Hydrologic Unit Geography

With completion of the Virginia portion of the National Watershed Boundary Dataset (NWBD) in 2006, the hydrologic unit designations within Virginia have once again changed. What follows explains what the NWBD is and how it differs from previous hydrologic unit references

Definitions

The terms watershed and hydrologic unit are used throughout DCR’s web pages. Do both terms refer to the same thing? Almost. A true watershed is an area of land and water defined by a boundary such that all surface drainage within the boundary converges to a single point. This point of convergence is usually the exit point, where the collected waters leave the watershed. There are, however, watersheds out of which no water flows.

In contrast, hydrologic units are drainage areas that are delineated so as to nest into a multi-level hierarchical drainage system. Aside from the surface waters that are collected within the boundary of a hydrologic unit, it may also accept water from one or more points outside of the unit’s boundary. The tidal portion of the James River is a good example. It can be a hydrologic unit but not a watershed, because water enters this unit from both the non-tidal (Piedmont) James River at Richmond and the Appomattox River at Hopewell. Additionally, hydrologic units may include associated surface areas whose drainages do not connect, thus resulting in multiple outlet points. This is usually the case with coastal frontage units such as those containing multiple outlets to the Chesapeake Bay or Atlantic Ocean.

According to the classic definitions then, all watersheds are hydrologic units but not all hydrologic units are watersheds. In the development of hydrologic units, watersheds are inherently preferred: They are the perfect hydrologic unit.

Pre-NWBD Federal Hydrologic Unit System

You may notice that there is no mention of size in the above definitions of watersheds and hydrologic units. How big or small may they be? Watersheds may be as big or as small as they need to be to correctly be referred to as a watershed. The hierarchical Hydrologic Unit System, on the other hand, has levels that are based on the size of the area within the boundaries of the units.

At one extreme are the very large units. For instance, you could correctly say that all of Virginia lies in just two hydrologic units - one that flows to and eventually drains into the Atlantic Ocean, and one that flows to and eventually drains into the Gulf of Mexico. However, using standards set in the 1970s to define hydrologic units, the large Atlantic unit defined above is divided into smaller areas. In Virginia these areas consist of the waters that flow into the Chesapeake Bay and the Atlantic Ocean north of the Chesapeake Bay mouth, and those that drain into the Atlantic south of the Chesapeake Bay mouth. In the Standards, hydrologic units that have this large of an extent are referred to as Regions, and the Regions defined above are the Mid-Atlantic Region and the South Atlantic - Gulf Region.

Obviously hydrologic units can be much more finely defined than by Region. A Region is the uppermost hydrologic unit level, which is the equivalent of saying that it is a 1st order hydrologic unit (see Table 1). Dividing Regions into the next smallest form produces Sub-Regions (2nd order). In the past, the standards further divided Sub-Regions into Accounting Units (3rd order), and further divided Accounting Units into Cataloging Units (4th order). As you will see later, the terms “Accounting Units” and “Cataloging Units” are no longer used, however the unit size per these orders remains the same.

Table 1. Pre-NWBD federal hydrologic unit system references

ORDER	DIGITS	ENGLISH NAME	UNIT SIZE
1	2	Region	Avg. 177,560 sq. miles
2	4	Sub-Region	Avg. 16,800 sq. miles
3	6	Accounting Unit	Avg. 10,596 sq. miles
4	8	Cataloging Unit	Avg. 703 sq. miles

A numeric string can identify any hydrologic unit, at any of these orders. Regions are designated by a two-digit code (i.e. 02), Sub-Regions by a four-digit code (i.e. 0208), 3rd order units by six-digit codes (i.e. 020801), and 4th order units by eight-digit codes (i.e. 02080105). The more digits required to identify a hydrologic unit, the smaller that unit is. Whereas all of Virginia is covered by parts of only five Regions, the same area now includes all or part of 50 4th order units. The hydrologic unit system is a nested hierarchical system: You can tell which Region and Sub-Region a 3rd order unit lies within by the first two and first four digits of the six-digit 3rd order unit code, respectively.

To view the Pre-NWBD Cataloging Unit geography, please click here (PDF).

