U.S. flag An official website of the United States government.
				Here's how you know we're official. **Official websites use .gov** A **.gov** website belongs to an official government organization in the United States.
 **Secure websites use HTTPS** A small lock or **https://** means you’ve safely connected to a .gov website. Share sensitive information only on official, secure websites.


Toggle navigation Toggle search National Ocean Service National Oceanic and Atmospheric Administration
 Search
					Search National Ocean Service


Home
				Ocean Facts

				Topics
					Explore All Topics 
                        Arctic 
                        Careers 
                        Economy 
                        Ecosystems 
                        Geodesy 
                        Hazards 
                        International
                        Navigation 
                        Observations 
                        Ocean 
                        Recreation 
                         Sea Level Rise 
                        Story Maps 
                        Tools
                
                			Education

				
				News
				Podcasts
				Video
				Images
				About Us


1. Home
2. Facts
3. What is a watershed?

**What is a watershed?**  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. aerial view of Drake Bay California*While some watersheds are relatively small, others encompass thousands of  square miles and may contain streams, rivers, lakes, reservoirs, and underlying  groundwater that are hundreds of miles inland. Shown here: an aerial view of Drakes Bay, part of California's Tomales-Drake watershed. Credit: Brian Cluer, NOAA Fisheries West Coast Region, California Coastal Office.* 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. **Did you know?**  The Chesapeake  Bay watershed is home to 18 million people and covers 64,000 square miles (165,759 square kilometers). Water from as far away as upstate New York eventually  finds its way to the Chesapeake—the nation’s largest estuary.
 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 (2,981,076 square kilometers) 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.
 More **Information** What is eutrophication? What  is nutrient pollution? What  is the biggest source of pollution in the ocean? National Estuarine Research Reserve  System   **Search** Our facts    Get **Social** Social
  **Last updated:** 01/20/23 **Author:** NOAA How to cite this article Contact Us


Feedback Provide Feedback
							Contact Us
							Report an error
 Social Instagram Logo/a>


*Science. Service.Stewardship.*
 About NOS About Us
							NOS Program Offices
							Media Inquiries
							Website Acknowledgements
                            Site Index
							Disclaimer
 More Resources Freedom of Information Act
							Accessibility
							Information Quality
							Privacy Policy
							NOS For Employees
							USA.gov
 Department of Commerce
									National Oceanic and Atmospheric Administration (NOAA)
									National Ocean Service