“In its historical definition, a watershed is the divide between two drainage streams or rivers separating rainfall runoff into one or the other of the basins. In recent years, the term has been applied to mean the entirety of each of the basins, instead of just the divide between them. The Continental Divide is a watershed according to the earlier definition, where rainfall runoff is directed towards the Gulf of Mexico or toward the Pacific Ocean. The Mississippi River basin and the Colorado River basin are watersheds under the new definition. Other parts of the world use the terms catchment, or river basin, to describe the drainage area between (historical) watersheds. It is from the earlier definition of watershed that we derive the phrase “watershed event” – an occurrence that changes the pattern of all that follows, moving the flow of events toward a different outcome.

A watershed embraces all its natural and artificial (manmade) features, including its surface and subsurface features: climate and weather patterns, geologic and topographical history, soils and vegetation characteristics, and land use. A watershed may be as small as a house’s roof, gutter and downspout, and as large as the Sacramento, San Joaquin or Klamath River basins.

Using watersheds as organizing units for planning and implementation of natural resource management means that:

• Large regions can be divided along topographical lines that describe a natural system more accurately than typical jurisdictional lines.
• Condition and trends analysis can be done on the basis of the entire natural system, in concert with economic and social conditions.
• Communities, including resource management and regulatory agencies, within and outside a particular watershed can better track and understand the cumulative impacts of management activities on the watershed system.
• Managers within each watershed can more effectively adjust their measures and policies to meet management goals across multiple scales, including regional and statewide goals.
• Multi-objective planning is facilitated by inclusion in, and reference to, a whole system context.

Effective management recognizes that mutually dependent interaction of various basic elements of a watershed system including the hydrologic cycle, nutrient carbon cycling, energy flows and transfer, soil and geologic characteristics, plant and animal ecology and the role of flood, fire an other large scale disturbance.