River Basins

Basins are commonly referenced levels of hydrologic units in Virginia. For instance, DCR frequently divides the commonwealth into 14 River Basins for program usage as follows: Potomac River, Rappahannock River, York River, James River, Atlantic Ocean Coastal, Chesapeake Bay Coastal, Chowan River, Albemarle Sound Coastal, Roanoke River, Yadkin River, New River, Clinch-Powell Rivers, Holston River and Big Sandy River. Except for offshore ocean claims, all of Virginia is accounted for in these basins.

Where do the Virginia River Basins fit in the hydrologic unit system? The short answer is that they don’t. They are not a level of that system. Most of them are 3rd order units, but others are just a collection of contiguous 4th order units. The York River Basin, for example, is a collection of three 4th order units (02080105, 02080106, and 02080107) found in the Lower Chesapeake 3rd order unit (020801). These three 4th order units not only form the York River Basin but also comprise, based on our previous definitions, the York River Watershed.

Why do we use an unofficial collection of hydrologic units such as these River Basins when previously defined levels of hydrologic units are the standard? DCR, like most Virginia state agencies, is primarily concerned with activities and occurrences within the state. State agencies in any other state have similar focus. For program purposes and the program needs of other agencies in the commonwealth, having the Rappahannock River Watershed combined with the York River Watershed, as well as with small coastal drainage basins to the Chesapeake Bay, the Chesapeake Bay itself and Atlantic Ocean drainage, is unacceptable. We basically created an unofficial system that suits our purposes.

Unfortunately, when a hydrologic unit system is unofficial, it is subject to alteration at will. What suits DCR’s needs one year may not the next. As well, other state agencies might have or might encounter different needs. Within DCR, the River Basin hydrologic unit system has been modified over the years in a manner that is not always consistent between programs. Therefore, as opposed to the River Basin system defined above, you will also find versions where the Upper Potomac and Shenandoah rivers are in a separate Basin from the Lower Potomac Basin, or where the Chowan River Basin is combined with the Albemarle Sound Coastal Basin, etc.

Are these versions correct? They are as correct as the system of basins we most frequently use as described above, since we are not talking about modifications to a standard hydrologic unit system, and because they do not make incorrect hydrologic units. There may not be a River Basin hydrologic unit standard, but there are still standards for developing hydrologic units. For instance, combining the Potomac River Watershed to the James River Watershed would be incorrect unless you also combined to them all the other surface waters that flow into the Chesapeake Bay that lie between them. Likewise combining the New River Basin to the Roanoke River Basin would require combining to this super-basin all surface drainage to the Gulf of Mexico and the Atlantic Ocean from the Mississippi River to the Albemarle Sound inclusive.

To view the aforementioned 14 River Basins in Virginia with related waters outside the state, please click here.

Pre-NWBD Virginia Hydrologic Unit System

Because the old Cataloging Units averaged 703 square miles in size nationally, they were too large an area to evaluate as a single entity in regard to water quality conditions at the state level. Evaluating such an expanse would result in generalizations that could completely mask problem areas. Therefore, in Virginia we delineated more detailed sets of hydrologic units for this purpose in the past.

DCR’s soil and water conservation program staff and the United States Department of Agriculture-Natural Resources Conservation Service (USDA-NRCS) delineated detailed sixth order hydrologic units for Virginia in 1990 and again in 1995 following the issuance of new hydrologic unit delineation standards in 1992. The 1995 delineation resulted in the creation of a hydrologic unit system containing 494 individual units in Virginia, each averaging about 54,000 acres. These units were subsets of the 48 nationally recognized Cataloging Units occurring in the state at that time. In accordance with the 1992 hydrologic unit system standards, a unique 14-digit string was created to identify each unit, thus these units are often referred to as the 14-digit hydrologic units. In Virginia a three-character code (e.g., A32) was also created for each unit for identification purposes, particularly for use on maps. The code could be related to the 14 digits as well as to a description of the hydrography contained within each unit.

This 14-digit system was the official 6th order set of hydrologic units for Virginia from 1995 to July of 2006. It was widely used as a geographical identifier for water related data and issues. DCR and the NRCS also produced maps, an atlas, and digital files of the boundaries, codes and descriptions of this system to promote its use. Click here for a map of the old 14-digit hydrologic system.

A selective merging of the 6th order units was done in 1996 to produce a 5th order hydrologic unit system, also referred to as the 11-digit hydrologic unit system. While both the 5th and 6th order systems were developed using established hydrologic unit standards, only the 5th order set was completed in all the states surrounding Virginia. Through the efforts of the NRCS, the 5th order units were made seamless between the states.

