What do we call an ecosystem where all water runoff drains into a single body of water?

A watershed is defined as an area of land where all the water that falls on it, either from precipitation or melting snow, drains into a common outlet, usually a river, lake, or ocean. This term encompasses both the surface runoff and the underground water flow, leading to a single body of water.

Watersheds are significant for understanding how water moves through the landscape and how it is affected by human activity, land use, and natural phenomena. The concept is essential for water resource management, environmental protection, and planning for sustainable ecosystems.

In contrast, the other terms refer to different specific locations or features. A lake is a specific body of water, a beach is a landform along the shoreline, and an estuary is where freshwater from rivers meets and mixes with saltwater from the ocean. These terms do not describe the broader concept of an area collecting water into one outflow, which is why a watershed is the correct answer.