Hydrologic Units of Virginia

region
sub-region
accounting unit
cataloging unit (CU)

As USGS describes the HUC numbering pattern:

The United States is divided and sub-divided into successively smaller hydrologic units which are classified into four levels: regions, sub-regions, accounting units, and cataloging units. The hydrologic units are arranged within each other, from the smallest (cataloging units) to the largest (regions). Each hydrologic unit is identified by a unique hydrologic unit code (HUC) consisting of two to eight digits based on the four levels of classification in the hydrologic unit system.

The first level of classification divides the Nation into 21 major geographic areas, or regions. These geographic areas contain either the drainage area of a major river, such as the Missouri region, or the combined drainage areas of a series of rivers, such as the Texas-Gulf region, which includes a number of rivers draining into the Gulf of Mexico. Eighteen of the regions occupy the land areaof the conterminous United States. Alaska is region 19, the Hawaii Islands constitute region 20, and Puerto Rico and other outlying Caribbean areas are region 21.

The USGS also provides online copies of the 1:250,000-scale Hydrologic Units of the United States for download and use in a Geographic Information System (GIS).

In the 8-digit hydrologic unit code (HUC), the first two digits are based on the region. The third and fourth digit reflect the sub-region, the fifth and sixth are based on the accounting unit, and the last two digits are the specific cataloging unit.

The Chesapeake Bay watershed numbers start with 02, because the rivers draining into the bay are part of the "Mid-Atlantic Region."

Region	Sub-region	Accounting Unit	Cataloging Unit
02 Mid Atlantic	0207 Potomac	020700 Potomac	02070001 South Branch Potomac 02070002 North Branch Potomac 02070003 Cacapon-Town 02070004 Conococheague-Opequon 02070005 South Fork Shenandoah 02070006 North Fork Shenandoah 02070007 Shenandoah 02070008 Middle Potomac-Catoctin 02070009 Monocacy 02070010 Middle Potomac-Anacostia-Occoquan 02070011 Lower Potomac
	0208 Lower Chesapeake	020801 Lower Chesapeake	02080101 Lower Chesapeake Bay 02080102 Great Wicomico-Piankatank 02080103 Rapidan-Upper Rappahannock 02080104 Lower Rappahannock 02080105 Mattaponi 02080106 Pamunkey 02080107 York 02080108 Lynnhaven-Poquoson 02080109 Western Lower Delmarva 02080110 Eastern Lower Delmarva
		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 Rapids 03010107 Lower Roanoke
		030102 Albemarle-Chowan	03010201 Nottoway 03010202 Blackwater 03010203 Ghowan 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
	0507 Big Sandy-Guyandotte	050702 Big Sandy	05070201 Tug 05070202 Upper Levisa 050702 Big Sandy
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 06010207 Lower Clinch