U.S. Department of the Interior U.S. Department of Agriculture

U.S. Geological Survey Natural Resources Conservation Service

Techniques and Methods 11–A3

Third edition, 2012

Federal Standards and Procedures for the 

National Watershed Boundary Dataset (WBD)

Chapter 3 of

Section A, Federal Standards

Book 11, Collection and Delineation of Spatial Data
Cover: Photo of Oahu, Hawaii, 

by Mike Damianakis.
Federal Standards and Procedures for the 

National Watershed Boundary Dataset (WBD)

By the U.S. Geological Survey and the U.S. Department of Agriculture, 

Natural Resources Conservation Service

Techniques and Methods 11–A3

Third edition, 2012

Chapter 3 of

Section A, Federal Standards

Book 11, Collection and Delineation of Spatial Data

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

First edition: 2009

Second edition: 2011

Third edition: 2012

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Suggested citation:

U.S. Geological Survey and U.S. Department of Agriculture, Natural Resources Conservation Service, 2012, Federal 

Standards and Procedures for the National Watershed Boundary Dataset (WBD) (3d ed.): U.S. Geological Survey 

Techniques and Methods 11–A3, 63 p.

Available on the World Wide Web at http://pubs.usgs.gov/tm/tm11a3/.
iii

Acknowledgments

In 2009, the first edition of the “Federal Guidelines, Standards, and Procedures for the 

National Watershed Boundary Dataset” was produced and released through contributions by 

the Watershed Boundary Dataset National Technical Coordinators (WBD–NTC), as requested 

by the Subcommittee on Spatial Water Data (a subcommittee of the Federal Geographic Data 

Committee, Advisory Committee on Water Information), to update and clarify a previous guidance docu¬ment, “Federal Standard for Delineation of Hydrologic Unit Boundaries, Version 

2.0, October 1, 2004.” The second edition of the “Federal Standards and Procedures for the 

National Watershed Boundary Dataset (WBD)” was published in 2011. The WBD–NTC Team 

has first-hand knowledge of the use of both documents through their work with WBD creators 

from different agencies and programs and through regional workshops that contributed to 

coordinated watershed management. As in previous versions, this third edition of the “Federal 

Standards and Procedures for the National Watershed Boundary Dataset (WBD),” would not 

have been possible without the work of the U.S. Department of Agriculture, Natural Resources 

Conservation Service (NRCS), the WBD–NTC, and numerous prior contributors and reviewers.
iv

Contents

Acknowledgments........................................................................................................................................iii

Abstract...........................................................................................................................................................1

1. Introduction.............................................................................................................................................1

1.1 Purpose.........................................................................................................................................1

1.2 Background..................................................................................................................................2

2. Coordination............................................................................................................................................4

2.1 Federal Geographic Data Committee Subcommittee on Spatial Water Data ..................4

2.2 National Steering Committee and WBD National Technical Coordinators.......................4

2.3 Intrastate and Interstate WBD Coordination..........................................................................4

2.4 Stewardship of the WBD and the National Hydrography Dataset .....................................4

3. Criteria and Considerations for Delineating Hydrologic Units.......................................................6

3.1 General Criteria............................................................................................................................6

3.2 Hydrologic Boundaries...............................................................................................................6

3.3 Number of Subdivided Hydrologic Units ...............................................................................10

3.4 Sizes of 10- and 12-Digit Hydrologic Units ............................................................................10

3.5 Geomorphic Considerations for Hydrologic Units ...............................................................11

3.5.1 Standard Hydrologic Units...........................................................................................11

3.5.2 Remnant Areas ..............................................................................................................12

3.5.3 Noncontributing Areas.................................................................................................12

3.5.4 Diverted Waters.............................................................................................................13

3.5.5 Karst Areas.....................................................................................................................13

3.6 Coastal Considerations for Hydrologic Units........................................................................13

3.6.1 Bays, Sounds, and Estuaries.......................................................................................15

3.6.2 Water and Frontal Hydrologic Units...........................................................................19

 3.6.3 Islands............................................................................................................................20

3.6.4 Reservoirs and Natural Lakes.....................................................................................22

3.7 Optional 14- and 16-Digit Hydrologic Units ...........................................................................25

3.7.1 Criteria for 14- and 16-Digit Hydrologic Units...........................................................25

3.7.2 Delineation Options.......................................................................................................25

3.7.2.1 Open Option........................................................................................................25

3.7.2.2 Standard and Remnant Area Option .............................................................26

3.7.2.3 Conventional Option..........................................................................................26

3.7.3 Review of 14- and 16-Digit Hydrologic Units and Inclusion in the WBD..............26

4. Required Geographic Data Sources and Recommended Techniques for Boundary 

Delineation.......................................................................................................................................27

4.1 Map Scale and Map Accuracy...............................................................................................27

4.2 Source Maps..............................................................................................................................27

4.2.1 Base Maps......................................................................................................................27

4.2.2 Hydrologic Unit Maps...................................................................................................28

4.2.3 Reference Maps............................................................................................................28

4.2.4 Digital Data.....................................................................................................................28

4.3 Hydrologic Unit Mapping Techniques....................................................................................28
v

4.3.1 Manual Techniques for Delineating Hydrologic Unit Boundaries.........................29

4.3.2 Digital Techniques for Delineating Hydrologic Unit Boundaries...........................29

4.3.3 Spatial Modeling Techniques for Delineating Hydrologic Unit Boundaries........29

4.3.4 Delineation Using High-Resolution Base Products ................................................29

4.4 Updating and Revising 2-, 4-, 6- and 8-Digit Hydrologic Units...........................................30

5. Coding and Naming 10-, 12-, 14- and 16-Digit Hydrologic Units..................................................31

5.1 Hydrologic Unit Levels .............................................................................................................31

5.2 Coding 10-, 12-, 14- and 16-Digit Hydrologic Units...............................................................32

5.2.1 Coding Coastal Areas That Contain Frontal Units....................................................33

5.2.2 Coding Islands ...............................................................................................................33

5.3 Naming Protocol for 10-, 12-, 14- and 16-Digit Hydrologic Units.......................................33

5.3.1 Sources of Names for 10-, 12-, 14-, and 16-Digit Hydrologic Units.......................34

5.3.2 Feature Priority of Names for 10-, 12-, 14-, and 16-Digit Hydrologic Units ..........34

5.3.3 Structure of Names for 10-, 12-, 14- and 16-Digit Hydrologic Units......................34

5.4 International Borders ...............................................................................................................35

5.4.1 United States Data Harmonization with Canada......................................................35

5.4.2 United States Data Harmonization with Mexico......................................................35

6. Geospatial Data Structure ..................................................................................................................36

6.1 Specifications for Geospatial Data Structure ......................................................................36

6.1.1 Hydrologic Unit Geometry............................................................................................36

6.2 Database Schema for Attributing Hydrologic Unit Polygons.............................................36

6.2.1 Changes to the Data Model Due to Integration with the NHD ..............................36

6.2.2 Base Classes..................................................................................................................36

6.2.2.1 PermanentIdentifier..........................................................................................36

6.2.2.2 SourceFeatureID ..............................................................................................37

6.2.2.3 MetaSourceID ..................................................................................................37

6.2.2.4 SourceDataDesc ...............................................................................................37

6.2.2.5 SourceOriginator ..............................................................................................37

6.2.2.6 WBDLine ............................................................................................................38

6.2.2.6.1 Common Fields.......................................................................................38

6.2.2.6.2 WBDLine: HULevel................................................................................38

6.2.2.6.3 WBDLine: HUClass................................................................................38

6.2.2.6.4 WBDLine: LineSource .............................................................................39

6.2.2.6.5 WBDLine: LoadDate..............................................................................40

6.2.2.7 WBDPoint ..........................................................................................................40

6.2.2.7.1 Common Fields.......................................................................................40

6.2.2.7.2 WBDPoint: HUC .....................................................................................41

6.2.2.7.3 WBDPoint: HULevel .............................................................................41

6.2.2.7.4 WBDPoint: HUClass..............................................................................41

6.2.2.7.5 WBDPoint: LoadDate............................................................................41

6.2.2.8 WBDAttributes...................................................................................................42

6.2.2.8.1 Common Fields ......................................................................................42

6.2.2.8.2 WBDAttributes: HUC.............................................................................42

6.2.2.8.3 WBDAttributes: GazID..........................................................................42

6.2.2.8.4 WBDAttributes: Name .........................................................................42
vi

6.2.2.8.5 WBDAttributes: HUClass ....................................................................43

6.2.2.8.6 WBDAttributes: HUType ......................................................................43

6.2.2.8.7 WBDAttributes: NContrbAcres...........................................................43

6.2.2.8.8 WBDAttributes: LoadDate ...................................................................44

6.2.2.9 WBDPointEvent .................................................................................................44

6.2.2.10 WBDNav ..........................................................................................................44

6.2.2.10.1 WBDNav: FromHUC ............................................................................44

6.2.2.10.2 WBDNav: ToHUC.................................................................................44

6.2.2.10.3 WBDNav: HUClass..............................................................................44

6.2.2.10.4 WBDNav: UpMain...............................................................................44

6.2.2.10.5 WBDNav: DownMain ........................................................................45

6.2.2.11 WBDFeatureToHUMod ..................................................................................45

6.2.2.11.1 WBDFeatureToHUMod for lines: HUMod........................................45

6.2.2.11.2 WBDFeatureTo HUMod for polygons: HUMod ..............................47

6.2.2.12 NWISLine..........................................................................................................48

6.2.3 Derived Classes.............................................................................................................48

6.2.3.1 WBDHU2.............................................................................................................48

6.2.3.2 WBDHU4.............................................................................................................48

6.2.3.3 WBDHU6.............................................................................................................49

6.2.3.4 WBDHU8.............................................................................................................49

6.2.3.5 WBDHU10...........................................................................................................49

6.2.3.6 WBDHU12...........................................................................................................49

6.2.3.7 WBDHU14...........................................................................................................50

6.2.3.8 WBDHU16...........................................................................................................50

6.2.3.9 WBDNWIS..........................................................................................................50

6.2.4 Metadata.........................................................................................................................50

6.2.4.1 MetaProcessDetail ...........................................................................................50

6.2.4.2 MetaSourceDetail.............................................................................................51

6.2.4.3 MetaID ................................................................................................................51

7. Quality Assurance and Quality Control............................................................................................52

7.1 Quality Assurance.....................................................................................................................52

7.2 Quality Control (Editing Checklist) ..........................................................................................52

7.2.1 Delineation......................................................................................................................52

7.2.2 Codes...............................................................................................................................52

7.2.3 Attributes ........................................................................................................................53

7.2.4 Data Format....................................................................................................................53

8. Data Revision, Checkout, Editing, Verification, and Submittal ....................................................53

8.1 State Data Revision ..................................................................................................................53

8.2 Data Checkout ..........................................................................................................................55

8.2.1 Stewardship Web Site..................................................................................................55

8.3 Data Editing ................................................................................................................................55

8.4 Data Verification........................................................................................................................55

8.4.1 File Names .....................................................................................................................55

8.4.2 Map Projection and Horizontal Datum ......................................................................55

8.5 Data Submittal............................................................................................................................55
vii

9. Metadata................................................................................................................................................55

10. Selected References .........................................................................................................................56

11. Appendixes..........................................................................................................................................57

11.1 Hydrologic Unit Names, Historical Names, Average Sizes, and Approximate Number 

of Hydrologic Units, Per Nested Level, in the United States..........................................57

11.2 Definitions.................................................................................................................................57

11.2.1 Hydrologic Definitions ................................................................................................57

11.2.2 Geospatial Data Definitions and Standards...........................................................59

11.2.2.1 Definitions.........................................................................................................59

11.2.2.2 Standards .........................................................................................................60

11.2.3 Compliance Definitions ..............................................................................................60

11.3 Abbreviations and Acronyms................................................................................................61

11.4 Data Type Crosswalk for Data Storage of Esri Products..................................................62

11.5 Previously Used Metadata Identification Attribute for Tracking Metadata..................62

11.6 Historical Sample Letter for Certification or Data Update Submittal ............................63

Figures

1. Watershed Boundary Dataset coordination participants and relationships......................5

2. This 12-digit hydrologic unit boundary meets the National Standards for the 

Spatial Data Accuracy and the Watershed Boundary Dataset standards 

because it is georeferenced to the minimum 1:24,000-scale topographic base 

map..................................................................................................................................................6

3. Hierarchy and areas for the six nested levels of hydrologic units are shown 

in the above example ...................................................................................................................7

4. Delineation of hydrologic features must be based on land surface, surface-water 

flow, and hydrographic features................................................................................................9

5. The two tributary outlets shown above are delineated at the bank of the 

receiving stream.........................................................................................................................10

6. In this example of an 8-digit hydrologic unit defined by the hydrology (Imnaha Subbasin), the main-stem stream has been split into “Upper,” “Middle,” and “Lower” 

watersheds and its tributary watersheds...............................................................................11

7. Noncontributing areas, caused by unique and unusual landforms, do not flow 

to the outlet of the hydrologic unit...........................................................................................12

8. To delineate the hydrologic unit boundary for frontal hydrologic units in open 

ocean areas, extend the boundaries of the frontal unit to intersect offshore 

boundaries ...................................................................................................................................14

9. The example depicts coastal delineation and coding for areas that drain 

into a water hydrologic unit (in this case, in North Carolina)..............................................15

10. A combination of depth (bathymetry), head of land, and variable distances 

(buffering) from the NHD coastline representation may be applied or frontal 

land and water units may be combined when nearshore hydrologic unit 

boundaries are delineated ........................................................................................................17

11. A gradual transition is needed between two offshore depths or distances, such 

as the 200-foot buffer in this bay and the 4,000-foot buffer in this open-water 

area of offshore Louisiana ........................................................................................................18
viii

12. The outer extent of these standard and frontal hydrologic units in the Santa 

Monica Bay area, California, is delineated to the first offshore boundary, the 

NOAA Three Nautical Mile Line...............................................................................................19

13. If an island falls within any offshore unit, then it can be included in that unit 

without further delineation or a nearshore buffer can be used to subdivide it 

from the larger unit providing it meets size criteria as shown in the Blue Hill Bay 

area in Maine ..............................................................................................................................20

14. For an island that is beyond the mainland NOAA Three Nautical Mile Line, such 

as the Channel Islands off the California coast, the outer extent of the island 

hydrologic unit is the NOAA Three Nautical Mile Line ........................................................21

15. To delineate the hydrologic unit boundary within a hydrologic unit that contains 

a major reservoir, such as the Lake Mead area of Arizona and Nevada, the 

preferred treatment is to use a topographic map showing preinundation contours 

of underwater features if such a map is available................................................................23

16. To delineate the hydrologic unit boundary within an area that contains a lake, 

such as Priest Lake on the border of Washington and Idaho, the preferred 

treatment is to break out standard hydrologic units of appropriate size and 

then combine nonstandard areas having direct flow into the lake with the lake 

hydrologic unit.............................................................................................................................24

17. Each hydrologic unit should be coded sequentially based on the location of the 

outlet, starting with the uppermost stream outlet.................................................................32

18. When high flows cause a natural diversion and the excess runs into an adjacent 

channel draining to a different hydrologic unit, the special condition is named 

overbank flow..............................................................................................................................46

19. The Metadata Process Details template within the WBD Edit Tool is where 

information is entered about the editor of a particular WBD edit and why the 

edit was made .............................................................................................................................50

20. The Metadata Source Detail template within the WBD Edit Tool is where 

information is entered about the base data used to make the edits and updates 

to the WBD...................................................................................................................................51

21. The Watershed Boundary Dataset revision process and review for national 

certification compliance and roles are depicted in the flowchart.....................................54
ix

Conversion Factors

Inch-pound to SI 

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

mile, nautical (nmi) 1.852 kilometer (km)

Area

acre 4,047 square meter (m2)

acre .4047 hectare (ha)

acre .4047 square hectometer (hm2) 

acre .004047 square kilometer (km2)

square mile (mi2) 2.590 square kilometer (km2) 

SI to inch-pound

Multiply By To obtain

Length

meter (m) 3.281 foot (ft) 

kilometer (km) .6214 mile (mi)

kilometer (km) .5400 mile, nautical (nmi) 

Area

square meter (m2) 10.76 square foot (ft2) 

square meter (m2) .0002471 acre 

Federal Standards and Procedures for the National 

Watershed Boundary Dataset (WBD)

By U.S. Geological Survey and U.S. Department of Agriculture, Natural Resources Conservation Service

Abstract

The Watershed Boundary Dataset (WBD) is a comprehensive aggregated collection of hydrologic unit data consistent with the national criteria for delineation and resolution. 

This document establishes Federal standards and procedures 

for creating the WBD as seamless and hierarchical hydrologic 

unit data, based on topographic and hydrologic features at 

a 1:24,000 scale in the United States, except for Alaska at 

1:63,360 scale, and 1:25,000 scale in the Caribbean. The data 

within the WBD have been reviewed for certification through 

the 12-digit hydrologic unit for compliance with the criteria 

outlined in this document. Any edits to certified data will be 

reviewed against this standard prior to inclusion. Although 

not required as part of the framework WBD, the guidelines 

contain details for compiling and delineating the boundaries of two additional levels, the 14- and 16-digit hydrologic 

units, as well as the use of higher resolution base information 

to improve delineations. The guidelines presented herein are 

designed to enable local, regional, and national partners to 

delineate hydrologic units consistently and accurately. Such 

consistency improves watershed management through efficient 

sharing of information and resources and by ensuring that 

digital geographic data are usable with other related Geographic Information System (GIS) data. 

Terminology, definitions, and procedural information are 

provided to ensure uniformity in hydrologic unit boundaries, 

names, and numerical codes. Detailed standards and specifications for data are included. The document also includes discussion of objectives, communications required for revising the 

data resolution in the United States and the Caribbean, as well 

as final review and data-quality criteria. Instances of unusual 

landforms or artificial features that affect the hydrologic units 

are described with metadata standards. Up-to-date information and availability of the hydrologic units are listed at http://

www.ncgc.nrcs.usda.gov/products/datasets/watershed/.

1. Introduction

1.1 Purpose

This document establishes Federal standards and procedures for creating a national, consistent, seamless, and 

hierarchical hydrologic unit dataset based on topographic and 

hydrologic features across the United States and territories. 

The WBD defines the perimeters of drainage areas (hydrologic 

units), formed by the terrain and other landscape characteristics, at a 1:24,000 scale in the United States, except for Alaska 

at 1:63,360 scale and 1:25,000 scale in the Caribbean, and it 

consists of digital geographic data that include six levels of 

detailed nested hydrologic unit boundaries. 

This document serves as interagency guidance for the 

improvement of digital geographic data for 10-digit hydrologic units (Watersheds, formerly referred to as fifth level) and 

12-digit hydrologic units (Subwatersheds, formerly referred 

to as sixth level), and for the optional delineation of 14- and 

16-digit hydrologic units. The guidelines in this document are 

intended to provide a consistent framework for local, regional, 

and national needs in States, Tribal Lands, Pacific Islands, 

Puerto Rico, and the U.S. Virgin Islands to accurately delineate watersheds. This document sets forth terminology, definitions, and procedural information to ensure the uniform development of hydrologic unit boundaries and numerical codes 

by the agencies, tribes, and other organizations that develop, 

manage, archive, exchange, and analyze data by hydrologic 

features. The information presented is intended to enable users 

from different agencies and programs to contribute to coordinated watershed management, to efficiently share information and resources, and to ensure the digital geographic data 

are usable with other related Geographic Information System 

(GIS) data. 
2 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

The use of these criteria and techniques for hydrologic 

unit selection and boundary delineation will permit use of the 

standardized hydrologic units by a diverse group serving multiagency programs. Some examples of these programs include 

watershed management, water-quality initiatives, watershed 

modeling, resource inventory and assessment, and Total Maximum Daily Load development. The usefulness of hydrologic 

units in a variety of sizes based on natural surface-water flow 

and topography cannot be overestimated for analytical and 

statistical purposes and applications. Instances of unusual 

landforms or artificial features that affect the hydrologic units 

are recorded in attributes and associated metadata, but in no 

way should they detract from the intent of these data to reflect 

surface-water flow.

The distribution of GIS layers and associated maps created by use of uniform guidelines and procedures improves 

the quality, consistency, and accessibility to hydrologic unit 

data nationwide, and thus becomes the framework for continued updates. Up-to-date information and availability of the 

hydrologic units are listed at http://www.nrcs.usda.gov/wps/

portal/nrcs/main/national/water/watersheds/dataset. This 

document addresses concessions and changes necessary to 

meet database integration standards between the WBD and the 

National Hydrography Dataset (NHD). Updates to this edition 

also include additional content for instruction on the use of 

the optional higher resolution base data for improvements, 

stewardship, Geographic Names Information System (GNIS) 

integration, expanded domains, changes in the population of 

the modification field, and current information on international 

border harmonization. Of particular interest to the user community is the inclusion of offshore areas in coastal hydrologic 

units, which enhances vertical integration and eliminates 

duplicate features in the combined NHD and WBD geodatabase. Further enhancements include minimizing future coastline updates in the WBD for long-term stability and improving 

flexibility for raster applications and modeling needs. 

This document refers to the nested hierarchy of the drainage areas by the hydrologic unit name ( 2-, 4-, 6-, 8-, 10-, and 

12-digit hydrologic unit) rather than the historical names of 

the general classes of the hydrologic units (Region, Subregion, 

Basin, Subbasin, Watershed, and Subwatershed). This is compatible with WBD names integration with GNIS.

1.2 Background

A standardized system for organizing and collecting 

hydrologic data was developed in the mid-1970s by the U.S. 

Geological Survey (USGS) under the sponsorship of the Water 

Resources Council (Seaber and others, 1987). This system 

divided and subdivided the country into successively smaller 

hydrologic units based on surface features; established associated codes, names, and boundaries for the units; and classified 

them into four levels: Regions, Subregions, Accounting Units 

(later referred to as “Basins”), and Cataloging Units (later 

referred to as “Subbasins”) (see section11.1). The hierarchical 

hydrologic unit code consists of two-digit numbers for each of 

the four nested hydrologic unit levels. These four levels aggregated form the 8-digit hydrologic unit code. The underlying 

hydrologic concept is a topographically defined set of drainage 

areas organized in a nested hierarchy by size and number of 

divisions per nested level.

The standardized 8-digit USGS Hydrologic Units are 

broadly used; however, the geographic area of the units was 

too large to adequately serve many water-resource investigations or resource-analysis and management needs. For 

example, the focus of many water-resource issues is pollutant 

loading and land-surface processes and the cumulative effects 

of pollution over space and time. Management of these issues 

required working with areas smaller than those defined by 

the 8-digit hydrologic units. Examples of programs requiring 

smaller hydrologic units include State river basin management plans; the U.S. Department of Agriculture (USDA), 

Natural Resources Conservation Service (NRCS) conservation 

and watershed programs; USDA Forest Service (USDA–FS) 

land-management-planning and watershed-management 

programs; various programs in the U.S. Environmental Protection Agency (USEPA), Office of Water; and programs in the 

USGS. 
1. Introduction 3

The NRCS is responsible for working with landowners 

to protect, improve, and sustain natural resources on private 

lands. In the early 1980s, the NRCS, which was known as the 

Soil Conservation Service (SCS) prior to 1994, completed 

mapping of 10-digit hydrologic units on small-scale State base 

maps for use in natural resource planning. In the mid-1990s, 

the NRCS, along with State agency conservation partners, 

began a national initiative to delineate and digitize 10- and 

12-digit hydrologic units using methods and procedures that 

result in data that meet national map accuracy standards. 

In 1992, NRCS developed National Instruction 170–304 to 

promote a standardized criterion for hydrologic unit determination and delineation that would serve as the agency’s policy 

for delineating and digitizing 10- and 12-digit hydrologic units 

(Soil Conservation Service, 1992). The NRCS updated the 

policy in 1995, incorporating changes from internal and external reviews. The NRCS has made considerable contributions 

to the development of nationally standardized 10- and 12-digit 

hydrologic unit boundaries (Natural Resources Conservation 

Service, 2002). 

