It’s a land area that channels rainfall and snowmelt to creeks, streams, and rivers, and eventually to outflow points such as reservoirs, bays, and the ocean.
The size of a watershed (also called a drainage basin or catchment) is defined on several scales—referred to as its Hydrologic Unit Codes (HUC)—based on the geography that is most relevant to its specific area. A watershed can be small, such as a modest inland lake or a single county.
Conversely, some watersheds encompass thousands of square miles and may contain streams, rivers, lakes, reservoirs, and underlying groundwater that are hundreds of miles inland. The largest watershed in the United States is the Mississippi River Watershed, which drains 1.15 million square miles from all or parts of 31 U.S. states and two Canadian provinces stretching from the Rockies to the Appalachians!
Water from hundreds, and often thousands, of creeks and streams flow from higher ground to rivers that eventually wind up in a larger waterbody. As the water flows, it often picks up pollutants, which may have sinister effects on the ecology of the watershed and, ultimately, on the reservoir, bay, or ocean where it ends up.
Not all water flows directly to the sea, however. When rain falls on dry ground, it can soak into, or infiltrate, the ground. This groundwater remains in the soil, where it will eventually seep into the nearest stream. Some water infiltrates much deeper, into underground reservoirs called aquifers. In other areas, where the soil contains a lot of hard clay, very little water may infiltrate. Instead, it quickly runs off to lower ground.
Rain and snowmelt from watersheds travel via many routes to the sea. During periods of heavy rain and snowfall, water may run onto and off of impervious surfaces such as parking lots, roads, buildings, and other structures because it has nowhere else to go. These surfaces act as "fast lanes" that transport the water directly into storm drains. The excess water volume can quickly overwhelm streams and rivers, causing them to overflow and possibly result in floods.
WHAT IS A WATERSHED?
FOR A DEEPER DIVE INTO WATERSHED EDUCATION:
DROP BY DROP DOCUMENTARY
Check out this educational video on Indiana's water infrastructure. This is great classroom material for teachers from grades 7-12. In "Drop by Drop: Protecting Indiana’s Water Supply," you can discover the mysteries of where our water comes from, and where it goes once it disappears down the drain.
Project WET is an international water-focused conservation education program to raise awareness for water resource management and encourage socially and economically sustainable environment. The program’s mission is to bring children, parents, teachers, and community
members around the world with water education that promotes awareness of water and empowers community action to solve complex water issues. Project WET contains widely used curriculum and activities, providing pre-kindergarten through twelfth grade educators with the tools they need to integrate water education into every school subject.