What is a Watershed?
A watershed, also called a drainage basin, is an area that contributes
surface water and groundwater to a specific stream. All land is part of
some watershed. Wherever you go, you are in a watershed. A watershed is
bounded in all directions (except down valley) by a drainage divide, which
is the line of separation between runoff that descends in the direction of
the watershed in question and that which goes toward an adjacent watershed.
The watershed for a principal river like the Rio Grande
encompasses the smaller drainage basins of all of its tributaries. The
major tributaries of the 1,825-mile, 3,042 kilometer Rio Grande are the
Pecos River in Texas and the Rio Conchos in Chihuahua. In all, the Rio
Grande watershed encompasses 335,000 square miles, or 862,500 square
kilometers, equal in size to 11 percent of the continental United States.
The Rio Grande watershed is the darkened area in this map. |
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How Does A Watershed Work?
The complex system of streams within a watershed is referred to as the
drainage net. In every drainage net, small streams join or come together
to form successively larger ones. Little streams join bigger streams, and
small valleys join more extensive ones. This relationship, although variable
in detail, holds true for watersheds of any size or extent. This systematic
characteristic makes it possible to recognize a natural organization within a
drainage net, and the concept of stream order has been devised to describe the
arrangement.
A first order stream is the smallest unit in the system and is thus conceptualized
as a stream without tributaries because it represents the smallest tributary in the
net. Where two first-order streams unite, a second-order stream is formed. At the
confluence of two second-order streams, a third-order stream begins, and this uniting
principle applies through successively higher orders in the hierarchy. The joining of
a lower-order stream with a higher-order stream does not increase the order below that
junction; for example, the confluence of a first-order stream with a second-order stream
does not produce a third-order stream. A third-order stream is formed only by the joining
of two second-order streams.
In the diagram, the steep, small segments are designated as 1 (or "first order"). Note that
"stream order" in a drainage network is not determined by the presence or absence of flowing
water, but by the shape of the land surface, which determines where flow will be concentrated
when water is present.
In a well-developed watershed, one can predict with some certainty that first-order streams and
valleys will be more numerous than all others combined and that each succeeding higher order will be
represented by significantly fewer streams. Other predictable relationships include that: (1) the
average length of streams increases regularly with increased order, (2) the average watershed area
drained by streams increases regularly with increased order, and (3) the average gradient of streams
decreases with increased order.
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When Does The Water Flow?
Many of the world's streams do not flow year-round. In humid regions, streams
and most tributaries are permanent, or perennial. In less well-watered regions many
of the major streams and most tributaries carry water only part of the time, during the
"wet season" or during and immediately after rains. These impermanent flows are called
ephemeral if they carry water only during and immediately after a rain, or intermittent
if they flow for only part of the year, although the term intermittent is sometimes used
to apply to both cases. In desert areas, virtually all streams may be intermittent or
ephemeral, with the notable exception of those that flow into the desert, bringing their
water from somewhere else. These are called exotic streams. |
How Does A Watershed Function?
Healthy watersheds provide three major functions. First, they transport and store
water, sediment, pollutants, and organisms. In general, a stream's load consists of
three kinds of materials: dissolved materials carried in solution, fine particles held
in suspension, and heavier or coarser materials pushed or bounced along the channel bottom.
Second, watersheds cycle and transform energy, as well as carbon, nitrogen, and phosphorus.
And finally, they provide ecological succession through changes in vegetation due to movement
of a watershed's energy, water, and materials from abiotic environment to biotic. The drainage
net of a watershed provides habitats for aquatic organisms, is an important component of
terrestrial ecosystems, and conveys runoff and sediment loads out of each stream's watershed.
Over time, a stream becomes graded. That is, a balance or equilibrium is reached among
channel slope (gradient), channel characteristics, available discharge, and load. Stream
banks and channels are relatively stable under graded conditions. This balance is upset,
however, by changes to the land cover and surface characteristics of the watershed.