The USDA–FS and the Bureau of Land Management 

(BLM) are the primary land-management agencies of Federal 

lands in the United States. These agencies, and coordinating 

States, delineated and digitized 10- and 12-digit hydrologic 

units within an 8-digit hydrologic unit. Earlier delineations of 

hydrologic units of federally administered public lands served 

administrative purposes, but they were often developed without full coordination between Federal and State agencies. 

The USGS and member agencies of the Federal Geographic Data Committee (FGDC), Subcommittee on Spatial 

Water Data (SSWD), coordinated and conducted a series of 

regional workshops to develop a nationally consistent hydrologic unit digital dataset. Member agencies of the SSWD 

assisted the NRCS and USGS in reviewing and certifying 

hydrologic units. Similarly, member agencies and others have 

assisted in researching techniques to employ digital elevation 

data for producing hydrologic unit delineations that function 

as draft lines to adjust to 1:24,000 contour lines where needed. 

Although the BLM and USDA–FS are responsible for 

managing Federal lands, many State programs are responsible 

for managing State-owned land. As keepers of the land, States 

have partnered with Federal agency offices located within their 

State to develop a WBD that meets map accuracy standards 

and reflects local knowledge of surface-water resources. 

A State agency is often the intermediary between local 

and Federal programs for land management. A representative 

example is North Dakota, where the Department of Health 

administers the nonpoint-source pollution control program. 

This watershed-based program provides EPA grant funds to 

local organizations for projects that reduce nonpoint-source 

pollution. In addition, the North Dakota Game and Fish 

Department uses the 12-digit hydrologic unit codes for their 

“Save Our Lakes” program, and the Private Lands Section of 

the Wildlife Division uses the 12-digit hydrologic unit codes 

for land-use inventories for their program. 

To improve the sharing of national data, minimize the 

duplication of effort of various agencies, and discourage the 

creation of disparate datasets, the USGS and member agencies 

of the FGDC SSWD support a formal Memorandum of Understanding between the USGS National Geospatial Program and 

the NRCS National Cartography and Geospatial Center, now 

the National Geospatial Management Center (NGMC), signed 

in June of 2008. This agreement between the two agencies 

identifies the roles of the agencies and establishes the cooperative enhancement, maintenance, integration, and distribution 

of the WBD with the NHD and the inclusion of the WBD as a 

component of The National Map. The agreement benefits the 

cooperative development of the WBD and the NHD, as well as 

the business needs of the agencies, by making optimum use of 

an escalating amount of national watershed and hydrography 

data. 

Since June of 2008, the USGS has started programmatic, 

data, editing tool, and stewardship integration of the WBD 

with the NHD. In 2012, USGS became the host of the WBD 

dataset of record, and NRCS is replicating this service. USGS 

and NRCS remain as copartners on the WBD. 
4 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

2. Coordination

2.1 Federal Geographic Data Committee 

Subcommittee on Spatial Water Data 

The SSWD was chartered and sponsored by the Advisory 

Committee on Water Information (ACWI) and the FGDC. 

The SSWD of the FGDC coordinates spatial water data and 

information activities among all levels of government and the 

private sector. Spatial water data include information about 

streams, hydrologic units, lakes, groundwater, coastal areas, 

precipitation, and other hydrologic information related to 

water resources. 

Federal and State agencies involved in development 

and use of hydrologic units for water-resource management 

responsibilities are encouraged to participate as members of 

the SSWD. The SSWD assists the ACWI and FGDC by facilitating the exchange and transfer of water data; establishing 

and implementing standards for quality, content, and transfer 

of water data; and coordinating standards and collection of 

geographic data to minimize duplication of efforts.

2.2 National Steering Committee and WBD 

National Technical Coordinators

To facilitate development of nationally consistent data, 

two groups were established in the late 1990s. The first is the 

National Steering Committee, consisting of Federal representatives led by the NRCS and the USGS. The second is the 

WBD National Technical Coordinators (WBD–NTC). The 

groups engage in program management and long-term planning, integrate and coordinate with other national projects, 

and facilitate intrastate and interstate cooperation. The teams 

regularly clarify guidance, provide oversight and training to 

States, review interim State geographic data, suggest solutions 

for complex hydrographic landscapes, and review final data 

for standard compliance and incorporation into the WBD. The 

Steering Committee oversees final review and acceptance of 

digital geographic data into the national framework after recommendation by the WBD–NTC, grants certification, and thus 

approves the data for public access via the Web.

The USGS facilitates the WBD effort by maintaining 

the standards presented in this publication and by providing 

delineation support and review to all States. The WBD-NTC 

reviews data for standard compliance, and then the USGS integrates and merges the individual State certified datasets into a 

seamless national layer and processes the data for delivery on 

the Geospatial Data Gateway.

2.3 Intrastate and Interstate WBD Coordination

During creation of the WBD, further subdivision of the 

8-digit hydrologic units into 10- and 12-digit hydrologic units 

provided an opportunity to develop consistent and commonly 

used nationwide digital geospatial data. States formed interagency hydrologic unit coordinating groups composed of Federal, State, local, and watershed agencies with an investment 

in developing hydrologic unit data, specifically including State 

agencies identified by State statute as having responsibility 

for the data. Each participating organization sought consensus appropriate to all interests and to obtain mutual technical approval. Each State coordinating group promoted the 

development, use, and maintenance of the WBD and identified 

a WBD In-State Steward to work with the WBD–NTC.

All States coordinated their delineation work within 8-, 

10-, and 12-digit hydrologic units so that the digital geographic data matched across State borders. This coordination 

included the locations of outlet points, the sizes of hydrologic units, and the coding sequence within each level of the 

hydrologic unit hierarchy, as well as other attribute fields. The 

mapping of hydrologic boundaries across political boundaries 

was coordinated to ensure a nationally consistent dataset.

The interaction between these groups and data originators 

is shown in figure 1.

2.4 Stewardship of the WBD and the National 

Hydrography Dataset

The NHD is the surface-water component of The 

National Map and a companion dataset to the WBD. The NHD 

is a comprehensive set of digital spatial data that represents 

the surface water of the United States using common features 

such as lakes, ponds, streams, rivers, canals, stream gages, 

and dams. Polygons are used to represent area features such as 

lakes, ponds, and rivers, lines are used to represent linear features such as streams and smaller rivers, and points are used to 

represent point features such as stream gages and dams.

Although the WBD and NHD are administered under a 

separate model, they are coordinated so that a single source 

of universal, useful, and reliable hydrography and hydrologic 

units is produced and maintained. The improvement of both 

datasets focuses on spatial integration, application, and maintenance through stewardship. 

Management of WBD–NHD data is distributed across the 

Nation, typically on a State-by-State basis. The WBD In-State 

Stewards coordinate and assume responsibility for identifying 

and implementing changes at the State level. Other organizations with specific local or topical interests may assume 

further stewardship under the auspices of the WBD In-State 

Stewards. In many cases, the State data stewards for the 

NHD and WBD are represented by different individuals and 

organizations. 
2. Coordination 5

Figure 1. Watershed Boundary Dataset coordination participants and relationships. (GIS, geographic information system)

The NHD approach to data stewardship provides documented roles and responsibilities between the USGS and NHD 

In-State Stewards. In most cases, existing NHD agreements 

will be modified to include WBD responsibilities, and new 

agreements will integrate stewardship for both datasets.

Because the WBD has national consistency standards, 

the WBD–NTC will continue to work closely with the WBD 

In-State Stewards. This will ensure that future revisions to the 

WBD follow the guidelines set forth in this document. Data 

revisions will become transactions provided for inclusion in 

the NHD geodatabase administered by the USGS on behalf of 

the entire user community. The interaction between the WBD 

and NHD data stewards after local enhancements are made is 

shown in figure 1.

Editing tools that allow data stewards to upgrade the 

individual datasets while preserving the integrity of the model 

and the geometric relationships in the hydrography datasets 

are the key to the implementation of data stewardship. Suites 

of editing tools are available for the NHD and the WBD. 

The USGS National Geospatial Technical Operations Center 

(NGTOC) is responsible for maintaining and improving these 

tools. The NHDGeoEdit tools work with a spatial geodatabase. 

To maintain the hydrologic units of the WBD in the future, the 

In-State Stewards will be provided with a similar, but unique, 

WBD tool set that facilitates editing WBD data and managing 

metadata. 

U.S. Geological Survey

National Geospatial 

Techical Operations 

Center

U.S. Geological 

Survey

National Spatial Data 

Infrastructure Office 

Geospatial Liaison

U.S. Geological Survey

National Hydrography

Dataset Geodatabase

National 

Technical

Coordinators

National Hydrography

Dataset In-State

Steward

State 

agencies

Counties

Federal

agencies

Native American

governments

and

organizations

GIS user groups

Other

groups

WBD In-State

Steward

National level guidance and support

State level coordination and support

(Principal Stewards)

Local

level

collaboration 

(Stakeholders)

In-State Interagency 

Hydrologic Unit 

Coordinating Group

(IHUCG)

High-resolution National Hydrography Dataset Watershed Boundary Dataset
6 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

3. Criteria and Considerations for 

Delineating Hydrologic Units

3.1 General Criteria

This section describes criteria for determining and 

delineating 10- and 12-digit hydrologic units and optional 

14- and 16-digit conventionally delineated hydrologic units. 

The 2- through 8-digit hydrologic units will be consistent with 

the new hydrologic units to create one dataset of 2- through 

12-digit hydrologic unit codes and boundaries at a 1:24,000 

scale in the United States, except for Alaska at 1:63,360 scale 

and 1:25,000 scale in the Caribbean. Selecting and delineating hydrologic units requires sound hydrologic judgment and 

must be based solely upon hydrologic principles to ensure a 

homogeneous, national, and seamless digital data layer. The 

diversity of hydrologic conditions nationwide, the complexity 

of surface hydrology, and the number of factors involved in 

the delineation process preclude an all-encompassing guideline. Variations will generally be limited to unusual hydrologic 

or landform features and dissimilar hydrologic or morphologic drainage-area characteristics. The intent of defining 

hydrologic units is to establish a baseline that covers all areas. 

At a minimum, the hydrologic units must be delineated and 

georeferenced to USGS 1:24,000-, 1:25,000-, or 1:63,360-

scale topographic maps, which meet the National Standard 

for Spatial Data Accuracy (NSSDA; Federal Geographic 

Data Committee, 1998b) (fig. 2). Digital Orthophoto Quarter 

Quadrangles (DOQQs) or higher resolution imagery contours 

of elevation, stream locations, and other relevant data are useful for keeping the hydrologic unit boundaries accurate and 

current. 

The delineation must be as simple as is practical and 

avoid creating hydrologic units that favor a particular agency, 

program, administrative area, or special project. Drainage 

boundaries generated for special purposes that do not follow 

these guidelines will not be accepted into the WBD.

3.2 Hydrologic Boundaries

Figure 3 illustrates the United States hydrologic unit 

boundaries, the six nested levels of the hydrologic unit hierarchy, and the aggregated sequence of hydrologic unit codes 

(from a 2-digit to a 12-digit number). The large geographic 

area of 8-digit hydrologic units is the basis for subdividing 10-digit hydrologic units, and the 10-digit hydrologic 

unit boundaries are the basis for further subdividing 12-digit 

Figure 2. This 12-digit hydrologic unit boundary meets the National Standards for the Spatial Data Accuracy 

and the Watershed Boundary Dataset standards because it is georeferenced to the minimum 1:24,000-scale 

topographic base map. The Appalachian Trail south of Enders, in Dauphin County, Pennsylvania, is shown.

Hydrologic unit boundary

12-digit hydrologic unit

(Subwatershed)

EXPLANATION

PENNSYLVANIA P E T E R S M O U N T A I N

Creek

Clark

Victoria Furnace

SCALE 1:24,000

0 0.25 0.5 0.75 1 MILE

0 0.25 0.5 0.75 1 KILOMETER
3. Criteria and Considerations for Delineating Hydrologic Units 7

Figure 3. Hierarchy and areas for the six nested levels of hydrologic units are shown in the above example. As they are successively 

subdivided, the numbering scheme of the units increases by two digits per level.

Pacific Northwest

Region 17

(273,647 square miles)

Lower Snake

Subregion 1706

(35,200 square miles)

Lower Snake

Basin 170601

(11,800 square miles)

South Fork Imnaha River 

Subwatershed 170601020101

(17,800 acres)

Upper Imnaha River

Watershed 1706010201

(141 square miles)

Imnaha Subbasin

17060102

(855 square miles)

2-digit hydrologic unit

First level 

Region

(177,560 square miles 

average)

4-digit hydrologic unit

Second level 

Subregion

(16,800 square miles 

average)

6-digit hydrologic unit

Third level 

Basin

(10,596 square miles 

average)

8-digit hydrologic unit

Fourth level 

Subbasin

(700 square miles

 average)

10-digit hydrologic unit

Fifth level 

Watershed

(227 square miles average,

40,000−250,000 acres)

12-digit hydrologic unit

Sixth level 

Subwatershed

(40 square miles average, 

10,000−40,000 acres)

Figure 3. Hierarchy and areas for the six nested levels of hydrologic units are shown in the above example. As they are successively subdivided, the numbering 

scheme of the units increases by two digits per level.

third edition 041112

Hydrologic unit boundary

2 digit, Region

4 digit, Subregion

6 digit, Basin

8 digit, Subbasin

10 digit, Watershed

12 digit, Subwatershed

EXPLANATION
8 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

hydrologic units. By delineating 10- and 12-digit hydrologic 

units at a 1:24,000 scale in the United States, except for 

Alaska at 1:63,360 scale, and 1:25,000 scale in the Caribbean, 

line precision and accuracy will be propagated throughout the 

dataset. 

If more accurate base reference information becomes 

available and editing suggests that more than minor changes 

are needed to the 8-digit hydrologic unit boundary, then the 

proposed edits must be coordinated with and preapproved by 

the WBD–NTC. If changes or revisions to 8-digit hydrologic 

unit boundaries are needed in addition to resolution improvements required for 1:24,000-scale data (section 4.4), then 

inform the WBD In-State Steward or interagency hydrologic 

unit group point of contact of the major change to prompt 

consultation and direction from the WBD–NTC.

The 10- and 12-digit hydrologic units, like other hydrologic units, are defined along natural hydrologic breaks based 

on land surface and surface-water flow. Boundaries delimit 

the land area of the 10- and 12-digit hydrologic units. A 

hydrologic unit has a single flow outlet except in frontal, lake, 

braided-stream, or playa (closed basin) hydrologic units (sections 3.5, 3.6). A hydrologic unit with an outlet at a delta or 

braided stream should be treated in a similar manner to that of 

a frontal hydrologic unit. Give priority to delineating 10- and 

12-digit hydrologic units that will be “standard” units having only one outlet (section 3.5.1). Because 10- and 12-digit 

hydrologic units are subdivisions of a higher level of hydrologic unit, they must share common boundaries with the existing hydrologic units defined in higher levels of the hydrologic 

unit hierarchy. A dam, diversion, or stream confluence may be 

used to divide a hydrologic unit into upper and lower parts.

Previous versions of 10- and 12-digit hydrologic unit 

boundaries may have used administrative boundaries as 

hydrologic unit boundaries. Now and in the future, hydrologic 

unit boundaries must be defined solely by examination of 

topography and hydrologic features. Do not use administrative 

or political boundaries, such as county, State, or national forest 

boundaries, as criteria for defining a hydrologic unit boundary unless the administrative boundary is coincident with a 

topographic feature that appropriately defines the hydrologic 

unit. Existing hydrologic unit data that include boundaries 

delineated solely by use of administrative or political boundaries will not be certified as meeting these guidelines until the 

hydrologic units are revised based on topography, surfacewater flow, and hydrologic features.

Hydrologic unit delineations along the international 

boundary of the United States are to be coordinated through 

the WBD–NTC because established processes, international 

efforts, and identified Federal partners are in place with Canada and Mexico. For units including offshore waters, use the 

National Oceanic and Atmospheric Administration (NOAA) 

Three Nautical Mile Line (section 3.6) as the offshore limit. 

Boundary delineations based on hydrology include 

land areas on both sides of a stream flowing toward a single 

downstream outlet, except in the case of open-water hydrologic units (section 3.6). Boundaries should not follow or run 

parallel to streams as delineated hypothetically in figure 4A, 

except where physical features such as levees, berms, incised 

channels, and similar structures prevent water from flowing 

directly to the outlet. Do not delineate boundaries down the 

middle of a stream. Figure 4B illustrates correct delineation. 

Boundaries will cross the stream perpendicular to flow at the 

hydrologic unit outlet (fig. 5). Where the main stem hydrologic 

unit outlet is defined at a confluence with a major tributary, 

the boundary should be placed on the downstream side of the 

confluence (fig. 5). The hydrologic unit boundary may use 

smaller tributaries as the delineation point to divide the hydrologic unit into suitably sized 10- or 12-digit hydrologic units. 

Delineating the boundary at a confluence accommodates the 

nesting of smaller units within the hydrologic unit for future 

site-specific planning, assessment, monitoring, or inventory 

activities. Information from stream gages, locations of major 

highway crossings, and NHD reach endpoints may aid in the 

identification of hydrologic unit divisions.

In addition to the primary criteria, there are general 

criteria for the number of hydrologic units subdivided from a 

higher level unit, the size of hydrologic units, and the treatment of noncontributing and remnant areas. 
3. Criteria and Considerations for Delineating Hydrologic Units 9

Figure 4. Delineation of hydrologic features must be based on land surface, surface-water flow, and hydrographic features. A. 

Incorrect—8-digit hydrologic unit boundaries follow the river and administrative boundaries. B. Correct—8-digit hydrologic unit 

boundaries cross the Chattahoochee River and end at the bank of the receiving stream for tributaries. The area includes the Walter F. 

George Reservoir south of Columbus, Georgia.

ALABAMA GEORGIA

ALABAMA

GEORGIA

ALABAMA

GEORGIA

0 3 6 MILES

0 3 6 KILOMETERS

Columbus

Columbus

Chattahoochee

River

Chattahoochee

River

Walter F. George 

Reservoir

Walter F. George 

Reservoir

N

Hydrologic unit boundary

10 digit, Watershed

12 digit, Subwatershed

EXPLANATION

A. Incorrect

B. Correct

Figure 4. Delineation of hydrologic features must be based on land surface, surface-water flow, and hydrographic features. A. Incorrect-8-

digit hydrologic unit boundaries follow the river and administrative boundaries. B. Correct—8-digit hydrologic unit boundaries cross the 

Chattahoochee River and end at the bank of the receiving stream for tributaries. The area includes the Walter F. George Reservoir south of 

Columbus, Ga.

third edition 041112
10 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

3.3 Number of Subdivided Hydrologic Units

As a general rule, subdivide each hydrologic unit into 

5–15 units. For example, 5–15 10-digit hydrologic units will 

be nested in each 8-digit hydrologic unit. This system accommodates geomorphic or other relevant basin characteristics 

and creates a fairly uniform size distribution of same-level 

hydrologic units within a broad physiographic area. This 

system also results in a smooth transition between sizes of 

same-level hydrologic units as topography changes between 

physiographic areas and maintains consistent delineations 

across State borders. 

There are exceptions to using the 5–15 rule, just as there 

are existing 4-digit hydrologic units that contain fewer than 

five 6-digit hydrologic units and 6-digit hydrologic units that 

contain fewer than five 8-digit hydrologic units. The number 

of 10-digit hydrologic units nested within 8-digit hydrologic 

units or the number of 12-digit hydrologic units nested within 

some 10-digit hydrologic units may occasionally be reduced 

or expanded to accommodate special areas. In some places, 

it is not possible to delineate 10- and 12-digit hydrologic 

units owing to the lack of hydrologic features or insufficient 

topography. Where offshore boundaries are required, 8-digit 

hydrologic units must be extended to the NOAA Three Nautical Mile Line.

3.4 Sizes of 10- and 12-Digit Hydrologic Units

The hydrologic units of any given level should be about 

the same size within a physiographic area for comparability 

and coordinating use and applications of hydrologic units. 

Hydrologic units should not be substantially different in size 

from the rest of the hydrologic units for a given level. 

Nationally, the typical size for a 10-digit hydrologic unit 

is 40,000–250,000 acres. Use this acreage range as a guide 

in subdividing 8-digit hydrologic units. Each 10-digit hydrologic unit must be completely contained within one 8-digit 

hydrologic unit. The typical size for a 12-digit hydrologic unit 

is 10,000–40,000 acres; however, in some areas with unique 

geomorphology the 12-digit hydrologic units may be greater 

than 40,000 acres or less than 10,000 acres, but never less than 

3,000 acres. A variance outside the size criteria of up to 10 

percent of the polygons within a State is allowed for both the 

10-digit and the 12-digit hydrologic unit.

In coastal areas where radial or centripetal drainage 

predominates, such as Hawaii, individual streams with outlets 

to the ocean, or remnants, may be less than 3,000 acres each. 

Their acreage can be combined into a single hydrologic unit of 

greater than 10,000 acres. In nonterrestrial coastal hydrologic 

units, subdivision to meet size criteria is not required, however the benefit of uniformity of sizes at any level should be 

considered.

Figure 5. The two tributary outlets shown above are delineated at the bank of the receiving stream. The boundary correctly crosses 

the Susquehanna River perpendicular to the river just downstream from the tributary confluence. The area is south of Bloomsburg, in 

Columbia County, Pennsylvania.

Figure 5. The two tributary outlets shown above are delineated at the bank of the receiving stream. The boundary correctly crosses 

the Susquehanna River perpendicular to the river just downstream from the tributary confluence. The area is south of Bloomsburg, in 

Columbia County, Pa.

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Hydrologic unit boundary

12 digit, Subwatershed

EXPLANATION

Catawissa

SCALE 1:60,000

0 0.5 1 MILE

0 0.5 1 KILOMETER

Susquehanna River

Boundary

crosses river

Outlet

at confluence

PENNSYLVANIA
3. Criteria and Considerations for Delineating Hydrologic Units 11

3.5 Geomorphic Considerations for Hydrologic 

Units

This section explains the most common geomorphic circumstances and details those that require additional considerations when hydrologic units are developed. 

3.5.1 Standard Hydrologic Units

The standard hydrologic unit is defined by the surfacewater drainage area (fig. 6). All of the surface drainage within 

the standard hydrologic unit boundary converges at a single 

outlet point. Larger standard hydrologic units may be subdivided into multiple hydrologic units in order to fit within the 

size criteria of a given level. 

Delineate by starting from the designated outlet (a point 

on a single stream channel that drains the area), and proceed 

to the highest elevation of land dividing the direction of water 

flow. This boundary connects back to the designated outlet, 

where it will cross perpendicular to the stream channel. Correctly selecting the outlet point is critical to delineating all 

hydrologic units accurately. The standard hydrologic units are 

subdivisions of higher level hydrologic units based on major 

tributaries. When choosing between tributaries for hydrologic 

unit delineation, larger streams are typically chosen over 

smaller streams. The downstream end of the hydrologic unit 

will be at the confluence with the main stem of the higher level 

hydrologic unit or the main stem of a hydrologic unit of the 

same level, when possible (fig 5.). 

A hydrologic unit may be divided at a lake outlet if the 

upstream drainage-area size is appropriate for the hydrologic 

unit level being delineated. The boundaries should be as 

simple as possible while capturing the topographically defined 

area that contributes to the outlet.

Identifying standard hydrologic units and delineating 

them with the recommended number of subdivisions and area 

sizes will cover much of the area of the higher level hydrologic unit to be subdivided. Where nonstandard areas such as 

remnant (section 3.5.2), noncontributing (section 3.5.3), and 

diverted (section 3.5.4) areas exist, they will need to be delineated by use of criteria described in the following sections. 

Nonstandard areas are typically added to adjacent hydrologic 

units, but they occasionally may have to exist as small, atypical hydrologic units.

