What is a drainage basin?

• A. The entire area that provides water to a given river
• B. The area around the river’s mouth
• C. The river’s channel
• D. A region drained by multiple oceans

Answer: (A)

A drainage basin is the land area from which all precipitation drains toward a single river system. The boundary is set by watershed divides—high ridges that separate one basin from another. Rain that falls inside this boundary eventually makes its way into the river and its tributaries through surface runoff and groundwater, all funneling toward one outlet, such as where the river reaches a lake or the ocean. The river’s channel is just the watercourse inside that area, not the whole region that contributes water. A region drained by multiple oceans isn’t correct because a drainage basin has one main outlet. For example, the Mississippi River basin collects water from a vast area and channels it toward the Gulf of Mexico.