What is the defining feature controlling water flow?

The defining feature controlling water flow is primarily the watershed. A watershed is an area of land where all of the water that falls within it drains into a common outlet, such as a river, lake, or ocean. This natural boundary influences how water moves through the environment, determining the direction and rate of flow based on the topography and other factors present in the landscape.

In contrast, while a ridge can influence water flow by acting as a barrier or guide, it is not the comprehensive feature that dictates how water collects and flows across a larger area like a watershed does. Valves and foot valves are mechanical devices used to manage water flow within specific systems, such as plumbing or irrigation, but they do not define the overall natural flow of water in the environment. Therefore, the watershed is the fundamental element to understand when discussing the controlling features of water flow.