Figure 6. In this example of an 8-digit hydrologic unit defined 

by the hydrology (Imnaha Subbasin), the main-stem stream has 

been split into “Upper,” “Middle,” and “Lower” watersheds and 

its tributary watersheds. The Imnaha River is in Wallowa County, 

Oregon.

Imnaha

Sheep Creek

River

Creek

Little Sheep

OREGON

N

Standard hydrologic unit

Lower watershed

Upper 

watershed

Middle 

watershed

Upper, middle, lower watershed

Hydrologic unit boundary

4 digit, Subregion

8 digit, Subbasin

10 digit, Watershed

EXPLANATION

(6 digit, Basin-level 

boundary not represented 

in this example)

Figure 6. In this example of an 8-digit hydrologic unit defined by the hydrology 

(Imnaha Subbasin), the main-stem stream has been split into “Upper,” “Middle,” 

and “Lower” watersheds and its tributary watersheds. The Imnaha River is in 

Wallowala County, Oreg.

third edition 041112

0 4 8 12 16 MILES

0 8 16 KILOMETERS
12 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

3.5.2 Remnant Areas

Delineating hydrologic units may result in remnant areas 

around the main stem of larger streams, even when sound 

hydrologic judgment and standard practices described above 

are used. Remnant areas typically occur as wedge-shaped 

areas along interfluvial regions between adjacent standard 

hydrologic units or as overbank areas along a stream between 

junctions with tributaries. These remnant areas, along with the 

mainstem, should be aggregated together into hydrologic units 

that meet the size criteria for any given level. These remnant 

areas are also referred to as “related contributing drainage 

areas” or “composite” areas. See section 3.6 for discussion of 

nonstandard areas along shorelines. 

3.5.3 Noncontributing Areas

Drainage areas that do not contribute flow toward the 

outlet of a hydrologic unit are called noncontributing areas 

(fig. 7). Such areas may be due to glaciated plains (potholes), 

closed basins, playas, cirques, depression lakes, dry lakebeds, 

or similar landforms. A noncontributing area may be designated as a hydrologic unit at any level of the hierarchy if it is 

within the size range for a given level. Semiconfined basins 

that contribute surface water to other areas in wet years but act 

as sinks in dry years may be defined as standard or noncontributing hydrologic units. These types of special situations 

require review, coordination, and agreement at the State level.

Assistance or consultation with climatologists or NOAA on 

prevailing precipitation regimes that may have a long-term 

influence on noncontributing areas should be explored.

If noncontributing areas are small and dispersed relative 

to the hierarchical level being delineated, then they should be 

considered as part of the encompassing delineated hydrologic 

unit. Because the precise definition of a noncontributing area 

will likely vary from State to State, document the criteria 

used to determine noncontributing areas in the metadata file, 

especially if a substantial number of noncontributing areas 

are defined. Include the total acreage of noncontributing areas 

within a hydrologic unit as an attribute of the data. Delineate 

noncontributing areas consistently. If a noncontributing area is 

on the boundary between two or more hydrologic units, then 

determine the low point along the noncontributing area boundary, and associate the noncontributing area with the hydrologic 

unit adjacent to the low point on the boundary.

Figure 7. Noncontributing areas, caused by unique and unusual 

landforms, do not flow to the outlet of the hydrologic unit. 

Isolated noncontributing areas such as these in Eastern Estancia 

Subbasin, Torrance County, New Mexico, should be delineated.

0 4 8 MILES

NEW MEXICO

0 4 8 KILOMETERS

Noncontributing area

Hydrologic unit boundary

4 digit, Subregion

8 digit, Subbasin

10 digit, Watershed

12 digit, Subwatershed

EXPLANATION

(6 digit, Basin-level 

boundary not 

represented in this 

example)

Figure 7. Noncontributing areas, caused by unique and unusual landforms, do 

not flow to the outlet of the hydrologic unit. Isolated noncontributing areas such 

as these in Eastern Estancia Subbasin, Torrance County, N. Mex., should be 

delineated.

third edition 041112

N
3. Criteria and Considerations for Delineating Hydrologic Units 13

3.5.4 Diverted Waters

Ditches and canals should be used to determine surfacewater drainage areas only when the artificial channel has 

permanently altered the natural flow. Many artificial drainage 

features in the United States were originally either perennial 

or intermittent channels that local government and private 

entities converted into permanent drainage features. Much of 

the surface drainage in these areas would disappear from local 

and State drainage maps if permanent, constructed diversions 

were not considered when delineating hydrologic units. If the 

present-day canal or ditch was once a legacy stream channel or 

has perennial flow, then it may be considered for delineating 

hydrologic units. Avoid delineating small, local ditch systems 

constructed for seasonal diversion of water or for irrigation of 

agricultural fields.

When all or part of the flow from one hydrologic unit is 

continuously discharged into another by constructed transbasin 

diversions, document the diverted flow as attributes to both 

the water-losing and water-gaining hydrologic units in the 

polygon 12-digit HUMod field (section 6.2. 2.11.2).

Information on the date of the diversion, flow rates, and 

water rights for both the receiving and losing hydrologic units 

may be included in the metadata. 

Avoid adjusting the location of hydrologic units because 

of interconnected flow from one hydrologic unit to another 

during high flow stages in streams.

3.5.5 Karst Areas

The delineation of hydrologic unit boundaries in karst 

areas is difficult because the potential exists to attribute the 

runoff from one hydrologic unit, or part of a hydrologic unit, 

to the wrong outlet. This difficulty occurs because the surface 

drainage pattern in karst areas is typically disrupted by sinkholes and sinking streams, making it difficult to choose a valid 

hydrologic unit boundary based on topography alone. Karst 

conduits frequently cross beneath topographically defined 

hydrologic unit boundaries, and drainage may be routed into 

or out of a topographically defined hydrologic unit. Because 

of the presence of karst conduits, groundwater and surfacewater interaction is relatively direct and rapid compared to 

that in nonkarst areas. The conduit networks transport most of 

the stormwater runoff, and the flow is both fast and turbulent. 

Each conduit network drains a finite area (karst basin) and 

discharges to a perennial spring. 

Sinkholes, sinking streams, springs, or cave entrances 

indicate karst hydrology. If sinkhole symbols or other map 

symbols associated with karst areas are not expressed on 

topographic maps that show areas with soluble rocks such as 

limestone, dolomite, gypsum, or salt near the land surface, 

then assume karst hydrology unless evidence indicates otherwise. The WBD is a surface-water dataset; it is not the intent 

of the WBD to determine or delineate in depth the pattern of 

underground seepage. Note all hydrologic units containing 

karst with the modifier “KA” in the relevant 12-digit hydrologic unit polygon HUMod field (section 6.2. 2.11.2). 

Additional details describing interpretation or identification of karst within the hydrologic unit should be documented 

in metadata (section 9).

3.6 Coastal Considerations for Hydrologic 

Units

The development of hydrologic units in coastal areas 

requires additional consideration, because coastal units may 

drain either to a single outlet (standard) or to multiple outlets 

(frontal drainage). Beginning in 2009, the shoreline feature 

was not included as a component of the WBD; therefore, an 

offshore boundary is now required for representing the outer 

extent of the hydrologic unit for an open ocean coastal area 

and an onshore topographically defined boundary is required 

for inland coastal areas. Nevertheless, the shoreline representation is retained within the NHD as part of the Hydrography 

geodatabase for areas such as inland lakes, large tidal rivers such as parts of the Mississippi, Columbia, and Potomac 

Rivers, and for ocean coastal areas. If the NHD shoreline is 

in conflict with the established State shoreline representation, 

then contact your In-State Steward to determine which version 

should be incorporated into the NHD. 

The offshore boundary can be generally categorized as 

open-ocean boundary or nearshore boundary. Open-ocean 

hydrologic units are closed by use of the nationally consistent NOAA Three Nautical Mile Line, as published on the 

NOAA nautical chart. Charts can be accessed at http://www.

nauticalcharts.noaa.gov/csdl/mbound.htm. For digital representation of this demarcation, contact the WBD–NTC. Nearshore delineation is optional. 
14 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

To eliminate the shoreline in bays, sounds, and estuaries, use criteria established by the State; these criteria may 

include the nearshore boundary delineation or the combination 

of a frontal and a water unit closed by a head of land boundary delineated across the mouth. The boundary should be far 

enough offshore that it does not interfere with other shoreline 

representations, for example, mean low water. 

Figure 8. To delineate the hydrologic unit boundary for frontal hydrologic units in open ocean areas, extend the boundaries of 

the frontal unit to intersect offshore boundaries. (NHD, National Hydrography Dataset; NOAA, National Oceanic and Atmospheric 

Administration)

Extend the boundaries of frontal coastal hydrologic units 

to intersect offshore boundaries. When the NOAA Three Nautical Mile Line is the outer extent, 8- and 10-digit hydrologic 

units must be delineated across open ocean, preferably along 

submerged ridges. Nonstandard units that flow directly into 

coastal waters may require extension to an offshore boundary 

(fig. 8). 

Figure 8. To delineate the hydrologic unit boundary within a hydrologic unit that contains a frontal hydrologic unit, such as one of the Channel Islands off 

of the coast of California, extend the boundaries of the frontal unit to intersect offshore boundaries. (NHD, National Hydrography Dataset; NOAA, 

National Oceanic and Atmospheric Administration)

Frontal 12-digt (subwatershed) 

boundaries that terminate on shore are 

extended out to the offshore boundary, 

whether the nearshore boundary or the 

NOAA Three Nautical Mile Line

8 digt, subbasin boundary extended to 

NOAA Three Nautical Mile Line

Outermost subbasin boundary moved offshore 

to the NOAA Three Nautical Mile Line

CALIFORNIA

Subbasin 18060013

Santa Barbara Channel

0203

0204

0201

0106

0202

Scale 1:170,000

0 2 4 Miles

0 2 4 Kilometers

Hydrologic unit boundary

4 digit, Subregion

8 digit, Subbasin

10 digit, Watershed

12 digit, Subwatershed

NOAA Three Nautical 

 Mile Line

NHD coastline

EXPLANATION

Hydrologic unit code

(6 digit, Basin-level boundary not 

represented in this example)

10 digit, Watershed

12 digit, Subwatershed

1806001302

180600130201
3. Criteria and Considerations for Delineating Hydrologic Units 15

across the mouth) is required, as agreed upon by State partners 

(fig. 9). Water levels in coastal areas can fluctuate substantially; therefore, consideration should be given to using either 

a nearshore buffer distance or the bathymetry of submerged 

morphologic features, such as stable shoals, ridges, shore 

faces, and flow channels (legacy channels), if the bathymetry 

is reliable and current. Transitory features, such as berms 

that are created by sediment flow from a tributary and later 

destroyed by other action, should not be used. Any boundary 

Figure 9. The above 

example depicts coastal 

delineation and coding 

for areas that drain into a 

water hydrologic unit (in 

this case, in North Carolina). 

Small frontal areas on each 

side of the river outlet are 

grouped with the open 

ocean water area. (NHD, 

National Hydrography 

Dataset; NOAA, National 

Oceanic and Atmospheric 

Administration)

3.6.1 Bays, Sounds, and Estuaries

The delineation of hydrologic unit boundaries in coastal 

areas is often complicated by the presence of large estuaries, 

bays, or sounds. Use the NHD to identify and define bays, 

sounds, and estuaries. Because coastline representations are 

not stored within hydrologic unit boundaries, either nearshore 

boundary delineation or a combination frontal and water (section 3.6.2) unit closed by a head of land boundary (delineated 

A l b e m a r l e S o u n d

Yeopim River

NORTH CAROLINA

.

.

Head of

land

Hydrologic unit boundary

4 digit, Subregion

2 digit, Region

8 digit, Subbasin

10 digit, Watershed

6 digit, Basin

12 digit, Subwatershed

NOAA Three Nautical 

 Mile Line

NHD coastline

EXPLANATION

Scale 1:35,000

0 0.5 1 Mile

0 0.5 1 Kilometer
16 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

chosen for delineation should be far enough offshore so that 

it will not interfere with other shoreline representations, for 

example, mean low water. 

To delineate nearshore hydrologic unit boundaries, a 

combination of depth (bathymetry), head of land, and variable 

distances (buffering) from the NHD coastline representation may be applied or frontal land and water units may be 

combined (fig. 10). When the nearshore boundary is a buffer 

distance from a shoreline, the buffer distance may differ from 

the shoreline representation in the NHD, which typically is 

the Mean High Water tidal datum supplied by NOAA. If the 

NHD shoreline representation is in conflict with the authoritative State shoreline, then contact your NHD In-State Steward. 

When the nearshore delineation is based on a depth, it may 

vary, for example, from 6 to 10 feet. When long sinuous 

estuaries or rivers are delineated, three options may be used: 

(1) truncate the river or estuary with a head of land boundary delineated across the mouth to separate the inland bay 

from the water unit (fig. 9); (2) reduce the buffer distance at 

the point where the buffers overlap, for example, at a narrow 

channel at the mouth of a bay; or (3) create an artificial closure 

line beyond the mouth at the outer buffer extent.
3. Criteria and Considerations for Delineating Hydrologic Units 17

Figure 10. A combination of depth (bathymetry), head of land, and variable distances (buffering) from the NHD coastline representation 

may be applied or frontal land and water units may be combined when nearshore hydrologic unit boundaries are delineated. (NHD, 

National Hydrography Dataset; NOAA, National Oceanic and Atmospheric Administration) Figure 10. A combination of depth (bathymetry), head of land, and variable distances (buffering) from the NHD coastline representation may be applied, 

or frontal land and water units may be combined when near-shore hydrologic unit boundaries are delineated. (NHD, National Hydrography Dataset; 

NOAA, National Oceanic and Atmospheric Administration)

third editioin 041112

. .

1801

1802

1803

1804

1805

1806

1807

1808

1809

1810

1812

1811

Head of

land

Atlantic

Ocean

Blue Hill

Bay

1810

1811

1808

Atlantic

Ocean

Blue Hill

Bay

EXPLANATION

Hydrologic unit type Hydrologic unit boundary

Standard 10 digit, Watershed

Frontal

Island

Water

12 digit, Subwatershed

NOAA Three Nautical Mile Line

NHD coastline

Hydrologic unit code

10 digit, Watershed

12 digit, Subwatershed

0105000218

010500021809

MAINE

8 digit, Subbasin

01050002

Blue Hill Bay Watershed - 0105000218

Scale 1:225,000

0 2 4 Miles

0 2 4 Kilometers
18 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

When delineating from bathymetric information, use 

the best available depth data for closure as confirmed by 

State partners. Extend the boundaries of coastal hydrologic 

units to intersect offshore boundaries. Delineations of 8-digit 

hydrologic units must be extended to nearshore and openocean boundaries, preferably along submerged ridges. Where 

bathymetry is not available, one must project an arbitrary 

perpendicular line out to the offshore boundary. 

Nautical charts from NOAA or local marine bathymetric 

data are available for many coastal areas. Help in obtaining 

bathymetric data is available through the WBD–NTC. Use 

the largest scale and most recent charts and maps that provide 

individual depth-sounding and depth-contour data. Digital 

elevation model data may be useful for identifying water 

hydrologic units. 

Nearshore delineations must transition seamlessly 

across State boundaries and physical features having edges 

that match. This may require negotiation between States to 

determine the transition from one nearshore representation to 

another. Nearshore delineations in bays, sounds, and estuaries 

that are adjacent to nearshore delineations in open-ocean areas 

may require a gradual transition between the two offshore 

depths/distances (fig. 11).

Figure 11. A gradual transition is needed between two offshore depths or distances, such as the 200-foot buffer in this bay and the 

4,000-foot buffer in this open-water area of offshore Louisiana. (NHD, National Hydrography Dataset; NOAA, National Oceanic and 

Atmospheric Administration)

At chaf al ay a Bay

0 2 4 Miles

0 2 4 Kilometers

Scale 1:20,000

Transition between 200-foot buffer 

in the bay and 4,000-foot buffer 

in open water .

Gulf of Mexico

Hydrologic unit boundary

4 digit, Subregion

8 digit, Subbasin

10 digit, Watershed

12 digit, Subwatershed

NOAA Three Nautical 

 Mile Line

NHD coastline

EXPLANATION

Hydrologic unit code

10 digit, Watershed

12 digit, Subwatershed

0809030204

080903020405

8 digit, Subbasin

08090302

0401

0405

0900

0900

0403

0404

0405

0406

0203

0204

LOUISIANA

Fi11A dl titiidd bttffhdthdith th200ft bffithibd th
3. Criteria and Considerations for Delineating Hydrologic Units 19

3.6.2 Water and Frontal Hydrologic Units

Hydrologic units in coastal areas may drain either to a 

single outlet (standard hydrologic unit) or to multiple outlets 

(frontal drainage). Frontal hydrologic units are areas that drain 

to multiple points along a coast, and they can be separated by 

standard hydrologic units flowing into the waterbody. After 

the standard 10- and 12-digit hydrologic units are delineated 

within the designated size criteria (section 3.4), coastal and 

small stream drainage areas, often with radial or centripetal 

drainage, will remain where mainland or island coastal outlet 

areas are fragmented. These frontal hydrologic units are typically included with offshore hydrologic units. The outer extent 

of these coastal units is delineated at an offshore boundary, 

either the NOAA Three Nautical Mile Line or by criteria 

as determined by State partners. At a minimum, extend the 

boundaries of coastal hydrologic units to intersect the first 

offshore boundary encountered (fig. 12). Nearshore delineations must transition smoothly between physical features, and 

the edges must match at State boundaries. When coastal areas 

transition from bays, sounds, and estuaries to open ocean, the 

nearshore delineation can be continued but it is not required. 

If a frontal unit includes an 8-digit hydrologic unit boundary, 

then the delineation must be extended to all offshore boundaries, preferably along submerged ridges.

Figure 12. The outer extent of these standard and frontal hydrologic units in the Santa Monica Bay area, California, is delineated to 

the first offshore boundary, the NOAA Three Nautical Mile Line. (NHD, National Hydrography Dataset; NOAA, National Oceanic and 

Atmospheric Administration) Figure 12. The outer extent of these classic and frontal hydrologic units in the Santa Monica Bay area, California, is delineated to the first offshore 

boundary, the NOAA Three Nautical Mile Line. (NHD, National Hydrography Dataset; NOAA, National Oceanic and Atmospheric Administration)

third edition 041112

0502

0402

0404

0403

0101

0200

0104

0601

0102

0103

0401

0501

0301

CALIFORNIA 8 digit, Subbasin 18070104

Hydrologic unit boundary

Hydrologic unit code

8 digit, Subbasin

10 digit, Watershed

12 digit, Subwatershed

NHD streams

Pacific Ocean

Santa Monica Bay

NHD coastline

NOAA Three Nautical Mile Line

EXPLANATION

1807010401 10 digit, Watershed 

180701040101 12 digit, Subwatershed

Scale 1:400,000

0 4 8 Miles

0 4 8 Kilometers
20 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

 3.6.3 Islands

Delineate island hydrologic units based on their size and 

proximity to each other and to adjacent land. An island large 

enough to be its own hydrologic unit can be delineated as such 

and drains into the surrounding water unit or ocean. The area 

of an island unit includes the land area and offshore extent. 

Subdivide island land area into hydrologic units consistent 

with unit size criteria. An island located between water 

hydrologic units may be bisected to indicate that surface water 

flows to the adjacent hydrologic units. This technique applies 

if a single barrier island or a string of islands along a shoal or 

reef functions as the hydrologic divide. The hydrologic-divide 

concept can be applied to any formation that provides a hydrologic barrier to flow, such as a peninsula or an isthmus.

Figure 13. If an 

island falls within 

any offshore unit, 

then it can be 

included in that 

unit without further 

delineation or a 

nearshore buffer 

can be used to 

subdivide it from 

the larger unit 

providing it meets 

size criteria as 

shown in the Blue 

Hill Bay area in 

Maine.

Figure 13 If an island falls within any offshore unit, then it can be included in that unit without further delineation or a 

near-shore buffer can be used to subdivide it from the larger unit providing it meets size criteria as shown in the Blue Hill Bay 

iMi

Atlantic Ocean

Blue Hill

Bay

Scale 1:150,000

0 1 2 Miles

0 1 2 Kilometers

.

.

.

.

.

.

Hydrologic unit boundary

Hydrologic unit type

8 digit, Subbasin

10 digit, Watershed

12 digit, Subwatershed

Standard

Frontal

Island

Water

NOAA Three 

Nautical Mile Line

NHD coastline

EXPLANATION

Hydrologic unit code

10 digit, Watershed

12 digit, Subwatershed

0105000218

010500021810

MAINE

Island grouped

with adjacent

land unit

Islands dissolved

 into Blue Hill Bay

water unit

Island group

is separate unit

1808

1809

1810 1811

Blue Hill Bay Watershed - 0105000218

8 digit, Subbasin

01050002
3. Criteria and Considerations for Delineating Hydrologic Units 21

An island too small to be its own hydrologic unit can be 

grouped with a nearby land unit or with another nearby island 

or group of islands, such as a barrier island. If an island falls 

within any offshore unit, then it can be included in that unit 

without further delineation or a nearshore buffer can be used 

to subdivide it from the larger unit providing it meets size 

criteria (fig. 13). If an island is beyond the mainland NOAA 

Three Nautical Mile Line, such as the Channel Islands off the 

California coast, then the outer extent of the island hydrologic 

unit is the NOAA Three Nautical Mile Line. The nearshore 

delineation is optional (fig. 14).

Figure 14. For an island that is beyond the mainland NOAA Three Nautical Mile Line, such as the Channel Islands off the California 

coast, the outer extent of the island hydrologic unit is the NOAA Three Nautical Mile Line. The nearshore delineation is optional.

P a c i f i c O c e a n

S a n t a B a r b a r a C h a n n e l

Scale 1:63,000

0 1 2 Miles

0 1 2 Kilometers

8 digit, Subbasin 18060014

CALIFORNIA

0101

0102

0103

0202

0201 San Miguel Island

Santa Rosa Island

Santa Cruz Island Anacapa Island

Hydrologic unit boundary

10 digit, Watershed

12 digit, Subwatershed

NOAA Three Nautical 

 Mile Line

NHD coastline

Flow off island based on 

 bathymetry

NHD streams

EXPLANATION

Hydrologic unit code

10 digit, Watershed

12 digit, Subwatershed

1806001401

180600140118
22 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

3.6.4 Reservoirs and Natural Lakes

Delineation of hydrologic unit boundaries is often complicated by the presence of reservoirs or large natural lakes, 

because they can obscure the natural drainage system. The 

effect of the pool area and its underlying drainage pattern on 

the delineation of hydrologic units depends on the size of the 

pool area and the amount of fluctuation in the normal pool 

area. To delineate hydrologic units that include reservoirs, use 

depth data or historic maps of legacy channels and ignore the 

reservoir pool. Let underwater features direct the flow and 

delineation. The order of priority for delineating reservoirs and 

lakes follows. 

If the uninterrupted natural drainage network would form 

a unit larger than one of the typical size at that level, then consider subdividing the hydrologic unit at a dam or natural outlet 

of the reservoir or lake. This decision will depend on adjoining 

hydrologic units and adjacent slope areas.

Avoid delineating boundaries to the reservoir’s normal, 

average, or high pool. To ensure that all of a reservoir’s pool 

area is accounted for, subdivide the hydrologic units to the 

legacy channel system underlying the pool area (fig. 15). 

For natural lakes, delineate tributary areas that flow 

directly into a lake and are of a size consistent with the hydrologic unit level being delineated as standard hydrologic units. 

Because shorelines are not used for hydrologic unit boundaries, nonstandard areas that drain into a lake can be combined 

with the lake (fig. 16). If the resulting unit is larger than the 

size guidelines, then it can be subdivided based on bathymetry 

or it may be left intact if the subdivision is not defensible. 

