Virginia rivers and boundaries of political jurisdictions
Source: National Atlas
Virginia is governed by political jurisdictions - 95 counties and 39 independent cities. However, water ignores the political boundaries and obeys instead the law of gravity. Water flows across the political boundaries, so managing water requires different counties/cities to cooperate.
Virginia is gradually implementing a regional approach to dealing with wastewater treatment, drinking water supplies, and pollution control. Urban areas such as Northern Virginia and Hampton Roads have confusing and overlapping relationships between counties/cities regarding how drinking water is obtained and wastewater is processed. (The Corps of Engineers provides drinking water to Falls Church and Arlington, and the District of Columbia processes sewage from some of Loudoun and Fairfax counties.)
Watersheds can be defined as large units (i.e., Chesapeake Bay watershed, Ohio River valley watershed), and those large units can be subdivided into smaller and smaller subwatersheds. In a few cases, water is transferred across watershed divides. Hydrologic units are defined by the water flow, in contrast to the topographic boundaries that define watersheds.
As noted by the National Resources Conservation Service, "A hydrologic unit can accept surface water directly from upstream drainage areas, and indirectly from associated surface areas such as remnant, non-contributing, and diversions to form a drainage area with single or multiple outlet points. Hydrologic units are only synonymous with classic watersheds when their boundaries include all the source area contributing surface water to a single defined outlet point."1
The Virginia Department of Conservation and Recreation phrased the distinction this way: "all watersheds are hydrologic units but not all hydrologic units are watersheds."1
As Geographic Information System (GIS) technology expanded through Federal agencies with different management challenges, those agencies defined watersheds at different levels of detail. The US Geological Survey (USGS) divided the Nation into 21 major geographic regions, using a 2-digit code. Virginia watersheds are included in 4 of the 21 regions: Mid-Atlantic Region code (02), South Atlantic-Gulf (03), Ohio (05), and Tennessee (06).
Regions were further subdivided into 4 separate levels starting with the 21 regions and ending with 2,264 Cataloging Units. Each Cataloging Unit was defined by a unique 8-digit Hydrologic Unit Code (HUC). Virginia had about 50 of the 8-digit Cataloguing Units; the Middle Potomac-Anacostia-Occoquan HUC code was 02070010.
The need for more-detailed, smaller hydrologic units triggered different agencies to create maps at different levels of detail. At one point, the HUC codes were being extended to 14 digits. After significant effort to coordinate requirements, Federal agencies have established the 12-digit (6th level) Watershed Boundary Dataset and are mapping according to common standards. The smallest, most-detailed 6th level (the "subwatersheds") are normally 10,000-40,000 acres in size.
Virginia water managers may discuss 14 "river basins" (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) in the state. These are not defined by the Watershed Boundary Dataset classifications, so the potential for cofusion remains high whenever a report references a "basin."
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