Recommended Watershed Terminology
Terms to Avoid
Bruce P. McCammon
USDA-Forest Service, Pacific Northwest Region
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.
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 - - - -
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 - -
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.
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.
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).
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.
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.
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
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).
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.
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.
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.
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.