Permanent, large-scale water bodies, such as lakes and 

reservoirs having historically documented permanent pools, 

may require exceptions to these priorities. When large water 

bodies are adjacent to several 8-digit hydrologic units, hydrologic unit delineations without subdivision along legacy channels or bathymetry may be accepted into the WBD. Where the 

boundaries are established and recognized within State law 

and a legal lake elevation has been extensively used for water 

management, the hydrologic unit delineation may be based on 

the legal lake elevation. The interior of the lake is delineated 

as a water hydrologic unit. Large lakes or reservoirs delineated as hydrologic units, such as the Great Salt Lake in Utah, 

should be listed with their legal lake elevation in the detailed 

overview section of the metadata and identified with a Line 

Modification attribute value of SL for shoreline. 

A unique coastal process was applied in the Great Lakes 

Basin. This treatment was developed by the WBD–NTC 

through months of coordination and collaboration with 

Canadian Federal and Provincial partners as well as all WBD 

In-State Stewards. This treatment is consistent with and 

complements the integration of the WBD and NHD with sister 

datasets in Canada, and it has been carefully developed to 

accommodate those datasets. WBD In-State Stewards should 

consult with the WBD–NTC prior to editing any coastal areas 

of the Great Lakes. Documents describing the evolution and 

agreed-upon method can be obtained through the WBD–NTC.
3. Criteria and Considerations for Delineating Hydrologic Units 23

Figure 15. To delineate the hydrologic unit boundary within a hydrologic unit that contains a major reservoir, such as the Lake Mead 

area of Arizona and Nevada, the preferred treatment is to use a topographic map showing preinundation contours of underwater 

features if such a map is available.

Figure 15. 

Hydrologic Unit

15010005

Boundary compiled using 

preinundation data currently

hidden by the reservoir

NEVADA

ARIZONA

NEVADA

ARIZONA

Mead Lake

Colorado River

Hydrologic unit boundary

2 digit, Region

4 digit, Subregion

8 digit, Subbasin

10 digit, Watershed

EXPLANATION

(6 digit, Basin-level boundary not 

represented in this example)

0 0.5 1 MILE

0 0.5 1 KILOMETER

SCALE 1: 48,000
24 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

Figure 16. To delineate the 

hydrologic unit boundary within 

an area that contains a lake, such 

as Priest Lake on the border of 

Washington and Idaho, the preferred 

treatment is to break out standard 

hydrologic units of appropriate size 

and then combine nonstandard areas 

having direct flow into the lake with 

the lake hydrologic unit.

Figure 16. To delineate the hydrologic unit boundary within an area that contains a lake, such as Priest Lake on the border of Washington 

and Idaho , the preferred treatment is to break out classic hydrologic units of appropriate size and then combine nonclassic areas having 

direct flow into the lake with the lake into a hydrologic unit.

third edition 041112

P r i e s t

L a k e

0 2 4 Miles

0 2 4 Kilometers

Scale 1:200,000

0304

0305

0307

0309

0309

0309

0302

0303

0306

0308

0301

0401

0701

0501

0104 0108

0202

0203

0201

0402

0403

IDAHO

WASHINGTON 8 digit, 

Subbasin

17010215

Subwatershed 170102150309 includes Priest Lake

and the associated remnant areas surrounding it.

Hydrologic unit boundary

Hydrologic unit code

4 digit, Subregion

10 digit, Watershed

12, digit, Subwatershed

NHD stream

EXPLANATION

8 digit, Subbasin

6 digit, Basin

10 digit, Watershed

12 digit, Subwatershed

1701021503

170102150309
3. Criteria and Considerations for Delineating Hydrologic Units 25

3.7 Optional 14- and 16-Digit Hydrologic Units

Development of 14- and 16-digit hydrologic units is 

optional, based on specific State needs, and should be undertaken only if the WBD In-State Steward and partner agencies 

agree that there is value in developing 14- and 16- digit hydrologic units for their area of interest. Delineations should be 

driven by the hydrology of the region and may vary by region. 

3.7.1 Criteria for 14- and 16-Digit Hydrologic 

Units

Delineations of 14- and 16- digit hydrologic units follow 

many of the same criteria for delineations of 10- and 12-digit 

hydrologic units. The following minimum criteria should be 

used when delineating 14- and 16- digit hydrologic units.

• Legacy delineations should be inventoried, examined, 

and used where applicable.

• 14-digit hydrologic units are nested within an associated 12-digit hydrologic unit.

• 16-digit hydrologic units are nested within associated 

14-digit hydrologic unit. 

• If 14-digit hydrologic units are delineated, then at 

a minimum they must be completed for the entire 

12-digit hydrologic unit and similarly for the 16-digit 

hydrologic units within a 14-digit hydrologic unit.

• At a minimum, 14- and 16-digit hydrologic units must 

be aligned with the 1:24,000 scale Digital Raster 

Graphic (DRG) contours, or with updated USGS Topographic Maps. 14- and 16-digit hydrologic units should 

be delineated with coordination from adjacent jurisdictions to create a seamless layer. Work with representatives in adjacent States to use a consistent approach in 

delineating 14- and 16-digit hydrologic units.

• Higher resolution or current base information should be 

used, if it is available, documented, and agreed upon 

by State partners. 

• 14- and 16-digit hydrologic units should not follow 

administrative boundaries but should instead be based 

on topography.

Include metadata to provide information about documented methodology and describe the procedures used, base 

information, reference, coordinating partners, and all other 

pertinent information.

3.7.2 Delineation Options

Several options are suggested for delineation of 14- 

and 16-digit hydrologic units. Because some of the current 

delineations were created before any national standards were 

in place, legacy data created to fit the specific needs of the 

agencies that developed these hydrologic units and delineations are tied to the current delineations. Differing landscapes 

and environments require flexibility in delineating 14- and 

16-digit hydrologic units. Three options for examining legacy 

delineations are described here, and all three options follow 

the guidelines stated in section 3.7.1. The first option is the 

“Open” option, where the 14- and or 16-digit hydrologic unit 

may be delineated in a sound manner in accordance with the 

area’s hydrology—this option provides the greatest amount 

of flexibility. The second and third options are subsets of this 

open option. The second option is a “Standard and Remnant 

Area” option that breaks out standard 14- and/or 16-digit 

hydrologic units based on major tributaries along the main 

stem and has a remnant area left over. This option may be the 

most beneficial for project work usually done for the 14- and 

16-digit hydrologic unit. The third option is the “Conventional” option, which uses the existing criteria stated for 

delineating 10- and 12–digit hydrologic units but uses smaller 

size criteria for the 14- and 16-digit hydrologic units. The 

third, conventional option would provide layers of consistent 

size and distribution that may be best for regional work, and it 

is the recommended option by the WBD–NTC because it fits 

most closely within the existing structure.

3.7.2.1 Open Option

The open option should be based on hydrology, but it 

does not have size and number criteria. The 14-digit hydrologic unit should nest within the 12-digit hydrologic unit. If 

any delineation is done within a 12-digit hydrologic unit, then 

all of the 14-digit hydrologic units should be completed within 

that 12-digit hydrologic unit. Because the 14- and 16-digit 

hydrologic units lack size restrictions, 14- and 16-digit 

hydrologic units could be the same size. For States that have 

delineated only 14-digit hydrologic units with size ranges 

from very small to quite large, but that still fit within a 12-digit 

hydrologic unit, the open option would fit into legacy delineations. This option would also work when a State has 12-digit 

hydrologic units that may have 14-digit hydrologic units delineated for just a few tributaries and the State has chosen to keep 

the rest of the 12-digit hydrologic unit as a large composite. 

Units are numbered in accordance with the “Federal Standards 

and Procedures for the National Watershed Boundary Dataset 

(WBD).”

The open option provides the most flexibility of the three 

options; however, it may be difficult for analysis over multiple 

hydrologic units. The open option also creates the least workload for stewardship in the short term. 
26 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

3.7.2.2 Standard and Remnant Area Option 

The standard and remnant area option examines major 

tributaries, breaking out standard hydrologic units along a 

main stem, and then classifies the remaining area as a remnant/

composite area. Standard 14-digit hydrologic units that bound 

major tributaries along a 12-digit hydrologic unit main stem 

are developed. Hydrography and local knowledge are utilized to determine which streams are considered major within 

the 12-digit hydrologic unit, and the remaining areas are the 

remnant area 14-digit hydrologic units. Standard 14-digit 

hydrologic units might also be found in 12-digit remnant area, 

closed basin, frontal, island, or unclassified hydrologic units.

Standard 16-digit hydrologic units that bound major 

tributaries along a 14-digit hydrologic unit main stem are 

developed. Hydrography and local knowledge are utilized to 

determine which streams are considered major, and remaining areas are remnant area 16-digit hydrologic units. Standard 

16-digit hydrologic units might also be found in remnant area 

14-digit hydrologic units.

There are no size criteria for these 14- and 16-digit 

hydrologic units. Standard 14- and 16-digit hydrologic unit 

sizes are based on the size of identified tributaries. Remnant 

area 14- and 16-digit hydrologic unit sizes are based on the 

remaining areas after standard unit sizes have been delineated. 

There are no minimum or maximum numbers of 14-digit 

hydrologic units within a 12-digit hydrologic unit, and there 

are no minimum or maximum numbers of 16-digit hydrologic 

units within a 14-digit hydrologic unit. 

Units are numbered in accordance with the “Federal Standards and Procedures for the National Watershed Boundary 

Dataset (WBD).”

3.7.2.3 Conventional Option

The conventional option uses the same criteria used 

for the delineation of 10- and 12–digit hydrologic units, but 

it alters the size limits to meet the 14- and 16-digit criteria. 

The 14-digit hydrologic units should typically be from 3,000 

to 10,000 acres in size and should nest within the 12-digit 

hydrologic unit. The 16-digit hydrologic units should typically 

be from 100 to 3,000 acres in size and should nest within the 

14-digit hydrologic units.

Units are numbered in accordance with the “Federal Standards and Procedures for the National Watershed Boundary 

Dataset (WBD).”

3.7.3 Review of 14- and 16-Digit Hydrologic 

Units and Inclusion in the WBD

Completed 14- and 16-digit hydrologic units are submitted under the same protocol as other WBD edits through the 

WBD In-State Steward. Metadata must accompany all 14- and 

16-digit hydrologic units submitted for review.

The WBD In-State Steward submits the State-approved 

14- and 16-digit hydrologic units and metadata to the WBD–

NTC for technical review and for final quality assurance; 

for example, the WBD–NTC will check to ensure that units 

nest correctly without overlaps or gaps and that all required 

attribute fields are complete. If the units pass this review, then 

they can be added into the WBD. At this time (2012), 14- and 

16-digit hydrologic units will not receive official certification.
4. Required Geographic Data Sources and Recommended Techniques for Boundary Delineation 27

4. Required Geographic Data Sources 

and Recommended Techniques for 

Boundary Delineation

There are many techniques for delineating hydrologic 

unit boundaries. In the past, delineation began with interpreting the hypsographic and topographic information found on 

1:24,000-scale paper or polyester film source materials and 

manually drawing boundary lines based on the interpretation 

of those materials. These manually drawn lines were subsequently digitized using scanning and vectorization or tablet 

digitizing.

Methods that are more current employ GIS software 

to combine delineation and digitization of hydrologic unit 

boundaries on a computer display. Instead of interpreting the 

hypsographic and topographic information found on paper 

or polyester film sources as used in the previous technique, 

digital sources are used as interpretive, background images for 

delineation. The boundary lines are digitized on the computer 

screen using functionality typically found in GIS software. 

Digital background sources include 1:24,000-scale Digital 

Raster Graphics (DRG), updated USGS Topographic Maps, 

and orthoimagery and orthophotographs such as Digital 

Orthophoto Quadrangles (DOQ) or DOQQs.

GIS software has functions for deriving drainage areas 

from elevation data, such as the National Elevation Dataset 

(NED) or light detection and ranging (LiDAR) and supplemental hydrographic and physiographic data. The intermediate boundaries are then verified and refined using on-screen 

digitizing to NSSDA. WBD tools have been developed by the 

USGS for editing.

4.1 Map Scale and Map Accuracy

Delineations need to meet a 1:24,000 scale in the United 

States, except for Alaska at 1:63,360 scale, and 1:25,000 scale 

in the Caribbean, described in the NSSDA (Federal Geographic Data Committee, 1998b), at a minimum. This can be 

accomplished by using recommended hardcopy maps, digital 

geographic data, or combinations. Hardcopy sources include 

current USGS 1:24,000-scale topographic quadrangles and 

higher resolution (larger scale) local hardcopy maps. If higher 

resolution local maps or other project-acquired base sources 

are used to generate the hydrologic unit boundaries, they must 

be provided to the WBD–NTC to use for verification and 

review. Digital data sources include at a minimum the NRCS 

County Mosaic DRGs or updated USGS Topographic Maps 

at a 1:24,000 scale in the United States, except for Alaska at 

1:63,360 scale, and 1:25,000 scale in the Caribbean, or larger 

scale DOQQs, and similar high-resolution digital data showing 

topography and surface-water features. Combinations of these 

geographic data should be used to delineate accurate, current 

hydrologic unit boundaries and to interpret areas of complex 

drainage regimes or geomorphologies. 

Digital elevation data at least equivalent in scale to the 

USGS 30-meter, Level 2 Digital Elevation Models (DEMs) or 

the NEDs (both 30 meter and 10 meter) are acceptable for the 

delineation of draft hydrologic unit boundaries. All derivative 

draft or raster-based delineations will require smoothing and a 

thorough verification and adjustments to 1:24,000-scale or better topographic data such as the NRCS 1:24,000-scale County 

Mosaic DRGs or updated USGS Topographic Maps. In Alaska 

and the Caribbean, USGS topographic maps (in digital and 

hardcopy formats) at 1:63,360 scale or 1:25,000 scale, respectively, may be used.

The 8-, 10- and 12-digit hydrologic units must be delineated from and georeferenced to a minimum horizontal accuracy of 1:24,000 scale in the United States, except for Alaska 

at 1:63,360 scale and 1:25,000 scale in the Caribbean, to meet 

the National Standard for Spatial Data Accuracy (Federal 

Geographic Data Committee, 1998b). For example, to quantify 

1:24,000-scale horizontal accuracy as it applies to the delineation of WBD-compliant hydrologic units, a hydrologic unit 

boundary must fall within a buffer of 40 feet or 12.2 meters 

of a well-defined point on a 1:24,000-scale topographic map. 

Geospatial positioning accuracy standards are defined and 

stated in documents by the Federal Geographic Data Committee (1998b).

4.2 Source Maps

This section describes the types of hardcopy and digitalformat source maps that can be used to delineate hydrologic 

units. The sources described are not an exhaustive list. Delineation of some hydrologic unit boundaries may require use 

of larger scale products or newly developed products. Each 

source map or source data product’s scale and series must be 

recorded in the metadata (section 6.2.4) and in the LineSource 

field (section 6.2.2.6.3). 

4.2.1 Base Maps

Use printed USGS 1:24,000-scale topographic quadrangles for hardcopy delineations and NRCS 1:24,000-scale 

County Mosaic DRGs, or updated USGS Topographic Maps

for compiling hydrologic unit boundaries on a computer display. USGS topographic quadrangles can be obtained at http://

www.usgs.gov/pubprod/maps.html, and NRCS County Mosaic 

DRGs can be obtained at http://datagateway.nrcs.usda.gov/. 

Blueprints or similar facsimiles of the USGS 1:24,000-scale 

topographic maps can be distorted and should not be used. 

In areas where 1:24,000-scale base maps are not available, 

the USGS 1:25,000-scale, 1:63.360-scale, and/or USGS 

1:100,000-scale maps may be used to generate lines. The NHD 

should always be referenced when generating hydrologic unit 

boundaries. Other data sources and map products can be used 

with the digital, 1:24,000-scale DRGs to facilitate the interpretation of hydrologic unit boundaries. Documentation of 

all supplemental base and source maps, digital and hardcopy, 

should be recorded in the LineSource field (section 6.2.2.6.3) 

as well as in the general metadata (section 6.2.4).
28 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

4.2.2 Hydrologic Unit Maps

Historically, smaller scale hydrologic unit maps provided 

a framework for hydrologic boundary information. These 

included the USGS State Hydrologic Unit Maps at 1:500,000 

scale, National Atlas Hydrologic Unit Boundary data at 

1:2,000,000 scale, or the Hydrologic Units of the United 

States data at 1:250,000 scale. These and other hydrologic unit 

maps produced by NRCS, USDA–FS, USGS, and State and 

local entities were used to determine the general location and 

level of complexity of 10- and 12-digit hydrologic units. The 

completed WBD supersedes all previously used information as 

the hydrologic unit framework for the Nation. 

4.2.3 Reference Maps

The following reference maps may be useful for delineating hydrologic unit boundaries where 1:24,000-scale base 

maps do not include sufficient detail for determining flow 

patterns based on topography. These supplemental reference 

layers or maps are useful for (1) documenting artificial flow 

delineations based on permanent features on the landscape, 

and (2) determining gravity flow in areas of extreme low relief 

or complex geomorphologies. In areas of flat terrain, interpolation between contours may be improved by reference to trails, 

old roads, or firebreaks in forested areas, all of which frequently follow drainage divides. The following types of digital 

and hardcopy maps may be used:

• county drainage maps;

• “as-built” plans, including diversions and ditches;

• flow-direction maps;

• NOAA nautical charts;

• ditch-canal maps;

• land-cover maps;

• soil-survey maps;

• orthophotos or other aerial photographs;

• major land-area resource maps;

• local highway or street profiles;

• local watershed-project maps.

When open-ocean delineation is required, use the NOAA 

Three Nautical Mile Line, as published on a NOAA nautical 

chart. Charts can be accessed at http://www.nauticalcharts.

noaa.gov/csdl/mbound.htm. Retain as a permanent record all 

maps, measurement data, and other supplemental reference 

data/maps used in delineations (section 6.2.2.6.3).

4.2.4 Digital Data

Where topographic products are used, the NRCS 

1:24,000-scale County Mosaic DRGs on-screen digital 

reference layer for delineating and digitizing hydrologic 

unit boundaries, or updated USGS Topographic Maps, are 

preferred. County Mosaics at 1:24,000 scale are available at 

http://datagateway.nrcs.usda.gov. Where these data are available, their use in delineation is acceptable.

The 30-meter and 10-meter NED DEMs are the preferred 

reference layers for using spatial modeling techniques to generate draft hydrologic unit boundaries. These DEMs can also 

be used with the County Mosaic DRGs and the NHD, as well 

as with local reference layers, to clarify challenging delineations. Both 30- and 10-meter NED data can be obtained from 

http://ned.usgs.gov/.

The 1:24,000-scale or better resolution NHD layers are 

the preferred hydrographic data layers to use with the NRCS 

1:24,000-scale County Mosaic DRGs to determine best placement of hydrologic unit boundaries, to verify the hydrologic 

unit connectivity, and to aid in the naming of hydrologic units. 

The NHD layers are available at http://nhd.usgs.gov/.

Interferometric synthetic aperture radar (IfSAR) and 

LiDAR products of 1:24,000 scale or better are also useful 

for determining the alignment of hydrologic unit boundaries, 

especially in areas of low relief. The use of these products for 

improvement of hydrologic unit boundaries is increasing as 

they become more widely available and affordable (section 

4.3.4). 

4.3 Hydrologic Unit Mapping Techniques

Drainage divides are usually determined by bisecting 

ridges, saddles, and contour lines of equal elevation. Hydrologic unit boundaries follow the middle of the highest ground 

elevation or the halfway point between contour lines of equal 

elevation (fig. 2). Where a tributary intersects the bank of a 

receiving stream, the hydrologic unit boundary should cross 

the tributary outlet parallel to the receiving stream channel.

The hydrologic unit has only one outlet point, except in the 

case of deltas, braided stream networks, and coastal and lakefront areas (section 3.6). Hydrologic unit boundaries cannot be 

streams (section 3.2).

Manual, digital, and semiautomated methods can be used 

to generate hydrologic unit boundaries. Procedures for completing delineation, mapping, and digitizing differ among these 

options, but all can produce boundaries that meet the 1:24,000 

scale in the United States, except for Alaska at 1:63,360 scale 

and 1:25,000 scale in the Caribbean. Delineating boundaries 

in areas of complex or flat terrain or complex hydrography 

requires careful attention to scale and source data for accurate 

interpretation. Sections 3.3 and 3.4 give the general size criteria for 10- and 12-digit hydrologic units and the recommended 

distribution of those units within an 8-digit hydrologic unit.
4. Required Geographic Data Sources and Recommended Techniques for Boundary Delineation 29

4.3.1 Manual Techniques for Delineating 

Hydrologic Unit Boundaries

When delineating hydrologic units by drawing boundary 

lines on hardcopy maps, use, at a minimum, USGS 1:24,000-

scale topographic map contours, elevations, and drainage patterns to interpret and delineate the hydrologic unit boundaries. 

Other supplemental geographic data such as county drainage 

maps, State hydrologic unit maps, and aerial photographs 

(section 4.2.3) may be used with the USGS 1:24,000-scale 

hardcopy topographic maps to facilitate interpretation of 

hydrologic units.

4.3.2 Digital Techniques for Delineating 

Hydrologic Unit Boundaries

When delineating and digitizing 1:24,000-scale hydrologic units on the computer screen, use the NRCS 1:24,000-

scale County Mosaic DRG contours, or updated USGS 

Topographic Maps, elevations, drainage patterns, and the 

NHD at a minimum, to interpret and delineate the hydrologic 

unit boundaries. Be advised that shifts have been detected 

when base information from online mapping services is used. 

In special cases—for example, where topography is complex 

or flat—the scale of digitizing may decrease depending on the 

resolution of the base or source data. When data layers such as 

the NED, NHD, and DOQQs are used with the 1:24,000-scale 

DRGs to facilitate interpretation of hydrologic units (section 

4.2.4), delineating on the computer display using GIS software may require digitizing 10- and 12-digit hydrologic units 

at a scale of approximately 1:7,500. Before delineation of an 

8-digit hydrologic unit is complete, the perimeter boundary 

associated with the hydrologic unit must also be reviewed 

and updated to meet NSSDA 1:24,000-scale map accuracy 

standards.

4.3.3 Spatial Modeling Techniques for 

Delineating Hydrologic Unit Boundaries

Image processing, GIS, and hydrologic modeling applications can be used to manipulate DEM data, creating derivative 

data that represent landform features and drainage-network 

patterns. DEMs are available in various horizontal and vertical 

resolutions. Digital elevation data with resolutions at least 

equivalent to the vertical and horizontal resolutions of either 

the 30-meter, Level-2 DEMs or NED (http://ned.usgs.gov/) 

can be used to develop a draft or preliminary delineation of 

hydrologic units, which will require further refinement to 

meet NSSDA 1:24,000-scale map accuracy standards. DEMs 

or NED are combined with other geospatial data, such as 

USGS NHD, using GIS to simulate drainage networks, stream 

courses, and direction of flow by applying hydrologic models. 

Maps generated from digital hydrography data with flowdirection arrows also are helpful for delineating hydrologic 

units. 

Depending upon the spatial modeling technique used, 

the data and its consistency, the software applications used, 

and other characteristics, the DEMs and NED will most likely 

provide a generalized depiction of landforms and drainage 

networks. This is especially apparent in areas of moderate and 

low topographic relief or complex hydrography. For this reason, all DEM- and NED-generated boundaries should be independently checked against 1:24,000-scale DRGs, or updated 

USGS Topographic Maps, DOQs, 1:24,000-scale NHD, or 

larger scale data. In some situations, particularly in flat areas, 

elevation data can be wrong or misleading and the drainage 

network in the NHD needs to be considered. Likewise, drainage patterns portrayed in the NHD, in similar situations, can 

be wrong or misleading and need to be checked against elevation data. Adjustments to boundaries and confluences at all 

levels will be required to ensure a 1:24,000 scale in the United 

States, except for Alaska at 1:63,360 scale and 1:25,000 scale 

in the Caribbean. A detailed description of the source elevation 

model must be documented in the metadata.

A similar semiautomated production and review process can be followed for the 30-meter-pixel-size bathymetric 

DEMs for major estuaries and sounds available nationally 

from NOAA.