National Watershed Boundary Dataset

In 2001 the NRCS, USGS, EPA and other federal agencies teamed with the Subcommittee on Spatial Water Data - part of the Advisory Committee on Water Information (ACWI) - and the Federal Geographic Data Committee (FGDC) to develop a new hydrologic unit delineation standard. With state inpute, the new Federal Standards for Delineation of Hydrologic Unit Boundaries was created. The new standards establish a new set of seamless 5th and 6th order hydrologic units for the entire US. The digital products resulting from the delineation and capture of these new units is the National Watershed Boundary Dataset. The NWBD became the official hydrologic unit system of Virginia in July 2006, replacing both the previous federal hydrologic unit system for 1st through 4th order units and the 5th and 6th order units of the pre-NWBD Virginia hydrologic unit system.

There are several major differences between the new standards and those used to develop the pre-NWBD Virginia Hydrologic Unit system:

The new standards call for smaller 5th and 6th order units. Sixth order units of the 1995 product averaged 54,000+ acres in size. The new requirements by order are in Table 2.
Additional specific attributes to polygons and arcs are required in the final digital product of the NWBD. This includes unit modifiers (dam, karst, drainage ditches, etc.) and types (standard, frontal, water, etc.), unit names, line source, official unit codes at the 4th, 5th and 6th order, and the official codes of the 5th and 6th order units downstream. Please click here (PDF) to view the main types of hydrologic units in the NWBD.
Water-side delineations of frontal units will now be defined to the toe of the shore face. In Virginia, this boundary has been set at a depth of 10 feet for the Chesapeake Bay and 30 feet for Atlantic waters. Both depths are based on research regarding where wave action first affects the shoreline.
Unit coding for 5th and 6th order units has changed from requiring 11 and 14 digits to requiring 10 and 12 digits respectively as shown in Table 2.
Order names have been changed from those established in the 1970s to a new system as shown in Table 2.
The Atlantic Ocean - out to the three nautical mile territorial limit - is now partitioned into 5th and 6th order hydrologic units.

Table 2. New versus old hydrologic unit system references

ORDER	NEW DIGITS	OLD DIGITS	NEW NAME	OLD NAME	UNIT SIZE
1	2	2	Region	Region	Avg. 177,560 sq. miles
2	4	4	Subregion	Sub-Region	Avg. 16,800 sq. miles
3	6	6	Basin	Accounting Unit	Avg. 10,596 sq. miles
4	8	8	Subbasin	Cataloging Unit	Avg. 703 sq. miles
5	10	11	Watershed		Range: 40,000 to 250,000 acres
6	12	14	Subwatershed		Range: 10,000 to 40,000 acres

The “New Name” reference to Basins in Table 2 is not the same geography as the previously referenced “River Basins.” Likewise the reference “Watershed” may refer to hydrologic units that are not actually watersheds as correctly defined at the top of this page. To avoid the confusion, therefore, that these unfortunate references create, DCR usually refers to the various levels of hydrologic units by their order instead of their name.

The Virginia Portion of the NWBD

As part of the NWBD development process in Virginia, 6th order units were delineated by DCR so as to preserve as much of the intent of the 1995 Pre-NWBD Virginia Hydrologic Unit boundaries as possible in order to make the transition between the two systems less complicated. Occasionally, however, unit boundaries had to be revised so as to be in compliance with the new standards or to fix previous flaws.

To uniquely identify NWBD units in Virginia without requiring the use of 10 or 12 digits, DCR developed a new four-character internal coding scheme for the 5th and 6th order units of the NWBD. This four-character code replaces the three-character code of the previous 14-digit system. The first two characters of the new code are based on the major stream name in the basin, or portion of the basin, where the unit is located (see Table 3). The two digits that follow these codes are a sequential numbering scheme based on the drainage flow (headwaters to mouth).

The new internal coding scheme for 5th and 6th order units of the Virginia NWBD, cross referenced to the three-character coding scheme of the Pre-NWBD Virginia hydrologic unit system, is shown in Table 3.