The urbanization of watersheds increases the imperviousness of land surfaces, alters
the density of channels, and diverts much of the surface drainage to underground storm
sewers. This, in turn, dramatically changes the volume of water and the amount and
type of material that streams in urbanized watersheds convey. Urbanization also alters
the physical configuration and stability of stream channels, reducing their value as
wildlife habitats. |
Why Are Riparian Wetlands Important?
Riparian wetlands are found in low-lying regions adjacent to rivers and
streams that are periodically subjected to overbank flooding. Since they are
hydrologically connected to both the river (downstream) and surrounding watershed
(upstream), riparian wetlands are of major importance in the watershed system.
Riparian wetlands intercept surface and subsurface (groundwater) runoff from the
upland regions of the wetland and thus function as buffers for the river systems.
These wetlands also interact periodically with floodwaters originating from rivers
and streams; these hydrologic interactions can have a significant effect on river
water quality.
Riparian wetlands have been shown to be highly effective in the reduction of non-point
source (NPS) loading of nutrients and sediments to rivers and streams. As a result,
many agricultural (including forestry) Best Management Practices (BMPs) are based on the
premise that riparian buffer zones, which include wetlands and non-wetland areas, are
essential components of the watershed that should be preserved or restored. Of particular
significance to downstream water quality are riparian wetlands associated with low-order
(smaller) streams, because of the large hydrologic throughput in these wetlands relative to
the flow in the river or stream. These wetlands generally occur in the upper reaches of
watersheds. Although the riparian zone of a single low-order stream may seem insignificant
to water quality in the watershed, the cumulative impact of the multitude of riparian wetlands
along low-order streams can be extremely significant.
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What is Groundwater?
Some precipitation infiltrates the ground and fills the pores in soil and rock.
The subsurface area where all available soil and rock spaces are filled by water is
called the zone of saturation, and the water in these pores is called groundwater.
The water table is the upper surface of the zone of saturation. It is the fuzzy and
fluctuating dividing line between saturated soil and rock, where every available pore
is full, and unsaturated (but still wet) rock and soil where the pores can absorb more
water. The water table falls in dry weather and rises in wet weather.
The ability of soil or rock to hold water depends on its porosity and permeability.
Porous, water-saturated layers of sand, gravel, or bed rock through which groundwater
flows and that can yield an economically significant amount of water are called
aquifers. |
Click for enlarged image
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Click for enlarged image
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In parts of Texas, more groundwater is being used than is being replenished
through natural means. If this practice continues, Texas water costs will rise,
land could subside, water quality could decline and people in some areas could run
out of water. To address this problem, the Texas Legislature has provided a way for
groundwater resources to be managed and protected locally, through the creation of
groundwater conservation districts. |
Some Websites for More Information:
American Water Resources Association
American Rivers
American Water Works Association Research Foundation
Discover a Watershed
"Encyclopedia of Water"
International Arid Lands Consortium
National River Restoration Science Synthesis (NRRSS) Database (pdf)
National Wetlands Inventory
Powell Consortium
River Network
Science in Your Watershed
Teaching Center
The Groundwater Foundation
University Council on Water Resources
USDA, Cooperative State Research, Education, and Extension Service, Water Outreach Education pilot web site
Water Environment Federation
Water on Tap
Water Science for Schools
Watershed Information Network
WaterWiser
Credits:
CGIS at Towson University, What are Watersheds, [online] available
http://chesapeake.towson.edu/landscape/impervious/all_watersheds.asp
New Mexico Water Resources Research Institute, New Mexico Watershed Maps, [online] available
http://wrri.nmsu.edu/wrdis/watersheds/nmmap.html
The Texas A&M University, Major Aquifers of Texas, [online] available
http://aggie-horticulture.tamu.edu/syllabi/315/water/aquifers.html
U.S. Geological Service, Rio Grande Delta in Texas—Sea-Level, Climate, Neotectonic and Anthropogenic Effects, [online] available
http://soundwaves.usgs.gov/1999/05/
Arthur N. Strahler and Alan H. Strahler, Modern Physical Geography, 3rd ed.,
New York: John Wiley & Sons, 1987.
Tom L. McKnight, Physical Geography: A Landscape Appreciation, 3rd ed.,
Englewood Cliffs, NJ: Prentice Hall, 1990.
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