4.3.4 Delineation Using High-Resolution Base 

Products 

New technology brings new opportunities for upgrading the hydrologic unit boundaries in the certified WBD with 

higher resolution versions derived from extremely detailed 

DEMs. The two preeminent high-resolution DEM types are 

called IfSAR and LiDAR. Updates to WBD using higher 

resolution base products are tracked on the existing boundaries 

using the LINESOURCE field (section 6.2.2.6.3). 

Using high-resolution elevation data from a trusted, 

authoritative data source is important to the success of any 

project. WBD In-State Stewards should consult with the 

WBD–NTC prior to beginning delineations using LiDAR or 

IfSAR base products to obtain the most recent documentation 

about supplementary metadata requirements. Final linework 

generated from LiDAR or IfSAR products should be smoothed 

to eliminate excess vertices to be cartographically compatible 

with all other WBD linework.
30 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

4.4 Updating and Revising 2-, 4-, 6- and 8-Digit 

Hydrologic Units

Hydrologic unit names and codes are common identifiers 

used by many agencies for reporting hydrologic unit characteristics. Many reports are tied solely to the name or code of 

the hydrologic unit. During the creation of the WBD, it was 

stated that the longstanding, existing 2-, 4-, 6-, and 8-digit 

hydrologic units would not have any substantial boundaries, codes, or names changed as the larger scale product was 

developed from the 1:250,000 scale. Because of significant 

hydrologic inaccuracies, international border harmonization, 

or coastal delineations, some of these boundaries, codes, and 

names changed during the creation of the higher resolution 

WBD. These rare instances have been reviewed and officially 

accepted by the WBD–NTC and WBD Steering Committee as 

of January 2012.

In the future, names and codes should not be changed for 

the 2-, 4-, 6-, and 8-digit hydrologic units. Boundaries should 

not be altered except for minor adjustments that will improve 

the data, such as refinements made because of the use of 

higher resolution base information. For any potential alteration 

other than these minor adjustments, consult the WBD–NTC. 

For coastal delineations, characteristics that may include a 

submerged river basin may warrant boundary, code, or name 

changes from legacy information. These instances should be 

carefully coordinated with the WBD–NTC on a case-by-case 

basis.

The 8-digit hydrologic units have been used and referenced so extensively in water-resource activities nationwide 

that major changes to them should be made only in cases of 

delineation error or major landform changes due to natural 

phenomena or human activity. Some examples include the 

removal of a dam, flow changes caused by earthquakes, construction of new reservoirs, embankments, or levees, volcanic 

eruptions, massive landslides, or hurricane damage. The identification of errors in the original digitized work may also lead 

to an update for some or all levels within an 8-digit hydrologic 

unit. For example, major revisions include those that place 

entire stream reaches (not small pieces of headwater reaches) 

in different 8-digit hydrologic units or those that recode contiguous areas approximating or exceeding the size of 12-digit 

hydrologic units of 10,000 to 40,000 acres. Inclusion of new 

offshore areas for coastal hydrologic units and revisions or 

additions to current terrestrial hydrologic unit delineation 

resulting from offshore delineations are considered revisions.

If the locations of 8-digit hydrologic unit boundaries need 

to be changed to be correct with the hydrology of an area, then 

notify the national WBD–NTC through the WBD In-State 

Steward. Obtain assurance from the WBD–NTC that proposed 

changes are acceptable before revising 8-digit hydrologic unit 

boundaries. When hydrologic unit boundaries have major 

revisions, update the area measurements, and note revisions as 

“revised” when the new data are released. Keep a record of all 

of the changes to the 8-digit hydrologic unit boundaries.
5. Coding and Naming 10-, 12-, 14- and 16-Digit Hydrologic Units 31

5. Coding and Naming 10-, 12-, 14- 

and 16-Digit Hydrologic Units

5.1 Hydrologic Unit Levels 

Guidelines for delineating the optional subdivisions of 

14- and 16-digit hydrologic units are available from the NRCS 

and the USGS. Delineation of 14- and 16-digit hydrologic 

units is not required; however, if a 14-digit unit is delineated, 

then subdivide that entire 12-digit hydrologic unit. If a16-

digit hydrologic unit is delineated, then subdivide that entire 

14-digit unit.

The eight different levels of hydrologic units and their 

characteristics are shown below. Figure 3 provides an example 

of this for the 2- through 12-digit hydrologic units.

Hydrologic unit 

name

Historical name Average size 

(square miles)

Approximate number 

of hydrologic units

2 digit Region 177,560 21 (actual)

4 digit Subregion 16,800 222

6 digit Basin 10,596 370

8 digit Subbasin 700 2,270

10 digit Watershed 227

(40,000–250,000 acres)

20,000 

12 digit Subwatershed 40

(10,000–40,000 acres)

100,000 

14 digit (None) Open Open

16 digit (None) Open Open
32 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

5.2 Coding 10-, 12-, 14- and 16-Digit Hydrologic 

Units

This section provides guidance on coding 10- and 

12-digit hydrologic units, and optional 14- and 16-digit 

hydrologic units, for WBD compliance, as well as their 

relation to the 1:250,000-scale coding. The 1:250,000-scale 

dataset does not contain a consistent coding structure. Avoid 

changing the existing numbering of the 8-digit codes unless 

there is a topographic or hydrologic justification to do so. If 

there is a legitimate reason to alter a location or code of an 

8-digit hydrologic unit, then the WBD In-State Steward should 

notify the WBD–NTC before making revisions (section 4.4).

Assign a new unique numbered code to each 10- and 12-digit 

hydrologic unit. Maintain the additional 2-digit field length for 

successive hydrologic units. Coordinate the coding within an 

8-digit hydrologic unit across State boundaries. 

Avoid complicating the coding of hydrologic units. 

Number the hydrologic units sequentially, beginning upstream 

at the hydrologic unit with the uppermost outlet, including 

along the mainstem, and proceed downstream. Downstream 

codes are in ascending order within each level (lower numbers always flow into higher numbers). All numbers from 

the lowest to the highest must be used. For example, one can 

start at the upstream end of the drainage and code the first 

10-digit hydrologic unit as 0908020301, code the next 10-digit 

unit downstream as 0908020302, and so forth. No numbers 

are skipped. The main-stem hydrologic unit is assigned the 

highest number when outlets are adjacent, or break at the 

same place, as shown in figure 17 (12-digit hydrologic units 

170601020104 and 170601020105).

Figure 17. Each hydrologic unit should be coded sequentially based on the location of the outlet, starting with the uppermost stream 

outlet. The downstream code should always be a higher number than the upstream code for 10- and 12-digit hydrologic units. The mainstem hydrologic unit should carry the higher code when outlets are adjacent or break at the same place. Codes are shown for the Upper 

Imnaha River 10-digit hydrologic unit. Figure 17. Each hydrologic unit should be coded sequentially based on the location of the outlet, starting with the uppermost stream outlet. The downstream code should 

always be a higher number than the upstream code for 10- and 12-digit hydrologic units. The main-stem hydrologic unit should carry the higher code when outlets are 

adjacent or break at the same place. Codes are shown for the Upper Imnaha River 10-digit hydrologic unit.

third edition 041112

Hydrologic unit boundary

Hydrologic unit name

8 digit, Subbasin

10 digit, Watershed

4 digit, Subregion

12 digit, Subwatershed

Direction of flow

EXPLANATION

Rock Creek-Imnaha River

(6 digit, Basin-level boundary not 

represented in this example)

N

0 2 4 6 8 MILES

0 2 4 6 8 KILOMETERS

Upper 

Imnaha River

1706010201

Middle 

Imnaha River

1706010204

Little Sheep

Creek

1706010202

Lower 

Imnaha River

1706010205 Big Sheep

Creek

1706010203

OREGON

Cliff Creek

Gumboot Creek

Mahogany Cr Crazyman Creek

Dry

Cree

R i v e r

k

I m n a h a

Blue Cr

Rock Cr

Dry Creek-Imnaha River

170601020105

Gumboot Creek

170601020104

South Fork Imnaha River

170601020102

North Fork

Imnaha River

170601020101

Crazyman Creek-Imnaha River

170601020106

Rock Creek-Imnaha River

170601020103

Imnaha Subbasin
5. Coding and Naming 10-, 12-, 14- and 16-Digit Hydrologic Units 33

If there is no scientific justification for subdividing a 

hydrologic unit, then code the hydrologic unit 00. For example, if a 10-digit hydrologic unit (1020305108) cannot be subdivided on the basis of hydrologic principles, then the 12-digit 

hydrologic unit should be coded with “00” (for example, 

102030511800). The “00” represents no subdivision from the 

next higher level.

An example of the numbering of hydrologic units: 

5.2.1 Coding Coastal Areas That Contain 

Frontal Units

This section provides coding rules for frontal hydrologic 

units. Frontal hydrologic units are areas that drain to multiple 

points along a coast, separated by standard watersheds flowing 

into the water hydrologic unit. Frontal hydrologic units that do 

not meet the size criteria using the combined area of the nearshore buffer and the frontal unit should be grouped together to 

make larger units. Frontals can also be grouped with nearshore 

or open-ocean hydrologic units. When aggregating these frontal hydrologic units, group those that are flowing into the same 

hydrologic feature, for example, a bay, lake, or estuary (see 

fig. 10, unit 1810).

Extend any boundaries that truncate onshore to offshore 

boundaries, based on underlying topography, provided this 

does not result in major alterations of the land codes. Code 

these coastal areas in a clockwise sequence from north or east, 

depending on the orientation of the hydrologic unit (fig. 10).

5.2.2 Coding Islands

Islands large enough to be subdivided can be coded in the 

same pattern for standard and frontal-unit coding, in a clockwise sequence from north or east, depending on the orientation of the island. Islands beyond the mainland NOAA Three 

Nautical Mile Line should use an island NOAA Three Nautical Mile buffer and be assigned a unique code. Islands can be 

grouped and need not be uniquely identified.

The coding of other islands depends on how the State 

chooses to delineate the outer extent of their offshore boundaries, which, along with proximity, determines island grouping. 

An island can be coded individually when the combined area 

of the nearshore buffer and the island meets the size criteria. 

Island groups containing several islands can be coded individually in a clockwise sequence from north or east, depending on the orientation of the islands. Smaller islands, whose 

offshore boundaries do not intersect another hydrologic unit, 

can be grouped with the closest island or land hydrologic unit 

and given the same code and name as the larger unit (fig. 13).

5.3 Naming Protocol for 10-, 12-, 14- and 

16-Digit Hydrologic Units

The numerical hydrologic unit code is the primary unique 

identifier for each hydrologic unit; however, the numerical 

identifier alone makes it difficult to relate a hydrologic unit 

to a geographic location. Therefore, the naming of 10- and 

12-digit hydrologic units, and optional 14- and 16-digit units, 

adds local and geographic identity to the hydrologic unit and 

is helpful for understanding the geographic location of the 

hydrologic unit. Hydrologic units are usually named after 

prominent water features in an area; however, if no water features are in an area, then hydrologic units may be named after 

prominent physical features. 

Identify each 10-, 12-, 14- and 16-digit hydrologic unit 

with a feature name from the area being subdivided. Maintain consistent structure when assigning a name to a unique 

hydrologic unit. Use the following guidelines for naming 10- 

and 12-digit hydrologic units and optional 14- and 16-digit 

hydrologic units. Do not change the names of the 2-, 4-, 6-, 

and 8-digit hydrologic units. 

Numbering 

sequence step Field Numbers Level Common name

1 2 digit 01 1 Region

2 4 digit 0108 2 Subregion

3 6 digit 010802 3 Basin

4 8 digit 01080204 4 Subbasin

5 10 digit 0108020401 5 Watershed

6 12 digit 010802040101 6 Subwatershed

7 14 digit 01080204010103 7 No historical name

8 16 digit 0108020401010302 8 No historical name
34 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

5.3.1 Sources of Names for 10-, 12-, 14-, and 

16-Digit Hydrologic Units

The 10-, 12-, 14-, and 16-digit hydrologic units should be 

named using feature names officially recognized in the GNIS.

Options are contained in the GNIS of the USGS at 

http://geonames.usgs.gov/, including names on USGS 

1:24,000-, 1:25,000-, or 1:63,360-scale topographic quadrangles, or in the NHD at http://nhd.usgs.gov. Avoid naming the 

hydrologic units after roads, streamflow-gaging stations, and 

political or administrative units.

5.3.2 Feature Priority of Names for 10-, 12-, 14-, 

and 16-Digit Hydrologic Units

Assign each 10-, 12-, 14-, and 16-digit hydrologic unit a 

name from the GNIS in this order of priority, advancing to the 

next level if no features of a type exist or are named: 

If no water or prominent physical feature name is assigned, 

then the hydrologic unit code is used as a placeholder.

Exceptions may be made to this priority order based 

on feature importance. For example, a canyon may be more 

prominent than a hydrologic feature, such as an upstream 

spring; this is typical in arid areas. Avoid using identical 

names within an 8-digit hydrologic unit. All 10-digit hydrologic unit names within an 8-digit unit must be unique to each 

other, and all 12-digit hydrologic unit names within an 8-digit 

unit must be unique to each other.

First–HYDROLOGIC FEATURE (Examples include rivers, lakes, 

dams, and falls) 

Second–GEOLOGIC FEATURE (Examples include 

canyons, mountains, buttes, and peaks)

Third–TOWN OR CITY (Town of Name, 

City of Name)

Fourth–OFFICIAL LOCAL 

NAME (Examples include 

cemeteries, municipal or 

international airports ) 

5.3.3 Structure of Names for 10-, 12-, 14- and 

16-Digit Hydrologic Units

Populate the hydrologic unit name attribute field using 

the following structure.

• Use the name of the major water feature within the 

hydrologic unit, spelled out exactly as listed in the 

GNIS, usually the water feature at the outlet of the 

hydrologic unit. Stream names are preferred, but 

sloughs, lakes, reservoirs, dams, bays, inlets, harbors, 

coves, falls, and springs may be used when they are the 

most important feature; for example, “Crescent Lake,” 

“Sequim Bay,” and “Grays Harbor.” 

• The word “Frontal” is reserved for coastal and lake 

areas that include multiple, nonconvergent streams 

associated with frontal hydrologic units. Name the 

hydrologic unit for the major hydrologic feature within 

the hydrologic unit and use “Frontal” as a prefix to 

the name of the hydrologic feature into which the unit 

drains, for example, “Squirrel Creek-Frontal Chesapeake Bay.”

• When the same primary water feature exists within 

equivalent-level hydrologic units or if a main-stem 

stream is subdivided into more than one hydrologic 

unit, use the following hyphenated naming structure 

to create unique names. Append the primary water 

feature name (Imnaha River) onto a secondary water 

feature such as a large tributary name (Rock Creek), 

for example, “Rock Creek-Imnaha River,” or “Dry 

Creek-Imnaha River” (fig. 17). 

• When a major stream is subdivided into three hydrologic units along the main-stem stream, use the words 

“Upper,” “Middle,” and “Lower” together, for example, “Upper Imnaha River,” “Middle Imnaha River,” 

“Lower Imnaha River” (fig. 17).Avoid using “Upper,” 

“Middle,” and “Lower” individually or “Middle” with 

only “Upper” or “Lower.”

• “Headwaters” or” Outlet” may be used with or without 

“Upper,” “Middle,” or “Lower.” If a main-stem stream 

is subdivided into more than five hydrologic units, 

then avoid this naming convention; use the standard 

hyphenated name.
5. Coding and Naming 10-, 12-, 14- and 16-Digit Hydrologic Units 35

delineations across the United States-Canada border. Completion of the 10- and 12-digit hydrologic unit delineations is 

estimated to continue through 2014.

Within the WBD, 12-digit hydrologic unit resides the 

international unique code, and for Canada this is the BinCNUS 

polygon attribute field. The coding is unique to this border. 

The code is populated within the 12-digits for every shared 

8-digit hydrologic unit along the border. The first three digits 

of the seven-digit number for the BinCNUS code represent 

the first harmonized level with Canada, which is at the United 

States 8-digit hydrologic unit level. The following two digits 

represent the 10-digit subdivision, and the subsequent two 

digits represent the 12-digit hydrologic unit level. These are 

merely sequential codes at each level, beginning in the north 

in Alaska, then south to Washington, and then east to Maine. 

Within the 8-digit hydrologic units, the numbering is sequential from headwaters to outlet for the next two nested levels.

5.4.2 United States Data Harmonization with 

Mexico

To facilitate hydrologic analysis applications, the USGS 

and the Mexican Instituto Nacional de Estadística y Geografía 

(INEGI) collaborated to harmonize shared hydrologic drainage areas along the United States-Mexico border and build a 

connected hydrographic network of surface-water features for 

the binational region. The harmonization process included participatory meetings between team leaders for the United States 

WBD and NHD and Mexico’s Red Hidrográfica and Cuencas 

Oficiales to agree on standards, content, connectivity, and 

binational “crosswalk” attribution tables from each country. 

These data are available within the WBD.

Within the WBD, 12-digit hydrologic unit resides the 

international unique code, and for Mexico, this is the BinMXUS polygon attribute field. The coding is unique for this 

border. The code is populated within the 12-digits for every 

shared 8-digit unit along the border. The first two digits of 

the six-digit number for the BinMXUS code represent the 

first harmonized level with Mexico, which is at the United 

States 8-digit hydrologic unit level. The following two digits 

represent the 10-digit subdivision, and the subsequent two 

digits represent the 12-digit hydrologic unit level. These are 

merely sequential codes at each level, beginning in the west 

in California and running east to the tip of Texas. Within the 

8-digit units, the numbering is sequential from headwaters to 

outlet for the next two nested levels.

• “Upper” and “Lower” or “Headwaters” and “Outlet 

“may be used in pairs when a stream is subdivided into 

two hydrologic units. 

• Do not use “Upper,” “Middle,” and “Lower” with the 

hyphenated naming structure.

• The words “Headwaters” and “Outlet” may be used 

with the hyphenated naming structure or the “Upper,” 

“Middle,” and “Lower” naming structure.

• When bathymetry is used to delineate submerged 

morphologic features, the resulting coastal water unit 

may not contain any other named features except 

the main water body name (for example, Atlantic 

Ocean). For these units, use the “hydrologic unit codeprimary water feature” for the name, for example, 

030102051406-Atlantic Ocean.

• Islands should be named according to the following 

conventions.

• When islands are large enough to be subdivided, use 

the standard naming convention for standard and 

frontal hydrologic units.

• Islands large enough to be their own unit use the 

island name.

• Hydrologic units composed of a group of islands use 

the name of the major island within that group. 

5.4 International Borders

Delineating or coding across international boundaries 

should be coordinated with the WBD–NTC, which will then 

provide the forum for coordinating further at the Federal, 

State, and Provincial level. Delineations or attributes along 

international borders may require future adjustment.

5.4.1 United States Data Harmonization with 

Canada

The USGS, in coordination with the International Joint 

Commission, Natural Resources Canada, Agriculture and 

Agri-Food Canada, Environment Canada, and many State and 

Provincial partners, has completed the 8-digit hydrologic unit 
36 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

6. Geospatial Data Structure

6.1 Specifications for Geospatial Data 

Structure

The structure of the WBD is based on well-known GIS 

concepts for modeling geographic features as vectors. Lines 

are used to model the linear features of hydrologic units, 

whereas polygons are used to model the areal features. For 

both linear and areal features, associated attribution is stored 

in tabular format. As part of the integration with the NHD, the 

WBD geospatial data model has been updated. The model was 

developed under the USGS common vector model practices. 

These standards have been implemented across the USGS to 

insure that all data maintained and delivered by the USGS is 

consistent in structure, naming conventions, and metadata. The 

common vector model also seeks to avoid duplication of data 

across the multiple datasets delivered by the USGS.

6.1.1 Hydrologic Unit Geometry

Hydrologic unit boundaries are delineated and digitized 

as lines. The digital lines are processed using GIS software to 

form polygons. Recommended techniques for data capture are 

described in section 4. All typical geometric problems should 

be resolved, including dangling lines, sliver polygons, and 

missing or duplicate labels. Note that some of these issues are 

specific to a particular spatial data format, such as Environmental Systems Research Institute (Esri) Coverage format, and 

do not necessarily apply to other formats such as Esri Geodatabase format or Esri Shapefile format. 

6.2 Database Schema for Attributing 

Hydrologic Unit Polygons

Each hydrologic unit polygon has associated attributes 

stored in tabular format. An attribute is defined by name, type, 

size, and valid values (or range of valid values). A collection 

of attributes is called a schema. There are two categories of 

schema: logical and physical. The logical schema refers in 

general terms to the information associated with hydrologic 

unit polygons; for example, every hydrologic unit polygon has 

a unique code. The physical schema, on the other hand, defines 

the specific format of the table used to store attribution; for 

example, one of the codes for a hydrologic unit polygon has 

the literal name, “HUC8,” and is required to have eight characters, a value is required (cannot be left blank), and if needed, 

preceding zeros fill out all the characters. The physical schema 

is immutable; that is, attribution must adhere to literal field 

names, field types, and the order of fields in the table.

6.2.1 Changes to the Data Model Due to 

Integration with the NHD

Prior to integration of WBD with the NHD, polygons 

and lines were edited within the WBD. Under the new model, 

base classes are edited, and then derived classes are created 

from these edits. Base classes include attributes, lines, and 

points. Derived classes are the attributed polygons, organized 

by hydrologic unit digit code. An example of a derived class 

would be a polygon layer consisting of all of the 12-digit 

hydrologic units for the Nation. The derived classes are 

updated whenever edits are submitted to the WBD from the 

base classes. 

6.2.2 Base Classes

Base classes include four feature classes and three 

tables. The feature classes include WBDLine, WBDPoint, and 

NWISLine. The tables include WBDAttributes, WBDNav and 

WBDFeatureTo HUMod. Base classes share five common 

attributes: PermanentIdentifier, SourceFeatureID, MetaSourceID, SourceDataDesc, and SourceOriginator. Some base 

classes are automatically created and assigned, and others 

must be populated by the WBD In-State Steward.

6.2.2.1 PermanentIdentifier

PermanentIdentifier is a unique 36-character field that 

identifies each element in the database exclusively. PermanentIdentifier is an automatically assigned code that always 

stays with each element. When an element is updated or 

changed, PermanentIdentifier links the element to the metadata 

record, documenting that change. PermanentIdentifier is also 

used to maintain relationship classes in the normalized data 

model. When an element is deleted or split, PermanentIdentifier stays with the original element and is not used again. In 

the case of a split, new permanent identifiers are assigned to 

the resultant parts.

Field Name PermanentIdentifier

Field Type Character

Field Width 40

Domain Randomly generated alpha-numeric character

Value required? Yes

Example {5DD21DC6-3692-4197-889B-49E652AA43D0}
6. Geospatial Data Structure 37

6.2.2.2 SourceFeatureID 

Like PermanentIdentifier, SourceFeatureID is a long, 

unique code. This code identifies the parent of the feature if 

the feature is the result of a split or merge, and it is automatically generated and assigned.

Field Name SourceFeatureID

Field Type Character

Field Width 40

Domain Randomly generated alpha-numeric character

Value required? No

Example {5DD21DC6-3692-4197-889B-49E652AA43D0}

6.2.2.3 MetaSourceID 

MetaSourceID is another unique identifier that links 

the element to the metadata tables. This ID is generated and 

assigned automatically by the database and remains with the 

object permanently.

Field Name MetaSourceID

Field Type Character

Field Width 40

Domain Randomly generated alpha-numeric character

Value required? Yes

Example {5DD21DC6-3692-4197-889B-49E652AA43D0}

6.2.2.4 SourceDataDesc

SourceDataDesc is a space provided for a brief description of the type of base data used to update or change the current WBD. The WBD In-State Steward completes this field as 

part of the metadata form.

Field Name SourceDataDesc

Field Type Character

Field Width 100

Domain User-entered text limited to 100 characters

Value required? Yes

Example Montgomery County 1-meter LiDAR

6.2.2.5 SourceOriginator 

SourceOriginator is the description of the agency that 

created the base data used to improve the WBD. The WBD 

In-State Steward completes this field as part of the metadata 

form.

