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The USGS Water Science School

Water Science Pictures
An example of a watershed (drainage basin)

Picture of a drainage basin (really a pool)You're expecting to see a picture of a watershed -- so why am I showing you a picture of my in-law's pool in winter? This covered pool is really a good example of a watershed.

The blue cover represents the watershed, the area in which precipitation that falls flows "down-gradient" towards the lowest part of the basin, the center of the pool cover in this case. Likewise, in nature, water flows towards a valley, river, or lake. The lake in the center of the pool forms for the same reason that a lake will form on the landscape -- it is the lowest area around and the water comes in, through a river, seepage into the ground, or by evaporation, faster than it can get out. In most valleys, the land slopes downhill somewhere, in which case, a river will form.

Do you see why this pool cover is not exactly the same as a real drainage basin?


What is a watershed?

A watershed is an area of land that drains all the streams and rainfall to a common outlet such as the outflow of a reservoir, mouth of a bay, or any point along a stream channel. The word watershed is sometimes used interchangeably with drainage basin or catchment. Ridges and hills that separate two watersheds are called the drainage divide. The water resources of a watershed include surface water--lakes, streams, reservoirs, and wetlands--and all the underlying groundwater.

A watershed is a precipitation collector

Most of the precipitation that falls within the Peachtree Creek watershed upstream of Northside Drive collects in the creek and eventually flows by the Peachtree Creek gaging site. Many factors, some listed below, determine how much of the streamflow will flow by the monitoring site. Imagine that the whole basin is covered with a big (and strong) plastic sheet. Then if it rained one inch, all of that rain would fall on the plastic, run downslope into gulleys and small creeks and then drain into Peachtree Creek. Ignoring evaporation and any other losses, then all of the approximately 1,512,000,000 gallons of water that fell (you can use our interactive rainfall calculator to find out how many gallons of water fall during a storm) as rainfall would eventually flow by the Peachtree Creek monitoring site.

Not all precipitation that falls in a watershed flows out

To picture a watershed as a plastic-covered area of land that collects precipitation is overly simplistic and not at all like a real-world watershed. A career could be built on trying to model a watershed water budget (correlating water coming into a watershed to water leaving a watershed). There are many factors that determine how much water flows in a stream (these factors are universal in nature and not particular to a single stream):

Related topics:

Watersheds  Rivers and the landscape

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Page Last Modified: Monday, 17-Mar-2014 11:03:38 EDT