What is a Watershed?
The definition of a watershed has varied over time and place and from one profession to another depending on need. The following are some common definitions currently in use.
Dictionary Definition (American Heritage Dictionary of the English Language, New College Edition 1976)
1. A ridge of high land dividing two areas that are drained by different river systems. Also called “water parting.”
2. The region draining into a river, river system, or body of water.
3. A crucially important or divisive factor, time or event.
Dividing Line Definition
A ridge of high land dividing two areas that are drained by different river systems. Also called “water parting.”
Sometimes a watershed is considered to be the actual ridge that divides drainage areas or catchment basins. For example, the “Great Divide” in North America formed by the crest of the Rocky Mountains is such a watershed. This dividing line definition is used to distinguish between “catchment basins” in America and Europe. This definition has word origin roots relating to cutting, parting, and sheds (Skei) as boundary lines between sun and shade.
Land Area Definition
The region draining into a river, river system, or body of water. This definition is commonly used in America and refers to the land area that has runoff into a river, river system, lake or the ocean.
Systems Definition (Bob Curry, Fred McPherson, Bruce Van Allen 2016)
Another important and useful definition, not included in the dictionary, but commonly used by hydrologists is the systems definition. When water in all of its forms is added to the physical landscape the watershed becomes a dynamic, living watershed system that can be defined in the following way:
A watershed is an area of land which catches water in the form of rain, snow, fog and frost and drains it and the suspended sediments, dissolved materials (like gasses and minerals) and potential and kinetic energy in it to a common outlet at some point along a stream, river, lake or ocean. It includes the surface water, ground water, metabolic waters of living things*, as well as the waters of transpiration and evaporation.
This dynamic definition includes the actual water that flows through the watershed as well as the geological and hydrological characteristics of the watershed land area. It is useful for looking at a watershed as an ecosystem and for an overall quantitative “water budget” analysis.
Catchment Basin Definition
Similar to the systems definition, when water in all of its forms is added to the physical landscape, the water can be perceived, recognized and valued as something that is “caught,” retained, captured or collected and absorbed as it travels through the landscape ecosystem area and all runoff is conveyed to the same outlet. The word catchment basin is often synonymous with the word watershed and only a slight distinction paid to the difference in meaning between catching and shedding.
Note:
* Most people only think of a watershed as something on the outside of the human body, but from a Human Ecology point of view, the watershed is interconnected with the metabolic water that occurs inside of an organism, which is involved in the structure and function at all internal levels of biological organization, from the organ system, organ, tissue, cellular and molecular levels. For example water is involved in the cytoplasm of individual cells, and the life processes in the circulatory, lymphatic, and excretory organ systems, as well as the interconnected watershed ecosystem.