Field Name SourceOriginator

Field Type Character

Field Width 130

Domain User-entered text limited to 130 characters

Value required? Yes

Example USDA–FS LiDAR
38 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

6.2.2.6 WBDLine 

WBDLine is the primary line representation of the WBD. 

This feature data class represents the official geometry of the 

WBD, and all polygon classes are derived from WBDLine. 

WBDLine is also the primary data class for geometry editing 

by WBD In-State Stewards. WBDLine is edited by changing 

the line geometry and attributes. The edits are then propagated 

throughout the other WBD-derived feature classes. WBDLine 

includes five common fields and four unique fields described 

below. WBDLine contains PermanentIdentifier, SourceFeatureID, MetaSourceID, SourceDataDesc, SourceOriginator 

fields. These common fields are automatically created and 

assigned. See the previous descriptions of these fields (sections 6.2.2.1–6.2.2.5).

6.2.2.6.1 Common Fields

WBDLine contains PermanentIdentifier, SourceFeatureID, MetaSourceID, SourceDataDesc, SourceOriginator 

fields. These common fields are automatically created and 

assigned. See the previous descriptions of these fields (sections 6.2.2.1–6.2.2.5)

6.2.2.6.2 WBDLine: HULevel

HULevel is a domain-based field, completed by the 

WBD In-State Steward, which indicates the WBD line level 

by use of a single digit. For example, a 12-digit hydrologic 

unit line would be a level 6 line, and a 4-digit hydrologic 

unit line would be a level 2 line. This field allows for crosswalking linked reports and external data that used the level 

classification.

The hydrologic unit level attribute indicates the relative 

position of each boundary line segment within the hydrologic 

unit hierarchy. Populate this field with the highest hydrologic 

unit level (smallest number) for each line as indicated by the 

record. A level is represented by numbers 0 through 8, with 0 

at the top and 8 at the bottom of the hierarchy. For example, 

if a line segment is the boundary of a 2-digit hydrologic unit, 

then the value is 1. If a line segment is the boundary of an 

8-digit hydrologic unit, then the value is 4, even though it is 

also a 10- and 12-digit hydrologic boundary. If a line segment 

ends at an international border and there is no information to 

complete the hydrologic unit beyond the United States boundary, then the value is 0. Where the NOAA Three Nautical Mile 

Line designation represents the outer extent of the hydrologic 

unit, the value is 1. Use one of the levels in the list below.

Level Name 

(number of digits) Common name

0 0 No data (international border)

1 2 Region 

2 4 Subregion 

3 6 Basin 

4 8 Subbasin 

5 10 Watershed 

6 12 Subwatershed

7 14 No historical name

8 16 No historical name

Field Name HULevel

Field Type Long Integer

Field Width 6

Domain 1, 2, 3, 4, 5, 6

Value required? Yes

Example 6

6.2.2.6.3 WBDLine: HUClass

HUClass is a domain-based field that indicates the minimum number of digits used to represent the hydrologic unit 

bounded by the line. The WBD In-State Steward completes 

HUClass entries.

Field Name HUClass

Field Type Long Integer

Field Width 16

Domain 2, 4, 6, 8, 10, 12, 14, 16 

Value required? Yes

Example 12 (Subwatershed)
6. Geospatial Data Structure 39

6.2.2.6.4 WBDLine: LineSource 

LineSource represents the code for the base data used 

for delineating hydrologic unit boundaries. There is an agreed 

upon standard for the abbreviation of the line source information, which indicates if the line was created using USGS 

1:24,000 DRG’s, a DEM, LiDAR, or some other base data 

source. The WBD In-State Steward completes this field, which 

is in the same format as the original NRCS Linesource field. 

Populate the field by using one or more of the standardized 

codes listed below in uppercase. If more than one code is used, 

then separate the values with a comma and no space with the 

most recent LineSource used listed first in the sequence. Other 

reference and source maps not listed should be noted in the 

metadata (section 6.2.4).

TOPO24 Delineated from hardcopy 1:24,000-scale topographic maps. 

TOPO25 Delineated from hardcopy 1:25,000-scale topographic maps for the Caribbean outlying areas.

TOPO50 Delineated from hardcopy 1:25,000-scale topographic maps.

TOPO63 Delineated from hardcopy 1:63,360-scale topographic maps for Alaska.

TOPO250 Delineated from hardcopy 1:250,000-scale topographic maps. 

DRG24 Delineated from 1:24,000-scale Digital Raster 

Graphics.

DRG25 Delineated from 1:25,000-scale Digital Raster Graphics for the Caribbean outlying areas.

DRG63 Delineated from 1:63,360-scale Digital Raster Graphics for Alaska. 

CAN50 Delineated from 1:50,000-scale Digital Raster 

Graphics (CanMatrix).

CAN250 Delineated from 1:250,000- scale Digital Raster 

Graphics (CanMatrix).

CDED50 Delineated from 1:50,000- scale Canadian Digital 

Elevation Data.

CDED250 Delineated from 1:250,000- scale Canadian Digital 

Elevation Data.

DEM10 Derived from 10-meter Digital Elevation Model.

DEM15 Derived from 15-meter Digital Elevation Model.

DEM30 Derived from 30-meter Digital Elevation Model.

NED10 Derived from 10-meter National Elevation Dataset 

Model.

NED30 Derived from 30-meter National Elevation Dataset 

Model.

NOAA3NM National Oceanic and Atmospheric Administration demarcation 3 nautical miles offshore. May be generalized by the user in complex configuration, such as islands 

where buffer overlaps.

EDNA30 Derived from 30-meter Elevation Derivatives for 

National Applications.

BATH “scale” Interpreted from NOAA 1:24,000-scale or 

other bathymetric data, for example, BATH24. 

BUFFER ”distance” “unit” A near offshore limit used in bays, 

sounds or estuaries. The distance offshore is determined by 

local groups in the State. For unit, use: “M” for meter, “F” for 

feet, or “N” for nautical mile; for example, BUFFER400F. 

NAIP “year” Delineated from aerial photography produced 

by the National Agriculture Imagery Program, for example, 

NAIP2005 

NHD24 Interpreted from 1:24,000-scale National Hydrography Dataset.

NHD25 Interpreted from 1:25,000-scale National Hydrography Dataset.

NHD63 Interpreted from 1:63,360-scale National Hydrography Dataset.

NHD100 Interpreted from 1:100,000-scale National Hydrography Dataset.

NHN10 Interpreted from 1:10,000-scale National Hydrography Network (Canada).

NHN20 Interpreted from 1:20,000-scale National Hydrography Network (Canada).

NHN50 Interpreted from 1:50,000-scale National Hydrography Network (Canada).

HYPSO “scale” Delineated from 1:24,000-scale or other 

contour data, for example, HYPSO24.

ORTHO “scale” Interpreted from 1:12,000-scale or other 

orthoimagery, for example, ORTHO12.

DEDEM10 Drainage-enforced 10-meter Digital Elevation 

Model.

DEDEM30 Drainage-enforced 30-meter Digital Elevation 

Model.

GPS Derived from Global Positioning System.

NRN Interpreted from GPS transportation data captured at 

10-meter accuracy (Canada).
40 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

LIDAR Derived from LiDAR (light detection and ranging) 

data. See required metadata form for details on resolution and 

acquisition.

IFSAR Derived from IfSAR (interferometric synthetic aperture radar) data.

TRIM Delineated from 1:20,000-scale British Columbia 

Terrain Resource Information Management three-dimensional 

data. 

“Province” “scale” Derived from Provincial GIS layers. 

Specify with a 2-digit Province abbreviation followed by 

scale, for example use MB20 for Manitoba 1:20,000-scale 

data. 

SPOT “year” Interpreted from System for Earth Observation 

imagery.

CAN Various Canadian base reference information was used. 

See FGDC metadata for more information.

MEX Various Mexican base reference information was used. 

See FGDC metadata for more information.

OTH Other.

If additional information is required, it can be stored in the 

accompanying FGDC metadata.

Field Name LineSource

Field Type Character

Field Width 30

Domain TOPO24, TOPO25, TOPO50, TOPO63, 

TOPO250, DRG24, DRG25, DRG63, 

CAN50, CAN250, CDED50, CDED250, 

DEM10, DEM15, DEM30, NED10, NED30, 

NOAA3NM, EDNA30, BATH “scale,” NOAA3NM, BUFFER “distance” “unit,” NAIP 

“year,” NHD24, NHD25, NHD63, NHD100, 

NHN10, NHN20, NHN50, HYPSO “scale,” 

ORTHO “scale,” DEDEM10, DEDEM30, 

GPS, NRN, LIDAR, IFSAR, TRIM, “Province” “scale,” SPOT “year,” CAN, MEX, 

OTH

Value required? Yes

Example DRG24

6.2.2.6.5 WBDLine: LoadDate

LoadDate automatically generates and assigns the date 

when the data were loaded into the official USGS WBD 

ArcSDE database. This field is the effective date for all feature 

edits.

Field Name LoadDate

Field Type Date

Field Width 22

Domain Calculated system date

Value required? Yes

Example 9/21/2011 10:41:40 AM

6.2.2.7 WBDPoint 

WBDPoint represents the centroid of each Polygon Unit 

and is the link between the WBDAttributes table and the 

derived polygons. Because some multipart hydrologic units in 

the WBD share the same attribution information, WBDPoint 

includes five common fields and four unique fields described 

below. 

6.2.2.7.1 Common Fields

Like WBDLine, WBDPoint contains PermanentIdentifier, 

SourceFeatureID, MetaSourceID, SourceDataDesc, SourceOriginator fields. These common fields are automatically created and assigned. See the previous descriptions of these fields 

(sections 6.2.2.1–6.2.2.5).
6. Geospatial Data Structure 41

6.2.2.7.2 WBDPoint: HUC

HUC is a text field, completed by the WBD In-State 

Steward, which contains the 2-, 4-, 6-, 8-, 10, 12-, 14-, or 

16-digit hydrologic unit code. This code is unique to each 

hydrologic unit at each nested level. The HUC codes use the 

number of digits to represent the level of the hydrologic unit. 

Each subsequent level gains two additional digits as it moves 

through the hierarchy. 

Field Name HUC

Field Type Character

Field Width 16

Domain (2-, 4-, 6-, 8-, 10-, 12-, 14-, or 16-digit 

hydrologic unit code, nested by area)

Value required? Yes

Example 1019000401

6.2.2.7.3 WBDPoint: HULevel 

HULevel is a domain-based field, completed by the WBD 

In-State Steward, which expresses the WBD line level as a single digit. For example, a 12-digit hydrologic unit line would 

be a level 6 line, and a 4-digit hydrologic unit line would be a 

level 2 line. This field allows for crosswalking linked reports 

and external data that utilized the level classification.

Field Name HULevel

Field Type Long Integer

Field Width 6

Domain 1, 2, 3, 4, 5, 6, 7, 8

Value required? Yes

Example 6

6.2.2.7.4 WBDPoint: HUClass

HUClass is a domain-based field, completed by the WBD 

In-State Steward, which indicates the minimum number of 

digits of the hydrologic unit that the line bounds.

Field Name HUClass

Field Type Long Integer

Field Width 16

Domain 2, 4, 6, 8, 10, 12, 14, 16

Value required? Yes

Example 12 (subwatershed)

6.2.2.7.5 WBDPoint: LoadDate

LoadDate represents the date when the data were loaded 

into the official USGS WBD ArcSDE database. The field is 

the effective date for all feature edits, and it is automatically 

generated.

Field Name LoadDate

Field Type Date

Field Width 22

Domain (Automatically calculated system date)

Value required? Yes

Example 9/21/2011 10:41:40 AM
42 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

6.2.2.8 WBDAttributes

WBDAttributes is the table where the hydrologic unit 

code, gazetteer identification, hydrologic unit name, hydrologic unit class, hydrologic unit type, noncontributing area, in 

acres, and load date are stored. This table also contains a permanent feature identification, provides a link to the metadata 

tables, and is used for the attributes of the derived polygon 

classes. WBDAttributes includes five common fields and 

seven unique fields described below.

6.2.2.8.1 Common Fields 

Like WBDLine and WBDPoint, WBDAttributes contains PermanentIdentifier, SourceFeatureID, MetaSourceID, 

SourceDataDesc, and SourceOriginator fields. These fields are 

automatically generated. See the previous descriptions of these 

fields (sections 6.2.2.1–6.2.2.5).

6.2.2.8.2 WBDAttributes: HUC

HUC is a text field, completed by the WBD In-State 

Steward, which contains the 2-, 4-, 6-, 8-, 10-, 12-, 14-, or 

16-digit hydrologic unit code. This code is unique to each 

hydrologic unit at each nested level. Each subsequent level 

gains two additional digits as it moves through the hierarchy. 

Field Name HUC

Field Type Character

Field Width 16

Domain (2-, 4-, 6-, 8-, 10-, 12-, 14-, or 16-digit 

hydrologic unit code, nested by area)

Value required? Yes

Example 1019000401

6.2.2.8.3 WBDAttributes: GazID

GazID is an automatically generated number field that 

uses a unique number to relate the name of the hydrologic unit 

to the GNIS names database. This field is not populated by the 

WBD In-State Steward.

Field Name GazID

Field Type Long Integer

Field Width 16

Domain (Unique sequential number assigned by GNIS)

Value required? Yes

Example 123456789

6.2.2.8.4 WBDAttributes: Name 

The name field contains the assigned name of the hydrologic unit, constructed with reference to a feature name that is 

officially recognized by the GNIS. The WBD In-State Steward 

completes this field, based on the rules described in section 

5.3, “Naming Protocol for 10-, 12-, 14-, and 16-Digit Hydrologic Units.” The value assigned to the 10- and 12-digit name 

attribute should be unique within the 8-digit hydrologic units 

that contain the10- and 12-digit hydrologic unit.

Field Name Name

Field Type Character

Field Width 255

Domain (GNIS official names)

Value required? Yes

Example Gerome Creek
6. Geospatial Data Structure 43

6.2.2.8.5 WBDAttributes: HUClass 

HUClass is a domain-based field, completed by the WBD 

In-State Steward, which indicates the number of digits in the 

hydrologic unit code assigned to the hydrologic unit.

Field Name HUClass

Field Type Long Integer

Field Width 16

Domain 2, 4, 6, 8, 10, 12, 14, 16

Value required? Yes

Example 12 (subwatershed)

6.2.2.8.6 WBDAttributes: HUType 

HUType contains a letter code that defines the type of 

hydrologic unit. This code is used only in the 10-, 12-, 14-, or 

16-digit hydrologic units. The HUType attribute is the singleletter abbreviation from the list of official names provided 

below. The HUType field is completed by the WBD In-State 

Steward. Use the single type that most closely describes the 

12-digit hydrologic unit (see coastal examples in fig. 10).

Field Name HUType

Field Type Text

Field Width 2

Domain S, C, F, M, W, I

Value required? No

Example S (standard)

S “Standard” hydrologic unit An area with drainage flowing 

to a single outlet point, excluding noncontributing areas. Some 

examples include “true,” “standard,” “composite,” and “remnant” hydrologic units (section 3.5.1 and section 3.5.2). 

C “Closed Basin” hydrologic unit A drainage area where 

all surface flow is internal (100-percent noncontributing); no 

overland flow leaves the hydrologic unit through the outlet 

point (section 3.5.3).

F “Frontal” hydrologic unit An area along the coastline of 

a lake, ocean, bay, etc. that has more than one outlet. These 

hydrologic units are predominantly land; however, they may 

include some water areas at or near the outlet(s) (section 

3.6.2).

M “Multiple Outlet” hydrologic unit An area that has more 

than one natural outlet, such as an outlet located on a stream 

that has multiple channels (for example, braided streams, deltas or alluvial fans). Multiple Outlet does not include frontal 

or water hydrologic units, hydrologic units with artificial interbasin transfers, drainage outlets through karst or groundwater 

flow, or outlets that cross a stream with an island. Use this 

code only in rare instances.

W “Water” hydrologic unit An area that is predominantly 

water with adjacent land areas; for example, a lake, estuary, or 

harbor (section 3.6). 

I “Island” hydrologic unit An area that is one or more 

islands, which includes surrounding water (section 3.6.3).

6.2.2.8.7 WBDAttributes: NContrbAcres

NContrbAcres is the area, in acres, of a hydrologic unit 

that does not drain outside of its own boundaries, in other 

words, the noncontributing area attribute represents the area 

that does not contribute to downstream accumulation of 

streamflow under normal flow conditions (section 3.5.3). If a 

noncontributing area is on the boundary between two or more 

hydrologic units, then determine the low point along the noncontributing area boundary, and associate the noncontributing 

area with the hydrologic unit adjacent to the low point on the 

boundary. The value is the total acreage of the noncontributing areas within a hydrologic unit. The WBD In-State Steward 

completes this field.

Field Name NContrbAcres

Field Type Double

Field Width 16

Domain 0–unlimited

Value required? No

Example 367
44 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

6.2.2.8.8 WBDAttributes: LoadDate

LoadDate represents the date when the data were loaded 

into the official USGS WBD ArcSDE database. This automatically generated field is the effective date for all feature edits. 

Field Name LoadDate

Field Type Date

Field Width 22

Domain (Automatically calculated system date)

Value required? Yes

Example 9/21/2011 10:41:40 AM

6.2.2.9 WBDPointEvent

This feature resides and is managed in the NHD.

6.2.2.10 WBDNav 

WBDNav is the flow table for the WBD, providing the 

downstream link for each hydrologic unit from its upstream 

unit or units. This table is also used to show main flow up- and 

downstream where multiple units flow into a single unit or 

where a single unit flows into multiple units.

6.2.2.10.1 WBDNav: FromHUC

FromHUC is an automatically generated 2-, 4-, 6-, 8-, 

10-, 12-, 14-, or 16-digit code that represents the originating 

hydrologic unit.

Field Name FromHUC

Field Type Text

Field Width 16

Domain (Automatically calculated)

Value required? Yes

Example 101900040101

6.2.2.10.2 WBDNav: ToHUC

ToHUC is a 2- to 16-digit number representing the hydrologic unit into which the FromHUC unit flows. The WBD 

In-State Steward completes this field only for the 12-digit 

hydrologic unit.

Field Name ToHUC

Field Type Text

Field Width 16

Domain None

Value required? No

Example 101900040102

6.2.2.10.3 WBDNav: HUClass

HUClass is a domain-based field, completed by the WBD 

In-State Steward, which indicates the number of digits that the 

FromHUC (see section 6.2.2.10.1) hydrologic unit contains. 

Field Name HUClass

Field Type Long Integer

Field Width 16

Domain (2-, 4-, 6-, 8-, 10-, 12-, 14-, 16-digit valid 

hydrologic unit codes)

Value required? Yes

Example 12 (subwatershed)

6.2.2.10.4 WBDNav: UpMain

UpMain is the main upstream hydrologic unit from 

the value listed in FromHUC (see section 6.2.2.10.1). The 

WBD In-State Steward completes this field. This capability is 

planned and not currently available.

Field Name UpMain

Field Type Long Integer

Field Width 16

Domain Yes, No

Value required? No

Example Yes
6. Geospatial Data Structure 45

6.2.2.10.5 WBDNav: DownMain 

DownMain is the downstream hydrologic unit from 

the value listed in FromHUC (see section 6.2.2.10.1). The 

WBD In-State Steward completes this field. This capability is 

planned and not currently available.

Field Name DownMain

Field Type Long Integer

Field Width 16

Domain Yes, No

Value required? No

Example Yes

6.2.2.11 WBDFeatureToHUMod 

WBDFeatureTo HUMod is a table that stores in a domain 

all of the modification attributes of lines and polygons, so that 

database integrity is maintained. The line and polygon modifications, described below, are stored in this table and related 

back to their respective WBDLine and hydrologic units by the 

PermanentIdentifier.

6.2.2.11.1 WBDFeatureToHUMod for lines: HUMod

The HUMod line modification attribute, completed by 

the WBD In-State Steward, is the two-character, uppercase 

abbreviation(s) code used to track either the modification to 

natural overland flow that alters the location of the hydrologic 

unit boundary or special conditions that are applied to a specific boundary line segment. The value identifies the type of 

modification, from the list provided, that has been applied to 

the boundary segment. If more than one abbreviation is used, 

then separate them by commas without spaces and list them in 

order of importance.

Field Name HUMod

Field Type Character

Field Width 20

Domain AW, DM, LA, LE, MA, NM, OC, OF, PD, PL, 

PS, SI, SL, TF, UA

Value required? Yes

Example PD, PS

AW Artificial Waterway An aqueduct, canal, ditch or drain 

used to transport surface water, altering the natural flow out 

of the hydrologic unit. (Options previously included AD 

Aqueduct, DD Drainage Ditch, GC General Canal/Ditch, ID 

Irrigation Ditch, IT Interbasin Transfer, SD Stormwater Ditch, 

SC Stormwater Canal, and BC Barge Canal, which are now 

grouped into this designation).

DM Dam A barrier constructed to control the flow or raise the 

level of water at a hydrologic unit outlet or on the hydrologic 

unit boundary; alters the natural boundary location.

LA Lava Field A lava field is a large expanse of nearly flat 

lying lava flows (usually deposited in the past).

LE Levee An artificial bank to confine a stream channel 

or limit adjacent areas subject to flooding; alters the natural 

boundary location. 

MA Mining Activity Heavy topographic modification of a 

hydrologic unit by surface mining; alters natural boundary 

location.

NM No Modifications No modifications are present. Use 

when no other options with the modification domain have 

been cited.

OC Overflow Channel or Flume An artificial channel built 

to control excess high flow from a natural channel; alters the 

natural boundary location.

OF Overbank Flow A natural condition in which a stream 

surpasses bankfull stage and the excess flows into a nearby 

channel draining to a different hydrologic unit (special condition; see example in fig. 18). 

PD Pipe Diversion A redirection of surface water by a 

pipeline from one hydrologic unit to another; alters the natural 

boundary location.

PL Playa A playa is sandy, salty or mud-caked flat floor of a 

dessert basin having interior drainage usually occupied by a 

shallow lake during or after prolonged, heavy rains.

PS Pumping Station A facility along a stream or other waterbody used to move water over a levee or other obstruction; 

alters the natural boundary location.

SI Siphon An artificial diversion, which is usually named 

“Siphon” on maps, to move surface water from one stream 

channel to another; alters the natural boundary location. 
46 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

Figure 18. When high flows cause a natural diversion and the excess runs into an adjacent channel draining to a 

different hydrologic unit, the special condition is named overbank flow. Overbank flow is usually found in areas of 

alluvial material, such as in this area of Graham County, Arizona.

0 0.5 1 MILE

N

0 0.5 1 KILOMETER

Hydrologic unit boundary

Subwatershed

EXPLANATION

C ree k

Overbank flow

Overbank flow

ARIZONA

Figure 18

third edition 041112
6. Geospatial Data Structure 47

SL Shoreline A demarcation of shoreline within WBD 

agreed upon by in-State groups. These instances are exceptions to the preferred method.

TF Transportation Feature A land transportation feature, for 

example, a road, railroad, dock, airport, etc., that alters the 

natural boundary location.

UA Urban Area Heavy modification of hydrologic unit 

topography by development beyond that described above 

in “Transportation Feature” that alters the natural boundary 

location. 

6.2.2.11.2 WBDFeatureTo HUMod for polygons: HUMod 

The HUMod 12-digit hydrologic unit polygon modification attribute, entered by the WBD In-State Steward, is a 

two-character, uppercase abbreviation for either (1) the type 

of modification to natural overland flow that alters the natural 

delineation of a 12-digit hydrologic unit or (2) the special 

conditions GF-groundwater flow, GL-glacier, IF-ice field, KAkarst, and NC-noncontributing area. The value of the HUMod 

field helps to indicate the areal location of the modification to 

the 12-digit hydrologic unit. If more than one abbreviation is 

used, then separate them by commas and list them in order of 

importance.

Previous versions of this guideline did not provide the 

same number of modification choices—check metadata or 

contact the WBD In-State Steward for more information.

