Answer: Tributary
A tributary is a water course or river that does not flow directly into the sea or ocean, but in another river (usually called main stem or parent river) that usually has a greater flow, being the joint point of both called confluence.
Thus, the tributary is a complementary part of a main river. Generally, the tributaries tend to have a shorter length and a less abundant flow than the main rivers into which they flow. However, there are exceptions such as the case of the Missouri River, which is a tributary of the Mississippi and has a considerable flow compared to the main river.