Which best describes the characteristics of a river basin?

• A. The land formed when rivers create estuaries and marshes
• B. The land at the mouth of a river where water flows into the ocean
• C. The land formed as a result of a river flooding
• D. The land drained by a river and its tributaries

Answer: (D)

A river basin is the land area from which all the water drains into one river and its tributaries. Imagine rainfall or melted snow in that area flowing downhill, gathering into smaller streams that feed the main river. The outer edge of the basin is formed by high ground that separates it from neighboring basins, so everything within that boundary eventually ends up in the same river system.

That description—land drained by a river and its tributaries—captures the whole drainage area that feeds the river. The land at the mouth of a river refers to the estuary or delta where the river meets the sea, not the whole basin. The land formed by river flooding describes features like floodplains, which are created by repeated floods but don’t define the entire drainage area.