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When we were married, my spouse and I, like so many others,
were presented with a set of mixing bowls. This is a great set of bowls that
takes little room on a shelf because they all nest nicely inside each other.
The problem with these bowls occurs when one of us asks the other to "hand
me a bowl, please." We can quickly determine if the need is for the "biggest"
or the "smallest" bowl but selecting among the others usually requires pointing
and head shaking. We have no standard terminology for the range of bowls in
our set. Much the same problem exists with respect to watersheds. Using the
bowl method, we can often communicate with each other adequately by simply
referencing "bowl" or "watershed." Often, however, it is necessary to be more
specific about the size or scale of nested "containers" in order to communicate
effectively. The terminology used to reference "watersheds" is not standardized
and often creates a miscommunication when people do not share the same sense
of scale for a given watershed term. My goal is to offer a suggestion for
watershed terminology that, in my experience, helps to communicate the relative
size of watersheds.
Technically, a watershed is the divide separating one drainage
area from another (Chow, 1964). The term "watershed" is commonly used to refer
to an area; specifically, the area in which all surface waters flow to a common
point. A great deal of confusion and misunderstanding is created by the inconsistent
use of terms to describe the relative size of watersheds-basin, watershed,
drainage, catchment. Use of the term "watershed" to describe the area drained
by the Columbia River as well as the area drained by the Deschutes River is
technically correct - it just does not provide insight to the fact that the
Deschutes is one small tributary to the Columbia River. More confusion is
introduced by referencing adjectives such as fifth-field watershed or fifth-order
watershed. One way to minimize the confusion is to use a consistent set of
terms that is based on established systems for subdividing large watersheds
into smaller ones. The preferred terms presented here - Region, Subregion,
River Basin, Subbasin, Watershed, Subwatershed, Drainage, and Site - are consistent
with the common interpretation of relative watershed size.
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REGION
The USGS recognizes 21 major geographic Regions (
Figure
1), 18 of which are located within the continental United States (Seaber,
et al., 1987). They assign the first two digits of an eight digit numeric
code to the Region. The Great Basin (16), the Pacific Northwest (17), and
the California Region (18) are most relevant to Federal agencies in Washington,
Oregon, and California.
Example: Pacific Northwest Region
17 - - - -
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SUBREGION
The USGS further subdivides these Regions into Subregions
(
Figure 2). Nationally, there are 222 Subregions;
18 of these lie within the States of Washington and Oregon and northern California.
A subregion includes the area drained by a river system, a reach of a river
and its tributaries to that reach, a closed basin, or a group of streams forming
a coastal drainage area. The numeric code for each of the subregions is composed
of four digits; two digits each for Region and Subregion.
Example: Willamette Subregion
17 09 - -
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RIVER BASIN
The USGS divides Subregions into yet smaller areas, resulting
in what is commonly referred to as River Basins (
Figure
3). There are 24 river basins completely or partially within the States
of Washington, Oregon and the northern portion of California. Like the Subregion,
a River Basin is composed of a river system (e.g., Willamette River), a reach
of a stream and its tributaries to that reach (e.g., Middle Snake-Powder),
a closed basin (e.g., Oregon Closed Basins), or a group of streams composing
a coastal drainage area (e.g., Northern California Coastal). The numeric code
for each River Basin is six digits long and is referred to as "third field"
watersheds.
Example: Willamette River Basin
17 09 00 -
The USGS hierarchy of Hydrologic Units refers to this level
of subdivision as Accounting Units (Seaber, et. al., 1987). This nomenclature
is very uncommon and confusing to the public and managers. The term Accounting
Unit is best avoided.
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SUBBASIN
The smallest subdivision in the USGS hierarchy is the Subbasin
(
Figure 4). There are 207 Subbasins within the
River Basins in Washington, Oregon, and northern California. Subbasins are
geographic areas representing part or all of a surface drainage area, a combination
of drainage areas, or a distinct hydrologic feature (Seaber, et al., 1987).
Almost all of the subbasins are larger than 700 square miles in size. Subbasins
in Washington, Oregon, and northern California range from 34 to 4100 square
miles with an average of 1143 square miles. The numeric code for the subbasins
is eight digits long and is composed of four two-digit fields. Subbasins are
equivalent to "fourth field watersheds."
Example: Middle Fork Willamette Subbasin
17 09 00 01
The USGS refers to the Subbasin level of the hierarchy as Cataloging
Units. Like Accounting Unit, the term Cataloging Unit has no common use or
meaning and should be avoided.