Table 3. Internal coding of 5th and 6th order units

1995 UNITS	5th ORDER NWBD UNITS (VAHU5)	6th ORDER NWBD UNITS (VAHU6)	DRAINAGE
A01-A34	PL-A - PL-U	PL01-PL74	Potomac River, Lower
B01-B09	PU-A - PU-F	PU01-PU20	Potomac River, Upper
B10-B58	PS-A - PS-T	PS01-PS87	Potomac River-Shenandoah River
C01-C16 and R01	CB-A - CB-O	CB01-CB47	Chesapeake Bay/Chesapeake Bay Coastal
D01-D07	AO-A - AO-H	AO01-AO26	Atlantic Ocean Coastal
E01-E26	RA-A - RA-R	RA01-RA74	Rappahannock River
F01-F27	YO-A - YO-S	YO01-YO69	York River
G01-G15	JL-A - JL-L	JL01-JL59	James River, Lower (Tidal)
H01-H22, H33-H39	JM-A - JM-U	JM01-JM86	James River, Middle (Piedmont)
H23-H32	JR-A - JR-E	JR01-JR22	James River- Rivanna River
I01-I38	JU-A - JU-T	JU01-JU86	James River, Upper (Mountain)
J01-J17	JA-A - JA-J	JA01-JA45	James River- Appomattox River
K01-K13	CM-A - CM-H	CM01-CM32	Chowan River-Meherrin River
K14-K36	CU-A - CU-R	CU01-CU70	Chowan River, Upper
K37-K38	CL-A - CL-C	CL01-CL05	Chowan River, Lower
K39-K42	AS-A - AS-D	AS01-AS20	Albemarle Sound
L01-L41	RU-A - RU-V	RU01-RU94	Roanoke River, Upper
L42-L74	RD-A - RD-S	RD01-RD77	Roanoke River- Dan River
L75-L82	RL-A - RL-G	RL01-RL24	Roanoke River, Lower
M01-M03	YA-A - YA-B	YA01-YA07	Yadkin River-Ararat River
N01-N37	NE-A - NE-Y	NE01-NE88	New River
O01-O14	TH-A - TH-L	TH01-TH46	Tennessee-Holston River
P01-P16	TC-A - TC-H	TC01-TC35	Tennessee-Clinch River
P17-P24	TP-A - TP-D	TP01-TP19	Tennessee-Powell River
Q01-Q14	BS-A - BS-H	BS01-BS35	Big Sandy River

The hydrologic unit products arising from compliance with the latest NWBD standards contain 1,247 6th order units and 315 5th order units in Virginia. This is a significant change from the 494 14-digit units and 211 11-digit units of the 1995 products.

There are also numerous improvements in the NWBD that arise from recapturing hydrologic unit boundaries using new geographic information technologies, from past experiences developing and using hydrologic unit systems, and from the opportunities that arise from a true multi-state effort. Such improvements include:

There was a more precise delineation and capture of hydrologic unit boundaries. Units of the NWBD were captured from heads-up digitizing on DRGs of the 7.5 minute topographic quadrangle maps and NOAA charts versus from the paper versions of those maps. The ability to zoom and pan made this process more precise at least in regards to capturing linework from these sources.
Linework and attribution are coordinated with all surrounding states so as to make and maintain seamless and sequentially coded units at all orders across all Virginia state borders. Unlike earlier efforts, the hydrologic units of all states are being modified to the same standard in the development of the NWBD.

Click here to see a the 5th and 6th order units of the Virginia NWBD along with the units of the 1995 Pre-NWBD Virginia hydrologic unit system.

The earlier two versions of 6th order hydrologic units developed for Virginia delineated units within the established 4th order units but, with one minor exception, did not affect the delineation of the existing 4th order units except to recapture them more precisely. Although continuance of that practice was requested in the NWBD, the final product includes a few significant modifications and redefinition of established 2nd through 4th order units. These changes, which affected multiple states, were requested to fix the more glaring problems created by imposing 5th and 6th order units from the new standards onto noncompliant larger units developed many standards ago.

The changes made had to eventually occur if any hydrologic unit system (NWBD or some future version) were to be delineated correctly. The long history of use of the 1st through 4th order hydrologic unit coding, however, meant that many past unit recordings would no longer correlate to the new hydrologic unit system codes of the NWBD. It is important to note where these unit designation changes occurred.