Field Name HUMod

Field Type Character

Field Width 20

Domain AW, GF, GL, IF, KA, LA, MA, NC, NM, OC, 

OF, PD, RC, RS, UA, WD

Value required? Yes

Example NC

AW Artificial Waterway A canal, ditch, or drain used to 

transport surface water that alters the natural flow out of the 

hydrologic unit. (The previously included designations AD 

Aqueduct, DD Drainage Ditch, GC General Canal/Ditch, ID 

Irrigation Ditch, IT Interbasin Transfer, SD Stormwater Ditch, 

SC Stormwater Canal, and BC Barge Canal have now been 

grouped into this designation). Withdrawing and receiving 

hydrologic units should carry this designation, as well as all 

hydrologic units in which the flow is altered by an artificial 

waterway.

GF Groundwater Flow A special condition, usually in locations with sandy soil, where most of the runoff in a hydrologic 

unit drains underground. 

GL Glacier A special condition where a hydrologic unit 

crosses or includes a body or stream of ice moving outward 

and downslope from an area of accumulation; area of relatively permanent snow or ice on the top or side of a mountain 

or mountainous area.

IF Ice Field A special condition where a hydrologic unit 

crosses or includes a field of ice, formed in regions of perennial frost.

KA Karst A special condition where a hydrologic unit is 

within an area of, or includes an area of, geologic formations of irregular limestone deposits with sinks, underground 

streams, or caverns. 

LA Lava Field A lava field is a large expanse of nearly flat 

lying lava flows (usually deposited in the past).

MA Mining Activity Heavy topographic modification of a 

hydrologic unit by surface mining; alters the natural flow out 

of the hydrologic unit.

NC Noncontributing Area A naturally formed area that does 

not contribute surface-water runoff to a hydrologic unit outlet 

under normal conditions, for example, a playa. This does not 

include groundwater flow.

NM No Modifications No modifications are present. Use 

when no other options with the modification domain have 

been cited.

OC Overflow Channel or Flume An artificial channel built 

to control excess high flow from a natural channel; alters the 

natural flow out of the hydrologic unit.

OF Overbank Flow A natural condition in which a stream 

surpasses bankfull stage and the excess flows into a nearby 

channel draining to a different hydrologic unit (special condition; see example in fig. 18). Both the losing and the receiving 

hydrologic unit should carry this designation.

PD Pipe Diversion A redirection of surface water by a 

pipeline from one hydrologic unit to another; alters the natural 

flow out of the hydrologic unit.

RC Receiving A hydrologic unit that receives diverted water.
48 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

RS Reservoir A constructed basin formed to contain and 

store water for future use in an artificial lake; alters the natural 

flow out of the hydrologic unit.

UA Urban Area Heavy modification of hydrologic unit 

topography by development beyond that described in section 

6.2.11.1, “Transportation Feature,” that alters natural flow out 

of the hydrologic unit.

WD Withdrawal A hydrologic unit from which water is 

diverted.

6.2.2.12 NWISLine

NWISLine is a feature dataset used for storing lines 

needed for the NWIS drainage areas but that are not part of the 

WBD.

6.2.3 Derived Classes

The WBD derived feature classes are probably the most 

familiar and frequently used WBD feature classes. These 

layers are “derived” because they are not edited by the WBD 

In-State Stewards; rather, they are derived from the edited 

data. The data are derived from the point and line features as 

well as the attribute and modification tables. Each time edits 

have been reviewed by the WBD–NTC and are successfully 

incorporated into the WBD, those areas where the edits took 

place are rederived and posted for downloading.

The following attributes are derived by the system:

AreaAcres This value is calculated from the intrinsic area 

value maintained by the GIS software; therefore, acreage 

values may vary from user calculations, depending on the projection of the data. North America Albers Equal Area Conic, 

North American Datum 1983 is the recommended projection. 

If the units of the area field are stored in square meters, then 

use the conversion factor 0.0002471. For example, 40,469,446 

square meters multiplied by 0.0002471 = 10,000 acres.

AreaSqKm This value is calculated from the intrinsic area 

value maintained by the GIS software; therefore, the square 

kilometers values may vary from user calculations, depending on the projection of the data. North America Albers Equal 

Area Conic, North American Datum 1983 is the recommended 

projection. 

States The State or outlying area attribute identifies State(s) 

or outlying areas that the hydrologic unit falls within or 

touches. The U.S. Census Bureau 1:100,000-scale State layer 

will be used to establish State boundaries in the derivation 

process. The two-letter U.S. Postal Service State abbreviation 

is assigned. If a hydrologic unit crosses into Canada, the twoletter Canada Post abbreviation for the Province is used. If a 

unit crosses into Mexico, the MX designation is used. If more 

than one abbreviation is used, they are separated by commas, 

without spaces, and are sorted in alphabetical order.

LoadDate A system-generated date that is assigned upon 

posting accepted edits to the national official database.

6.2.3.1 WBDHU2

WBDHU2 represents the 2-digit hydrologic unit boundaries (previously referred to as Regions). There are 22 2-digit 

hydrologic units (Regions) in the WBD, and each has the following attribute fields.

• GazID Gazetteer identification number (also referred 

to as GNIS)

• AreaAcres Area, shown in acres

• AreaSqKm Area, shown in square kilometers

• States States included in the 2-digit hydrologic unit 

• LoadDate Date when the data were loaded

• HU2Name Region name

• HUC2 Unique 2-digit hydrologic unit code

6.2.3.2 WBDHU4

WBDHU4 represents the 4-digit hydrologic unit boundaries (previously referred to as Subregions). There are 220 

4-digit hydrologic units (Subregions) in the WBD, and each 

has the following attribute fields.

• GazID Gazetteer identification number (also referred 

to as GNIS)

• AreaAcres Area, shown in acres

• AreaSqKm Area, shown in square kilometers

• States States included in the 4-digit hydrologic unit

• LoadDate Date when the data were loaded

• HU4Name Subregion name

• HUC4 Unique 4-digit hydrologic unit code
6. Geospatial Data Structure 49

6.2.3.3 WBDHU6

WBDHU6 represents the 6-digit hydrologic unit boundaries (previously referred to as Basins). There are approximately 

378 6-digit hydrologic units (Basins) in the WBD, and each 

has the following attribute fields.

• GazID Gazetteer identification number (also referred 

to as GNIS)

• AreaAcres Area. shown in acres

• AreaSqKm Area. shown in square kilometers

• States States included in the 6-digit hydrologic unit

• LoadDate Date when the data were loaded

• HU6Name Basin name

• HUC6 Unique 6-digit hydrologic unit code

6.2.3.4 WBDHU8

WBDHU8 represents the 8-digit hydrologic unit boundaries (previously referred to as Subbasins). There are approximately 2,283 8-digit hydrologic units (Subbasins) in the WBD, 

and each has the following attribute fields. 

• GazID Gazetteer identification number (also referred 

to as GNIS)

• AreaAcres Area, shown in acres

• AreaSqKm Area, shown in square kilometers

• States States included in the 8-digit hydrologic unit

• LoadDate Date when the data were loaded

• HU8Name Subbasin name

• HUC8 Unique 8-digit hydrologic unit code

6.2.3.5 WBDHU10

WBDHU10 represents the 10-digit hydrologic unit 

boundaries (previously referred to as Watersheds). There are 

approximately 17,828 10-digit hydrologic units (Watersheds) 

in the WBD, and each has the following attribute fields.

• GazID Gazetteer identification number (also referred 

to as GNIS)

• AreaAcres Area, shown in acres

• AreaSqKm Area, shown in square kilometers

• States States included in the 10-digit hydrologic unit 

• LoadDate Date when the data were loaded

• HU10Name Watershed name

• HU10Type Watershed type

• HUC10 Unique 10-digit hydrologic unit code

6.2.3.6 WBDHU12

WBDHU12 represents the 12-digit hydrologic unit 

boundaries (previously referred to as Subwatersheds). There 

are approximately 97,443 12-digit hydrologic units (Subwatersheds) in the WBD, and each has the following attribute fields.

• GazID Gazetteer identification number (also referred 

to as GNIS)

• AreaAcres Area, shown in acres

• AreaSqKm Area, shown in square kilometers

• States States included in the 12-digit hydrologic unit

• LoadDate Date when the data were loaded

• HU12Name Subwatershed name

• HU12Type Subwatershed type

• HUC12 Unique 12-digit hydrologic unit code

• HU12Mod Polygon modifications

• NContrbAcres Noncontributing area, in acres

• NcontrbSqKm Noncontributing area, in square kilometers
50 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

6.2.3.7 WBDHU14

WBDHU14 represents the 14-digit hydrologic unit 

boundaries. There is no common name for this hydrologic 

unit. This is a new subdivision in the WBD and data have yet 

to be added to it. Each 14-digit hydrologic unit has the following attribute fields.

• GazID Gazetteer identification number (also referred 

to as GNIS)

• AreaAcres Area, shown in acres

• AreaSqKm Area, shown in square kilometers

• States States included the 14-digit hydrologic unit

• LoadDate Date when the data were loaded

• HU14Name 14-digit name

• HU14Type 14-digit type

• HUC14 Unique 14-digit hydrologic unit code 

• HU14Mod Polygon modifications

• NContrbAcres Noncontributing area, in acres

• NcontrbSqKm Noncontributing area, in square kilometers

6.2.3.8 WBDHU16

WBDHU16 represents the 16-digit hydrologic unit 

boundaries. There is no common name for this hydrologic 

unit. This is a new subdivision in the WBD, and data have yet 

to be added to it. Each 16-digit hydrologic unit has the following attribute fields.

• GazID Gazetteer identification number (also referred 

to as GNIS)

• AreaAcres Area, shown in acres

• AreaSqKm Area, shown in square kilometers

• States States included in the 16-digit hydrologic unit

• LoadDate Date when the data were loaded

• HU16Name 16-digit name

• HU16Type 16-digit type

• HUC16 Unique 16-digit hydrologic unit code

• HU16Mod Polygon modifications

• NContrbAcres Noncontributing area, in acres

• NcontrbSqKm Noncontributing area, in square kilometers

6.2.3.9 WBDNWIS

WBDNWIS is the newest feature class in the WBD, and 

it represents the drainage area boundaries of gaging stations 

stored within the NWIS database.

6.2.4 Metadata

Metadata in the new model are kept not only at the 

dataset level but also at the feature level. Each time an item is 

updated or edited in any way, metadata are required to show 

how, when, and why the feature was modified. The WBD Edit 

Tool will not allow editing without correctly populated metadata. The WBD Edit Tool provides a template and tool needed 

to create metadata properly.

6.2.4.1 MetaProcessDetail

MetaProcessDetail is the template that defines the 

overarching process related to the associated set of edits and 

to provide contact information for and instructions about questions of those specific edits (fig. 19).

Figure 19. The Metadata Process Details template within the 

WBD Edit Tool is where information is entered about the editor of 

a particular WBD edit and why the edit was made. This example 

shows the contact information portion of the template, should an 

end user need more information about a particular change.
6. Geospatial Data Structure 51

Figure 20. The Metadata Source Detail template within the WBD Edit Tool is where information is entered about the base data 

used to make the edits and updates to the WBD. The example shows that GNIS data is being used to help name a hydrologic unit.

6.2.4.2 MetaSourceDetail

MetaSourceDetail is the metadata template where information is entered about the base data used to make the edits 

and updates to the WBD (fig. 20). This information includes 

the type of data (LiDAR, DEM, DRG, NAIP), the source of 

the data, and the creator of the data, the relevant dates associated with the creation of the data, the scale and accuracy of the 

base data, and a brief description of the source data.

6.2.4.3 MetaID

Prior to integration of the WBD with the NHD, the 

metadata identification attribute was the code used for tracking 

changes made to a specific boundary (line or arc) segment or 

polygon attribute. When a State received provisional certification status, all boundary segments and polygon attribute values 

were 01. After provisional certification, the MetaID value 

was incrementally updated for that feature whenever changes 

were made to a boundary segment or polygon attribute. The 

change was documented in the updated FGDC metadata, and 

it referred back to the MetaID.

The four-character MetaID began with the two-letter U.S. 

Postal Service State abbreviation, in uppercase, followed by 

a two-digit sequence number; for example, “OK01,” “ID02.” 

All State polygons had a MetaID of (State abbreviation 01) at 

the time of provisional certification. Appendix 11.5 documents 

the previously used method for tracking metadata.
52 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

7. Quality Assurance and 

Quality Control

7.1 Quality Assurance

Hydrologic units are delineated and verified through 

an interagency process. Before newly delineated hydrologic 

unit boundaries are submitted for the WBD, the boundaries 

must be reviewed for conformance to these guidelines by the 

WBD In-State Steward originating agency and/or designated 

members of an interagency hydrologic unit coordinating 

group within the State. Qualified reviewers typically would 

be hydrologists or natural resource and GIS specialists with 

background and experience in hydrologic unit delineation. 

Representatives of the interagency hydrologic unit coordinating group and/or regional/local parties should participate in 

the development and review of delineations before the data are 

submitted for national review for compliance with the required 

standard and release to the public. The WBD In-State Steward 

should thoroughly review the data before submitting it to the 

WBD NTC for national review.

It is recommended that reviews and edit checks be done 

throughout the delineation process. At a minimum, edit checks 

should be made after the hydrologic units are delineated, 

mapped, and digitized.

7.2 Quality Control (Editing Checklist)

A recommended list of quality-control items to be 

checked during the delineation, coding, documentation, and 

digitizing process is shown below. This quality-control process 

is necessary for maintaining a consistent national database of 

hydrologic units. This is not an exhaustive list, but it covers 

most of the items requiring verification.

7.2.1 Delineation

• Do the hydrologic unit boundaries match at State 

boundaries, and are coding and size criteria consistent 

across boundaries? Resolve differences in delineation 

at State boundaries before the data are submitted for 

agency certification. 

• Does linework meet current NSSDA?

• Are hydrologic units consistently delineated across the 

State?

• Are the hydrologic units correctly delineated against 

the minimum required scale NRCS County Mosaic 

DRGs with respect to hydrography and elevation contours? (See section 4.)

• Is the NHD shoreline representation the basis for 

distance-based nearshore delineation? If not, explain 

what was used and why.

• Do coastal delineations in bays, sounds, and estuaries 

meet the guidance criteria?

• Are open-ocean boundaries complete where bays, 

sounds, and estuaries do not predominate? 

• Are 8-digit hydrologic units extended to the openocean NOAA Three Nautical Mile Line?

• Do hydrologic units that were difficult to delineate 

include a description of the procedure used? Make 

notes about how certain boundaries were established 

or field checked so that they can be documented in 

the field LINESOURCE (section 6.2.2.6.4) and in the 

metadata (section 6.2.4).

• Were local sources used to resolve the locations of 

questionable boundaries? Make notes where local 

knowledge supersedes source maps, and document 

this information in the field LINESOURCE (section 

6.2.2.6.4) and in the metadata (section 6.2.4).

• Although number and area criteria per level are listed, 

exceptions for extreme geomorphology will be considered.

• Are terrestrial hydrologic units subdivided into 5 to 

15 units? (See section 3.3.)

• Are the areas of terrestrial 10-digit hydrologic units 

within the recommended range of 40,000–250,000 

acres?

• Are the areas of terrestrial 12-digit hydrologic units 

within the recommended range of 10,000–40,000 

acres, with none less than 3,000 acres? When a 

hydrologic unit is divided by a State boundary, check 

the total area including both States’ portion. (See 

section 3.4.) A combined total of 10 percent of the 

polygons (combined 10- and 12-digit hydrologic 

units) outside the specified size criteria is allowed 

for the State.

7.2.2 Codes

• Is the coding of hydrologic units consistent within 

an 8-digit hydrologic unit that covers more than one 

State? Check the coding system for correctness, duplication, or missing codes. Each polygon must have a 

single label. (See section 5.2.)

• Does the coding of hydrologic units within a given 

level meet the guidelines for starting upstream and 

progressing downstream? (See section 5.2.)
8. Data Revision, Checkout, Editing, Verification, and Submittal 53

7.2.3 Attributes

• Are the attribute fields complete, with valid values? 

(See section 6.2.2.8)

• Do polygons have the required values in these fields? 

• HUClass, HUC (at both the 10- and 12-digit, and 14- 

and 16-digit, if applicable; see section 6.2.2.8.5)

• Name (at both the 10- and 12-digit, and 14- and 

16-digit, if applicable; see section 6.2.2.8.4)

• HUMod, HUType (at both the 10- and 12-digit, and 

14- and 16-digit, if applicable; see sections 6.2.2.8.6 

and 6.2.2.11.1) 

• Do lines have the required values in these fields? 

• HUClass (see section 6.2.2.8.5)

• HULevel (see section 6.2.2.7.3)

• Linesource (see section 6.2.2.6.4)

• HUMod (See section 6.2.2.11.1)

• Do coastal hydrologic units have the correct values in 

these fields? 

• HULevel (see section 6.2.2.6.2)

• Do artificial modifications or special conditions exist? 

Check and document in the fields HUMod for both 

lines and polygons, if the natural boundary location has 

been modified or special conditions exist. (See section 

6.2.2.11.1.)

7.2.4 Data Format

Does the file format conform to the guidelines? Before 

submitting data for national review and certification, verify 

that the coordinate system used in the data is correct. Data 

submitted for certification must be in geographic coordinates 

stored in decimal degrees. The horizontal datum is the North 

American Datum of 1983 (NAD 83). The data should be in 

double-precision accuracy.

8. Data Revision, Checkout, Editing, 

Verification, and Submittal 

This section presents guidelines and procedures to be 

followed for successful checkout, editing, and submittal of 

State contributions to the WBD. Digital geospatial data and 

attributes must match adjoining States. State submissions to 

the WBD will be made by use of the WBD stewardship Web 

site, using provided tools.

8.1 State Data Revision 

Agencies involved in the development of the WBD 

should establish organizational responsibility for reviewing the 

data before submittal. After the revisions are completed, the 

WBD In-State Steward will submit the data to USGS through 

the WBD Edit Tools for review and assurance of compliance 

with the standards by the WBD–NTC. Supporting material, 

such as text documents describing the procedure used to delineate and digitize the hydrologic units, is documented in the 

LINESOURCE field and metadata. In addition, a summary of 

decisions for placement of hydrologic unit boundaries in complex hydrographic areas and access to digital base reference 

maps will benefit and expedite review by the WBD–NTC. 

The NGMC and the USGS will make the data and metadata available electronically to the member agencies of the 

SSWD for review and decisions about acceptance, when necessary. The process for the submittal, review, and acceptance 

is shown in figure 21. The data will be reviewed for adherence to the current “Federal Standards and Procedures for the 

National Watershed Boundary Dataset (WBD)” interagency 

guidelines, particularly the following:

• mapping and delineation accuracy and consistency;

• coordination of boundaries and codes that match across 

State or Subregion boundaries;

• hydrologic unit size, nesting, coding protocol (including format), and completion of required fields in the 

attribute table;

• completion of required metadata.
Figure 21. The Watershed Boundary Dataset revision process and review for national certification compliance and roles are depicted in the flowcha

Figure 21.third edition 041112 

Process stepContributing activity to  process step

Product transfer pathInteractive collabation pathEvaluation

EXPLANATION

1.

a.

WBD In-State 

Steward

and team

4. WBD In-State Steward submits edits to WBD-NTC for review and notifies WBD-NTC of submittal.

WBD–NTC

Public 

milestones,

status maps

3. WBD In-State Steward edits the data.

8. WBD-NTC notifies WBD In-State Steward and consults on plan for resubmittal. 

Subcomittee on

Spatial Water

Data or 

Steering

committee

5. WBD-NTC reviews submitted edits against WBD Standards

6. WBD-NTC submits them for inclusion in the National database 

7. WBD-NTC notifies WBD In-State Steward and preset list and interested parties.

10. Steto thSubDatreq

a.

a.

b.

c.

b.

2. WBD In-State Steward checks out data.

1. State recognizes a need to edit WBD and coordinates a completion plan with interagency hydrologic unit coordinating group.

If area to be edited 

falls along an international 

border, then WBD In-State 

Steward coordinates with the 

WBD-NTC to check out data. 

WBD 

In-State Steward 

distributes notification of 

area editing to preset list 

and interested parties.

WBD In-State 

Steward initiates editing 

process by using the WBD Edit 

Tools and completing the 

required metadata. 

WBD In-State 

Steward solicits feedback 

from interagency hydrologic 

unit coordinating group and 

WBD-NTC as needed

WBD In-State 

Steward completes and 

validates edits.

If edits

do not meet

WBD certification  standard,  then

If edits

meet

 WBD certification  standard,  then
9. Metadata 55

The WBD–NTC will refer review comments from the 

interagency team to the office that submitted the data. After 

discrepancies are resolved, the WBD–NTC will notify the 

WBD In-State Steward, specifying that the hydrologic unit 

data are approved and meet the national guidelines. 

After the initial completion of WBD, some States 

were given a Provisional Certification status, which usually 

indicated that work such as providing metadata or matching boundaries across State lines remained to be completed. 

Provisional Certification was not given when obvious corrections needed to be made. If the data complied with the 

“Federal Standard for Delineation of Hydrologic Unit Boundaries” (Natural Resources Conservation Service, 2004) and 

were approved, the NGMC and the USGS sent a letter to the 

originating office stating that the WBD met the national standards, was fully certified, and would be added to the national 

database. 

8.2 Data Checkout 

WBD data can be checked out for editing from the NDH/

WBD Stewardship Web site at http://usgs-mrs.cr.usgs.gov/

stewardbeta/index.html. Every WBD In-State Steward will 

be required to obtain a valid username and password prior to 

login and checkout of data for editing. Contact the WBD-NTC 

for assistance in obtaining a login.

Once logged in, the WBD In-State Steward selects by 

8-digit hydrologic unit code (Subbasin) the area(s) to be 

checked out and edited. Information related to the nature 

of the editing is required in the checkout form. The size of 

checkout should be limited to an area that can be successfully 

completed within a 90-day period.

When the checkout process is completed, the WBD InState Steward will be notified by email that the job is ready for 

editing. Each job receives a unique job number, which is used 

to “get” the job from USGS servers prior to editing the data 

with the WBD Edit Tools.

8.2.1 Stewardship Web Site

An additional new site for WBD stewardship is the WBD 

stewardship Web site hosted by USGS http://nhdpreview.

er.usgs.gov/wbd.html. This new site provides links for downloading WBD and NHD data, information about national and 

State WBD contacts, historical information, FGDC metadata, 

and other pertinent information. 

8.3 Data Editing

The WBD In-State Steward begins the editing process by 

using the WBD Edit Tools. Instruction for using the WBD Edit 

Tools is provided on the WBD stewardship website at 

http://nhdpreview.er.usgs.gov/wbd.html. Users will also use 

the tools for populating related metadata, validating edits, and 

submitting data for national review by the WBD–NTC.

8.4 Data Verification

This section sets forth the rules that WBD In-State Stewards need to follow for handling data check-out and check-in 

through the USGS stewardship process.

8.4.1 File Names 

All dataset checkouts are named according to USGS job 

process identification, and the dataset name should never be 

changed. Copying, projecting, or renaming checked out datasets will prevent them from being submitted successfully. 

8.4.2 Map Projection and Horizontal Datum

Data submitted for certification must be in geographic 

coordinates stored in decimal degrees. The horizontal datum is 

the North American Datum of 1983 (NAD 83). All projection 

information must be included in the metadata file. Projecting 

a WBD dataset that has been checked out will prevent it from 

being submitted successfully. If areal calculations are required, 

a projected copy for personal reference may be needed.

8.5 Data Submittal

Data are submitted through the WBD Edit Tool by the 

WBD In-State Steward. The tools perform the appropriate 

compression and delivery of the data to USGS.

9. Metadata

According to an Executive Order 12906 signed by 

President William J. Clinton on April 11, 1994, all Federal 

agencies developing geospatial data are required to document newly created data by completing metadata. The most 

recent FGDC content standards for metadata must be followed. A sample of a completed FGDC-compliant metadata 

template for hydrologic units is available at http://www.nrcs.

usda.gov/wps/portal/nrcs/detail/national/water/watersheds/

dataset/?&cid=nrcs143_021631.

