries because the river collects water from multiple tributaries along its course. Tributaries are streams or smaller rivers that flow into a larger river. As the river flows downstream, it collects water from these tributaries, increasing its volume and carrying capacity. Therefore, the mouth of the river is where all the collected water from the tributaries is discharged, resulting in more water compared to an individual tributary.