WATERSHED
The USGS hierarchy does not continue subdividing or provide
terms for areas smaller than the Subbasin. There is some resistance to labeling
the next level within the terminology hierarchy with the term, but the next
logical subdivision is the Watershed. If we continue with the coding scheme
used by the USGS, Watershed would be the fifth two-digit field. Currently,
there is no universally accepted delineation of watersheds in the states of
Washington, Oregon, and California
(Editor's Note: California now has the "CALWATER" watershed
system, brought to fruit by WMC Prez Clay Brandow. It will likely be widely
accepted soon).
SUBWATERSHED
Within any watershed, there are logical stratifications or
subdivisions that help orient people, especially analysts, based on geography
or a distinctive feature or use. Typically, references like "Lick Creek" or
"Upper Crow Creek" are used to identify major tributary areas within a watershed.
Similarly, a specific use, such as a diversion for a municipal water supply,
may be used to identify a geographic area. Specific features, like a collection
of springs within the same geographic area, may be referred to and would logically
help orient a person to a Subwatershed area within a bigger watershed.
DRAINAGE
Generally, within a Subwatershed, there are logical stratifications
based on the development of the stream channel network. Identifying smaller
drainage areas is particularly helpful for organizing and presenting analysis
of physical processes within the larger Subwatershed.
SITE
The smallest hierarchical delineation is the Site. This level
is not based on hydrography, but represents an area that is appropriate to
an existing or proposed use. Examples would be: allotment X, administrative
site Y, or campground Z.
Terms to avoid
Terms to avoid when referring to different levels or sizes
of watersheds include:
- "x" Field (i.e., fifth field)
- "y" Order
- Stream Class
Field
The term "field" is not recommended for three reasons:
(1) very few people are aware of the USGS national standard hydrologic unit
hierarchy,
(2) the term "field" does not convey a visual image of size,
(3) there is no universally accepted system to subdivide below the USGS's
Subbasin level (fourth field).
Stream Order
The term "order" is commonly used to refer to the relative
size of a specific watershed. While this is technically appropriate for a
specific watershed, the term causes problems when we try to use the term to
compare or equate, even on a relative scale, the size of different watersheds.
An explanation is in order (pardon the pun). Stream order refers to a systematic
process for describing the degree of branching of a stream network within
a watershed (Strahler, 1952).
The order of any stream segment is determined by starting at
the headwaters and labeling each unbranched tributary as order one. Where
two order one streams come together, an order 2 stream is created. Similarly,
when two second order streams merge, a third order stream is created. The
junction of any two streams of equal order results in a stream of the next
higher order. Stream density and branching patterns are determined by many
factors including geology, soils, relief, and precipitation. Two watersheds
of the same size can have very different stream densities.
A consistent relationship between stream order and watershed
size does not exist...particularly at the Region or Subregion scale.
Determining stream order is a function of map scale and the
delineating process used. Different depictions and, therefore, stream orders,
will be derived if different scale maps are used. Similarly, if one analyst
uses only the "blue lines" on the quads and another analyst extends the stream
network based on contour crenulations, very different stream orders will result.
A universally accepted procedure for delineation of tributaries
for the determination of stream density or stream order does not exist.
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Stream Class
Stream class refers to the relative value of a stream based
on the need for protection of beneficial uses. Class I streams typically represent
streams that are very important for water supply, fisheries, or recreation
values. Other stream classes denote streams of lesser value or streams that
are intermittent or ephemeral. Stream class generally represents a sense of
values that may be totally independent of watershed size.
Conclusions
The term "watershed" refers to areas of land from which surface
waters flow. Unfortunately, the term is used without consistent size discrimination.
Some existing terminology, such as Accounting Unit, does not connote any relationship
land areas. Use of adjectives and terminology associated with geomorphic or
social or biologic value criteria often confuse people who are trying to sort
out the relative sizes of watersheds within an area. Use of a common terminology
to describe the nested and relative sizes of successively smaller areas would
help to minimize the confusion that exists today. The terms Region, Subregion,
River Basin, Subbasin, Watershed, Subwatershed, Drainage, and Site are recommended.
References
Chow, Ven Te, 1964. Handbook of applied hydrology. McGraw-Hill.
Seaber, Paul, F. Paul Kapinos, and George Knapp, 1987. Hydrologic
Unit maps. United States Department of Interior, US Geological Survey, Water
Supply Paper 2294, 63 pp.
Strahler, A. N. 1857. Quantitative analysis of watershed geomorphology.
Transactions American Geophysical Union, vol. 38, pp. 913-920.
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