Which term refers to the boundary that separates drainage basins of a landscape?

• A. Confluence
• B. Reservoir
• C. Clouds
• D. Watershed

Answer: (D)

The boundary that separates drainage basins is defined by the high ground that divides how water flows across the landscape. This dividing line follows the topographic divide—ridges and hills—so that rain on one side drains into one outlet and rain on the other side drains into a different outlet. That boundary is called a watershed (also known as a catchment): it encompasses all the land whose rainfall will eventually flow to a common river or stream, within that basin.

So why is watershed the best term here? It directly describes the area of land contributing to a single drainage outlet and the boundary that separates such areas. Terms like confluence refer to where two streams join, a reservoir is water stored for use, and clouds concern atmospheric moisture. None of those describe the dividing boundary that governs how surface water is collected and routed across the landscape.