Click here to see a graphic comparison of the most radical changes between the new 4th order Sub-basin units of the NWBD and the old 4th order Cataloging Units. You will see that the major changes are meant to fix two previous problems:

The boundary between the Upper Chesapeake Bay Subregion and the Lower Chesapeake Bay Subregion previously followed the Virginia-Maryland state border across the Chesapeake Bay and the Eastern Shore. This was hydrologically incorrect. As a result of using submerged morphological feaures in delineating coastal hydrologic units in the NWBD, that division is now farther north. Thus the Pocomoke River watershed and Tangier Sound drainageare now part of the Lower Chesapeake Bay Subregion.
The Atlantic Ocean drainage in the Eastern Shore was previously part of the Lower Chesapeake Bay Subregion. Atlantic Ocean drainage is clearly separate from Chesapeake Bay drainage. It is now more correctly the southern extent of the Delaware - Mid-Atlantic Subregion, which includes Atlantic Coastal drainage that begins in New Jersey.

Table 4 lists the English references for all 1st through 4th order units in Virginia and indicates in red which of these units have had their references altered as a result of the above change.

Table 4. English references for 1st through 4th order units in Virginia

1st Order	2nd Order	3rd Order	4th Order
02 - Mid-Atlantic	0204 - Delaware - Mid-Atlantic	020403 - Mid-Atlantic Coastal	02040303 - Chincoteague 02040304 - Eastern Lower Delmarva
	0207 - Potomac	020700 - Potomac	02070001 - South Branch 02070003 – Cacapon-Town 02070004 - Conococheague-Opequon 02070005 - South Fork Shenandoah 02070006 - North Fork Shenandoah 02070007 - Shenandoah 02070008 - Middle Potomac-Catoctin 02070010 - Middle Potomac-Anacostia-Occoquan 02070011 - Lower Potomac
	0208 - Lower Chesapeake	020801 - Lower Chesapeake	02080101 - Lower Chesapeake 02080102 - Great Wicomico-Piankatank 02080103 - Rapidan-Upper Rappahannock 02080104 - Lower Rappahannock 02080105 - Mattaponi 02080106 - Pamunkey 02080107 - York 02080108 - Lynnhaven-Poquoson 02080110 - Tangier 02080111 - Pokomoke-Western Lower Delmarva
	0208 - Lower Chesapeake	020802 - James	02080201 - Upper James 02080202 - Maury 02080203 - Middle James-Buffalo 02080204 - Rivanna 02080205 - Middle James-Willis 02080206 - Lower James 02080207 - Appomattox 02080208 - Hampton Roads
03 - South Atlantic-Gulf	0301 - Chowan-Roanoke	030101 - Roanoke	03010101 - Upper Roanoke 03010102 - Middle Roanoke 03010103 - Upper Dan 03010104 - Lower Dan 03010105 - Banister 03010106 - Roanoke Rapid
	0301 - Chowan-Roanoke	030102 - Albemarle-Chowan	03010201 - Nottoway 03010202 - Blackwater 03010203 - Chowan 03010204 - Meherrin 03010205 - Albemarle
	0304 - Pee Dee	030401 - Upper Pee Dee	03040101 - Upper Yadkin
05 - Ohio	0505 - Kanawha	050500 - Kanawha	05050001 - Upper New 05050002 - Middle New
05 - Ohio	0507 - Big Sandy	050702 - Big Sandy	05070201 - Tug 05070202 - Upper Levisa
06 - Tennessee	0601 - Upper Tennessee	060101 - French Broad-Holston	06010101 - North Fork Holston 06010102 - South Fork Holston 06010104 - Holston
		060102 – Upper Tennessee	06010205 - Upper Clinch 06010206 - Powell

Please click here (PDF) to view the complete, new set of NWBD Subbasin Units in Virginia, which replaces the previous Cataloging Units.

NWBD Product Downloads

Version 3 of the Virginia NWBD is available in both an ESRI cover format and as a shapefile for those wanting to use this layer in their geographic information systems (GIS). Version 4 is a September 2009 update to the October 2008 attribute update (Version 3). Version 3 is an October 2008 update to the May 2007 attribute update (Version 2). Version 2 was an update to the initial July 2006 release of the Virginia NWBD.

County and city maps showing the 6th order NWBD units have also been created - and updated - to facilitate the geocoding of features and events with the VAHU6 codes for those who need to but cannot use GIS files.

Click here (MS Excel spreadsheet) to see the changes that were made between versions and which county maps were updated.

Click here (PDF) for instructions on retrieving the GIS files, the maps or both.