Hydrologic Unit Geography

Definitions | Earlier Hydrologic Unit Systems | River Basins | National Watershed Boundary Dataset | The Virginia Portion of the NWBD | NWBD Product Downloads | Interactive Map of Virginia Hydrologic Units

The Virginia portion of the National Watershed Boundary Dataset (NWBD) was completed in 2006. 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.

Click here to view the main types of hydrologic units in the NWBD.

According to the classic definitions, 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.

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. A 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.

Earlier Hydrologic Unit Systems

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

A numeric string can identify any hydrologic unit, at any of these orders. For instance, a Sub-Region is identifiable by a four-digit code (i.e. 0208). The more digits required to identify a hydrologic unit, the smaller that unit is. This 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.

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. 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 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.

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:

• Both 5th and 6th order units are smaller. 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 include 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.
• Water-side delineations of frontal units extend 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 changed, 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.

The new unit name reference “Watershed” above may refer to hydrologic units that are not actually watersheds as correctly defined at the top of this page. Therefore, to avoid the unfortunate confusion these references create, DCR 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.

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.

Numerous improvements in the NWBD also arose from recapturing hydrologic unit boundaries using new geographic information technologies, from past experiences developing and using hydrologic unit systems, and from this being true, 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 map of the 5th and 6th order units of the Virginia NWBD along with the units of the 1995 Pre-NWBD Virginia hydrologic unit system.

Although it was a goal of the new standards to not affect delineation of the existing 4th order units during NWBD development, except to more precisely recapture them, 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 Coastal 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.

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

River Basins

Although 3rd order units of the NWBD are called "Basins," these units are not necessarily the equivalent of river basins as described in many state programs. 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. Click here to view a map of the described River Basins in detail, or click here for a map depicting major drainages - some outside the state - that are associated with Virginia's waters.

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.

While there may no River Basin hydrologic unit standard, river basins are composed of contiguous hydrologic units that do meet standards. 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.

NWBD Product Downloads

Version 4 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 was 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 to see the changes that were made between versions and which county maps were updated.

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

Click here for an interactive map of Virginia's hydrologic units of the NWBD.