Hydrologic Unit Geography Origins

The Virginia National Watershed Boundary Dataset (VaNWBD) was completed in 2006 and has been updated several times since then. What follows describes earlier versions of the hydrologic unit systems covering Virginia and how they differ from the current VaNWBD.

Earlier Hydrologic Unit Systems

Initial federal hydrologic unit standards were set in the 1970s. These standards divided the U.S. into the units listed in Table 1.

Table 1. Previous federal hydrologic unit system references
LEVEL	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 levels. For instance, a Sub-Region is identifiable by a four-digit code (e.g., 0208). The more digits required to identify a hydrologic unit, the smaller that unit is in area. This is a nested hierarchical system: You can tell which Region and Subregion a 3rd level unit lies within by the first two and first four digits of the six-digit 3rd level unit code, respectively.

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 U.S. Department of Agriculture Natural Resources Conservation Service (USDA-NRCS) delineated detailed 6th level 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. 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.

This 14-digit system was the official 6th level 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.

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

Although it was a goal of the hydrologic unit development standards of the time to not affect delineation of the existing 4th level units during 6th level development (except to more precisely recapture them), a few significant modifications and redefinition of established 2nd through 4th level units were made in cooperation with the NRCS. These changes, which affected multiple states, were requested to fix the more glaring problems created by imposing 5th and 6th level units from the new standards onto noncompliant larger units developed in the 1970’s.

This modification had to eventually occur if any hydrologic unit system was to be delineated to new higher levels correctly. The long history of use of the 1st through 4th level unit coding, however, meant that many past unit recordings would no longer correlate to the new hydrologic unit system. It is important to note where these unit designation changes occurred, as they persist through all subsequent hydrologic unit systems, even while the distribution of old hydrologic unit system maps with now incorrect codes continues:

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 features in delineating coastal hydrologic units, that division is now farther north. Thus the Pocomoke River watershed and Tangier Sound drainage are 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 Coastal Subregion, which includes Atlantic Coastal drainage that begins in New Jersey.

Table 2 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 2. English references for 1st through 4th level units in Virginia
1st Level	2nd Level	3rd Level	4th Level
02 - Mid-Atlantic	0204 - Delaware - Mid-Atlantic Coastal	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 - Pocomoke-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
06 - Tennessee	0601 - Upper Tennessee	060102 – Upper Tennessee	06010205 - Upper Clinch 06010206 - Powell

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