Wherever You Are, You’re in a Watershed


The term “watershed” is commonly used to describe areas of land associated with water features, such as lakes or rivers. Misuse of the term is also quite common, leaving many with an unclear idea of what a watershed is or isn’t. Strictly speaking, a watershed is an area of land where all the water that falls as precipitation (rain, snow, sleet, hail)  flows to a common outlet; it is the land area that is drained by a body of intermittently or continually flowing water. The names used for these bodies of flowing water depend not only on its size and physical characteristics, but also on regional customs and cultural differences. Common names include brook, stream, creek, river, gulch, wash, draw, slough, rigolet, gully, ravine, arroyo, and fork; all refer to roughly the same thing. “Watershed” as used here refers to surface watersheds rather than ground watersheds. Underground watersheds (aquifers) behave differently and are less understood than those on the surface, even though the two types are connected through the process of infiltration (percolation).

Watersheds, also known as “catchments”, “basins”, or “drainage basins”, are generally defined by their topography. Consider a high mountain peak, with melting snow flowing down its east and west flanks. Water flowing down the east face enters one watershed, while the water flowing down the west face enters another distinct watershed. Picture a mountain range opposite the peak, with a river valley between. Both the peak and the ridge on the opposite side of the valley drain to the river below, and so constitute major catchment boundaries or divisions. Major drainage basins are generally composed of the faces of opposing peaks and mountain ranges, as well as the valleys into which they drain.

A single watershed can cover millions of square miles, or it may be smaller than an acre. Smaller divisional lines and topographical boundaries (such as hills and small ridges) define smaller basins within larger ones. These lesser units are referred to as subwatersheds. In order to be classified as such, each subwatershed must drain into a common larger watershed. In addition to topographical barriers, different watersheds are characterized by different types of vegetation, soils, and climate. Although adjacent watersheds are usually much more similar than they are different from one another, no two watersheds are ever exactly alike.

So what’s in a watershed? The short answer is “Everything!” Every portion of the earth’s land surface lies within at least one watershed. Trees and other plants, mountains, wildlife, roads, lakes, rivers, all our cities and buildings, all our farms – all these things and more are located in watersheds. Even someone who lives in the middle of the desert, hundreds of miles from the nearest stream, lives in a watershed. While it may not be obvious, actions taken by this desert-dweller can affect other people who live downstream in the basin – even if there is no body of continually flowing water nearby. It’s important to be cautious about what chemicals are applied to the earth, and what substances are allowed to soak into it, because they almost always end up in the groundwater. More people around the world depend on groundwater for their fresh water supplies than on any other source.

Watersheds are invaluable and irreplaceable. They collect and provide fresh water, and they store fresh water for future use. Watersheds filter pollutants and contaminants from water, while providing habitat for all the living things in terrestrial ecosystems – which includes humans. Take a moment to contemplate the wonder and necessity of our watersheds, and perhaps to assess what impacts your everyday actions might have on watershed health including changes, if any, you might make. After all – wherever you are, you’re in a watershed.