Beginning in the spring of 2012, all required metadata 

entry is collected at the beginning of an editing session within 

the WBD Edit Tools. Examples of metadata templates are 

available through the WBD Edit Tools.
56 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

10. Selected References

Dana, P.H., 1995a, Coordinate systems overview: Boulder, 

Colo., University of Colorado at Boulder, Department 

of Geology, accessed October 23, 2007, at http://www.

colorado.edu/geography/gcraft/notes/coordsys/coordsys.

html.

Dana, P.H., 1995b, Geodetic datum overview: Boulder, Colo., 

University of Colorado at Boulder, Department of Geology, 

accessed October 23, 2007, at http://www.colorado.edu/

geography/gcraft/notes/datum/datum_f.html.

Environmental Systems Research Institute, 2007, Data in 

the DBMS—Learn how data converts from one type 

to another: Redlands, Calif., accessed August 3, 2007, 

at http://webhelp.esri.com/arcgisdesktop/9.2/index.

cfm?TopicName=Data_types_in_the_DBMS.

Federal Geographic Data Committee, 1998a, Content standard 

for digital geospatial metadata (revised June 1998): Washington, D.C., FGDC–STD–001–1998, 78 p., accessed February 14, 2007, at http://www.fgdc.gov/standards/projects/

FGDC-standards-projects/metadata/base-metadata/

v2_0698.pdf.

Federal Geographic Data Committee, 1998b, Geospatial 

positioning accuracy standards, pt. 3—National standard 

for spatial data accuracy: Washington, D.C., FGDC–

STD–007.3–1998, 25 p., accessed February 14, 2007, at 

http://www.fgdc.gov/standards/projects/FGDC-standardsprojects/accuracy/part3/chapter3.

Lamke, R.D., Brabets, T.P., and McIntire, J.A., 1994, Alaska 

hydrologic units (revised 1996): U.S. Geological Survey, 

scale 1:250,000, accessed February 14, 2007, at http://agdc.

usgs.gov/data/projects/anwr/datahtml/akhuc.html.

National Oceanic and Atmospheric Administration, National 

Ocean Service, Office of Coast Survey, n.d., U.S. Maritime 

Zones/Boundaries: accessed December 22, 2008, at http://

www.nauticalcharts.noaa.gov/csdl/mbound.htm.

Natural Resources Conservation Service, 2007, Watershed 

Boundary Dataset (WBD) 1: accessed October 23, 2007, 

at http://www.ncgc.nrcs.usda.gov/products/datasets/

watershed/.

Seaber, P.R., Kapinos, F.P., and Knapp, G.L., 1987, Hydrologic unit maps: U.S. Geological Survey Water-Supply 

Paper 2294, 63 p., accessed February 14, 2007, at 

http://pubs.usgs.gov/wsp/wsp2294/.

Snyder, J.P., 1987, Map projections, a working manual: U.S. 

Geological Survey Professional Paper 1395, 383 p., available online at http://pubs.er.usgs.gov/usgspubs/pp/pp1395.

U.S. Bureau of the Budget, 1947, United States national map 

accuracy standards: Washington, D.C., accessed February 29, 2012, at http://nationalmap.gov/standards/pdf/

NMAS647.PDF.

U.S. Geological Survey [n.d.], United States Geographic 

Names Information System (GNIS): accessed, at http://

geonames.usgs.gov/pls/gnispublic/f?p=gnispq:8

U.S. Geological Survey [n.d.], United States Geographic 

Names Information System (GNIS) feature class definitions: 

accessed February14, 2007, at http://geonames.usgs.gov/

domestic/feature_class.htm.

U.S. Geological Survey, 1994, 1:250,000-scale hydrologic 

units of the United States: accessed February 14, 2007, at 

http://water.usgs.gov/GIS/metadata/usgswrd/XML/huc250k.

xml.

U.S. Geological Survey, 1999a, Map accuracy standards: 

U.S. Geological Survey Fact Sheet FS–171–99, accessed 

October 23, 2007, at http://erg.usgs.gov/isb/pubs/factsheets/

fs17199.html.

U.S. Geological Survey, 1999b, Standards for National 

Hydrography Dataset: accessed February 29, 2012, at 

http://nationalmap.gov/standards/nhdstds.html.

U.S. Geological Survey, 2005, Elevation Derivatives for 

National Applications (EDNA): accessed October 23, 2007, 

at http://edna.usgs.gov/. 

U.S. Geological Survey, 2006a, Digital Elevation Models 

(DEMs): accessed March 13, 2007, at http://edc.usgs.gov/

products/elevation/dem.html.

U.S. Geological Survey, 2006b, Digital Raster Graphics 

(DRGs): accessed March 13, 2007, at http://topomaps.usgs.

gov/drg/.

U.S. Geological Survey, 2006c, National Elevation Dataset 

(NED): accessed March 13, 2007, at http://ned.usgs.gov/.

U.S. Geological Survey, 2006d, National Hydrography Dataset 

(NHD): accessed March 13, 2007, at http://nhd.usgs.gov/.

U.S. Geological Survey, 2007, The National Map—Orthoimagery layer: accessed October 23, 2007, at http://erg.usgs.

gov/isb/pubs/factsheets/fs20073008/index.html.

U.S. Geological Survey, 2009, National Hydrography Database stewardship handbook: U.S. Geological Survey: 

accessed May 27, 2009, at http://webhosts.cr.usgs.gov/

steward/index.html.
11. Appendixes 57

11. Appendixes

11.1 Hydrologic Unit Names, Historical 

Names, Average Sizes, and Approximate 

Number of Hydrologic Units, Per Nested Level, 

in the United States

The historical names of the general classes of hydrologic 

units are being replaced in the Geographic Name Information System (GNIS) with numerical names that refer to the 

number of digits that indicate the place of a hydrologic unit in 

the nested hierarchy of drainage areas organized by size and 

position. This publication uses the same numerical names for 

hydrologic units that will be used in the GNIS.

Hydrologic unit

name

Historical

name

Average size

(square miles)

Approximate number 

of hydrologic units

2 digit Region 177,560 21 (actual)

4 digit Subregion 16,800 222

6 digit Basin 10,596 370

8 digit Subbasin 700 2,270

10 digit Watershed 227

(40,000–250,000 acres)

20,000 

12 digit Subwatershed 40

(10,000–40,000 acres)

100,000 

14 digit (None) Open Open

16 digit (None) Open Open

11.2 Definitions

The hydrologic (Office of Water Research and Technology, 1980) and compliance terms in the following lists are 

defined with reference to the WBD process. The definitions 

may not be the only valid ones for these terms. Boldfaced 

terms within definitions are defined elsewhere in the lists. 

11.2.1 Hydrologic Definitions

14-digit hydrologic unit A subdivision of a 12-digit hydrologic unit (Subwatershed). 

16-digit hydrologic unit A subdivision of a 14-digit hydrologic unit, and the smallest of the hydrologic unit hierarchy.

Basin A subdivision of Subregion. A Basin is the third-level, 

6-digit unit of the hydrologic unit hierarchy. Basins were 

formerly named “accounting units” in USGS terminology.

Composite hydrologic unit A topographically defined area 

where all the surface drainage converges to a single point, usually along the main stem of a stream between outlets of standard hydrologic units. This includes areas or small triangular 

wedges (remnant areas) between adjacent drainage areas that 

remain after standard hydrologic units are delineated.

Contiguous boundary A hydrologic unit boundary shared in 

whole or in part by an adjacent, different hydrologic unit.

Estuary The region of interaction between streams and 

nearshore ocean waters, where tidal action and streamflow mix 

freshwater and saltwater.

Frontal hydrologic unit A land and water area where surface 

flow originates entirely within the hydrologic unit and drains 

to multiple points along a large waterbody, such as the ocean 

or large lake. 
58 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

Head of land A projection of land extending into a waterbody that interrupts the coastal trend of that waterbody. The 

point where the ridgeline meets the waters’ edge is connected 

to a similar point on the opposite bank of the channel to form a 

boundary across the mouth of an outlet.

Hydrography The scientific description, study, and analysis 

of the physical conditions, boundaries, measurement of flow, 

investigation and control of flow, and related characteristics of 

surface water such as streams, lakes, and oceans. 

Hydrologic unit (HU) An identified area of surface drainage 

within the United States system for cataloging drainage areas, 

which was developed in the mid-1970s under the sponsorship 

of the Water Resources Council and includes drainage-basin 

boundaries, codes, and names. The drainage areas are delineated to nest in a multilevel, hierarchical arrangement. The 

hydrologic unit hierarchical system has four levels and is the 

theoretical basis for further subdivisions that form the Watershed Boundary Dataset fifth and sixth levels. A hydrologic 

unit can accept surface water directly from upstream drainage 

areas and indirectly from associated surface areas, such as 

remnant areas, noncontributing areas, and diversions, to 

form a drainage area with single or multiple outlet points. 

Hydrologic unit code (HUC) The numerical identifier of a 

specific hydrologic unit or drainage area consisting of a twodigit sequence for each specific level within the delineation 

hierarchy.

Hydrologic unit name A standardized name assigned to a 

hydrologic unit used to identify the geographic location of 

the area. Hydrologic units are typically named after significant or prominent water features in an area; however, in some 

instances, they may also be named after other features. 

Hydrology The science dealing with the properties, distribution, and circulation of water on the surface of the land, in the 

soil and underlying rocks, and in the atmosphere.

Karst areas Areas of carbonate-rock formations (limestone 

and dolomite) characterized by sinks, underground streams, 

and caverns. 

Nearshore boundary An offshore closure line for hydrologic 

units, determined by State subject matter experts, based on a 

depth or a distance from the mean high water designation or 

legally defined lake shoreline. (The outermost closure line is 

the NOAA Three Nautical Mile Line required for open-ocean 

hydrologic units.)

Noncontributing area A naturally formed area that does not 

contribute to the downstream accumulation of streamflow 

under normal flow conditions.

Offshore boundary Any boundary in water, including near 

shore, head of land, the NOAA Three Nautical Mile Line, 

and adjustments or generalizations where applying a buffer or 

distance guideline is problematic.

Open-water hydrologic unit An area delineated within an 

ocean. Land is not a major portion of the hydrologic unit, but 

land may be included, as in the case of islands.

Region A Region is the first-level, 2-digit unit and is the 

largest in the hydrologic unit hierarchy. 

Remnant area A topographically defined area that is residual 

after delineation of standard hydrologic units. Remnant 

areas include small triangular wedges between adjacent 

drainage areas. These areas are commonly incorporated into 

composite hydrologic units.

Shoal A natural accumulation of sand, gravel, or other material forming a shallow underwater or exposed embankment.

Sound (A) A relatively narrow sea or stretch of water 

between two close landmasses that connects two larger bodies of water. (B) A deeper part of a moving body of water (as 

bays, estuaries, or straits) through which the main current 

flows or that affords the best passage through an area otherwise too shallow to navigate.

Standard hydrologic unit An area with drainage flowing to a 

single outlet point. Examples include composite and remnant 

hydrologic units.

Subbasin A subdivision of a Basin. A Subbasin is the fourthlevel, 8-digit unit of the hydrologic unit hierarchy. Subbasins 

were formerly named “cataloging unit” in USGS terminology. 

The average size is about 450,000 acres. 

Subregion A subdivision of a Region. A Subregion is the 

second-level, 4-digit unit of the hydrologic unit hierarchy. 

The hydrologic unit category name is retained for the Federal 

Standards and Procedures for the National Watershed 

Boundary Dataset (WBD).

Subwatershed A subdivision of a Watershed. A Subwatershed is the sixth-level, 12-digit unit of the hydrologic unit

hierarchy. Subwatersheds generally range in size from 10,000 

to 40,000 acres. 
11. Appendixes 59

Toe of the shore face A demarcation depth to which seasonal 

storms, prevailing winds, and resultant waves and currents 

move shallow sediments to and from the shore. From this 

geomorphic feature toward the shore, water depth decreases 

rapidly for a short distance and then slowly for the remaining 

distance.

True hydrologic unit See standard hydrologic unit.

Water hydrologic unit A body of water that can receive flow 

from adjacent frontal hydrologic units, composite hydrologic units, and standard hydrologic units. Examples include 

inland lakes and nearshore ocean waters. Generally, these units 

are not delineated to the next lower hydrologic unit level.

Watershed (A) In the hierarchy of hydrologic units, a 

10-digit hydrologic unit (fifth level) also is known as a 

“Watershed,” and it is a subdivision of an 8-digit (fourth level) 

unit, also known as a “Subbasin.” These 10-digit hydrologic 

units range in size from 40,000 to 250,000 acres. (B) The 

hydrologic term “watershed” refers to the divide that separates 

one drainage basin from another or to a combination of hydrologic units of any size.

11.2.2 Geospatial Data Definitions and 

Standards

11.2.2.1 Definitions

Attribute A defined characteristic of a geographic feature or 

entity.

Base classes The points, lines, attributes and related tables 

that are updated during the editing process (as defined by the 

USGS National Geospatial Program Office).

Contour line A line (as on a map) connecting the points on a 

land surface that have the same altitude. 

Coordinates Pairs of numbers expressing horizontal distances along orthogonal axes; alternatively, triplets of numbers 

measuring horizontal and vertical distances.

Coordinate system A system in which points on the Earth’s 

surface are located with reference to a pair of intersecting lines 

or grid. For more information, see Dana (1995a) and Snyder 

(1987).

Compilation The act of composing new or revised materials 

from existing documents or sources.

Crosswalk (Crosswalking) The use of a table to show the 

relationship between elements in other tables.

Dataset A collection of related data.

Datum A reference surface for a geodetic survey. Refers to a 

direction, level, or position from which angles, heights, depths, 

and distances are normally measured. Datum, as applied to a 

horizontal geodetic survey, is a reference based on the shape 

of the Earth. For more information, see Dana (1995b).

Delineation The act of indicating or representing by drawn 

lines.

Derived classes The attributed polygons, organized by 

hydrologic unit level, starting with the 2-digit hydrologic 

units and ending with the 16-digit hydrologic units (as defined 

by the U.S. Geological Survey National Geospatial Program 

Office). The derived classes also include the attributed polygons of the NWIS drainage areas.

Edge matching A digital editing procedure equivalent to 

joining adjacent features at hardcopy map edges; used to 

ensure that features crossing political boundaries or adjoining 

maps connect.

Geographic information system (GIS) A computer system 

designed to collect, manage, manipulate, analyze, and display 

spatially referenced data and associated attributes. 

Geospatial data Information that identifies the geographic 

location (coordinate system) and characteristics (attributes) of 

natural or constructed features and boundaries on the Earth. 

The numerical scale associated with geospatial data refers to 

the spatial accuracy, the smallest scale of delineation, and the 

scale of other data that are spatially compatible. 

Hypsography The study of the distribution of elevations on 

the surface of the Earth with reference to a datum, traditionally 

sea level.

Metadata The description and documentation of the content, 

quality, condition, and other characteristics of geospatial data. 

Polygon A sequence of alternating line segments and angled 

vertices that form a closed two-dimensional loop, thus defining the boundary of an area. 

Resolution The minimum difference between two independently measured or computed values that can be distinguished 

by the measurement or analytical method being considered or 

used.

Topology The spatial relations between geometric entities, 

including adjacency, containment, and proximity. 

Vector data A coordinate-based data structure used to 

represent positional data in spatial units of line, point, and 

polygon.
60 Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)

Vector digitizing The act of tracing a line with a device to 

capture and store the locations of geographic features by converting their map positions to a series of x-y coordinates. 

Watershed Boundary Dataset (WBD) A national geospatial 

database of drainage areas consisting of the 2- through12-digit 

hydrologic units, with optional 14- and 16-digit hydrologic 

units. The WBD includes the required attribute and metadata

information. For more information, see Natural Resources 

Conservation Service (2007).

11.2.2.2 Standards

Federal Geographic Data Committee (FGDC) The FGDC 

develops geospatial data standards for implementing the 

nationwide data publishing effort known as the National Spatial Data Infrastructure (NSDI). For the most recent information, see http://www.fgdc.gov/standards. 

National Map Accuracy Standards (NMAS) Specifications formerly governing the accuracy of topographic, base, 

orthophoto, and other maps produced by Federal agencies. For 

more information, see U.S. Geological Survey (1999a). 

National Standards for Spatial Data Accuracy 

(NSSDA) Specifications superseding the NMAS as the 

standard for governing the accuracy of topographic, base, 

orthophoto, and other maps produced by Federal agencies. The 

WBD is delineated and georeferenced to USGS 1:24,000-, 

1:25,000-, or 1:63,360-scale topographic quadrangle maps that 

meet the most current NSSDA standard. For more information, 

see Federal Geographic Data Committee (1998a,b).

11.2.3 Compliance Definitions

Certification Formal acknowledgment that hydrologic 

units have been reviewed and meet the criteria as stated in the 

“Federal Standards and Procedures for the National Watershed 

Boundary Dataset (WBD),” 2011, as agreed to by member 

agencies of the SSWD.

Provisional Certification Acknowledgment that hydrologic units for large regional areas or whole States have been 

reviewed and meet the criteria as stated in “Federal Standards 

for Delineation of Hydrologic Unit Boundaries; Version 2.0 

October 1, 2004,” as agreed to by member agencies of the 

SSWD. Hydrologic unit boundaries and attributes are edge 

matched to the fullest possible extent at that time. States are 

encouraged to work with neighboring States to resolve boundary issues and resubmit the data to attain final certification.

Prior to April 2006, Provisional Certification was known as 

Certification or Verification.

Full Certification Hydrologic units have been reviewed, and 

meet the National requirement as stated in “Federal Standards for Delineation of Hydrologic Unit Boundaries; Version 

2.0, October 1, 2004,” agreed to by member agencies of the 

SSWD.
11. Appendixes 61

11.3 Abbreviations and Acronyms

ACWI Advisory Committee on Water Information

BLM Bureau of Land Management

DEM Digital Elevation Model

DOQ Digital Orthophoto Quadrangle

DOQQ Digital Orthophoto Quarter Quadrangle

DRG Digital Raster Graphic

EDNA Elevation Derivatives for National Applications

Esri Environmental Systems Research Institute

FGDC Federal Geographic Data Committee

FTP File Transfer Protocol

GIS Geographic Information System

GNIS Geographic Names Information System

HUC Hydrologic unit code

IHUCG Interagency Hydrologic Unit Coordinating Group

IFSAR or 

IfSAR

Interferometric synthetic aperture radar

INEGI Instituto Nacional de Estadística y Geografía

LIDAR or 

LiDAR

Light detecting and ranging

NED National Elevation Dataset

NGMC National Geospatial Management Center 

(formerly National Geospatial Cartographic 

Center (NCGC))

NGTOC National Geospatial Technical Operations Center

NHD National Hydrography Dataset

NMAS National Map Accuracy Standards

NOAA National Oceanic and Atmospheric Administration

NRCS Natural Resources Conservation Service 

(U.S. Department of Agriculture)

NSSDA National Standards for Spatial Data Accuracy

QA/QC Quality assurance/quality control

SSWD Subcommittee on Spatial Water Data

STDS Spatial Data Transfer Standard

USDA U.S. Department of Agriculture

USDA–FS U.S. Department of Agriculture, Forest Service

EPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

WBD Watershed Boundary Dataset

WBD–NTC Watershed Boundary Dataset National Technical 

Coordinators
11.5 Previously Used Metadata Identification Attribute for Tracking Metadata

Attribute Field 

name

Field 

type 

Field 

width Domain Is value required ? Example

Metadata 

identification

MetaID Character 4 AL, AK, AR, AZ, CA, CO, CT, DE, FL, GA, HI, IA, 

ID, IL, IN, KS, KY, LA, MA, MD, ME, MI, MN, MO, MS, MT, NC, ND, NE, NH, NJ, NM, NV, NY, OH, OK, OR, PA, RI, SC, SD, TN, TX, UT, VA, VT, WA, WI, WV, WY, PR,VI, AS, FM, GU, MH, MP, PW, UM;01-99

Yes OK01

11.4 Data Type Crosswalk for Data Storage of Esri Products

SQL standard Arc GIS Esri INFO (coverage)

Esri 

ArcView 

(shape 

file)

Esri ArcSDE 

personal 

(access)

Esri ArcSDE 

file based

Esri ArcSDE enterprise

Oracle MS SQL (server) DB2 

EXACT NUMERIC SHORT 

INTEGER

I Number, 

Boolean

Integer SHORT INTEGER NUMBER (4) SMALLINT (2) SMALLINT 

EXACT NUMERIC LONG 

INTEGER

B,I Number Long integer LONG INTEGER NUMBER (38) INT (4) INTEGER (4

APPROXIMATE 

NUMERIC

FLOAT F,N Number Single FLOAT NUMBER 

(38,8)

REAL DECIMAL 

(31,8)

APPROXIMATE 

NUMERIC

DOUBLE F,I,N Number Double DOUBLE NUMBER 

(38,8)

DOUBLE DECIMAL 

(31,8)

CHARACTER 

STRING

TEXT C String Text TEXT VARCHAR VARCHAR (N) VARCHAR 

DATE TIME DATE D Date Date/time DATE DATE DATETIME TIMESTAMBIT STRING BLOB — — OLE object BLOB BLOB IMAGE BLOB EXACT NUMERIC GUID — — Number GUID CHAR (38) UNIQUE 

IDENTIFIER (16)

CHARACTE

(38)
11. Appendixes 63

11.6 Historical Sample Letter for Certification or Data Update Submittal 

To help the review and certification process, consider including these categories of information about the history and 

development of the dataset in the transmittal letter. Contact the WBD–NTC for questions or assistance.

Date:

Subject: Submittal of the Watershed Boundary Dataset for 

To: Director

USDA Natural Resources Conservation Service

National Geospatial Management Center (NGMC)

501 West Felix Street, Building 23 Fort Worth, TX 76115

From: [Name]

[Title]

[Agency and address]

I am pleased to inform you that [State] has completed its Watershed Boundary Dataset (WBD) in accordance with the 

[standards document used]. This dataset, along with its metadata, has been posted at the following FTP site for retrieval: 

Describe the timeframe, major cooperators and agencies involved in development, and formal agreements in place. For 

example,

“The dataset was created by the coordinated effort between multiple agencies. NRCS provided the oversight and did a 

major portion of the delineations and attribution in [specify] part of the State. Over [timeframe] [other agencies and locations] in [State] as well as [WBD–NTC agent] have, under an agreement between [agencies] and [State], completed the 

remainder of the State’s boundaries and attributes, and appended the dataset together.”

Describe general production methods and reference or source materials used. For example,

“This dataset was produced digitally on screen in ArcView or ArcMap, referencing the Digital Raster Graphics for the 

method of delineation. As the dataset evolved, and especially in the northeastern portion of the State, the National Hydrography Dataset, along with the Digital Ortho Quarter Quads (DOQQs), provided very useful as a reference for determining 

breaks in the flood plains. DOQQs were cited frequently along the [specify major features] as well. Every effort has been 

made to create new boundaries and revise the historical linework to meet the National Standards for Spatial Data Accuracy 

(NSSDA) for 1:24,000 meter scale.”

Describe edge-matching activities and outcomes. Include desired scenario involving States without current WBD. For 

example,

“Edge-matching with [list surrounding States] has been completed. [List States without existing WBD delineation]. 

[Explain desired scenario involving States without current delineation]. It was agreed upon with adjacent States that [your 

State] would delineate to the [special areas] and the bordering States would include the remainder of the [State] Subbasin 

boundaries within those States WBDs.” Itemize other unique areas and circumstances.

Final QA/QC was performed by [WBD–NTC agent] under this latest agreement, as well as [State interagency hydrologic unit group or lead agency].

If there should be any technical questions during the review process, or edits to be made, please feel free to contact me 

(information listed below). Or you may contact [WBD–NTC agent] by [phone] or [email]

Sincerely,

[Name]

[Agency and address]

cc: Please fill in list of major people to notify


USGS and USDA, NRCS—Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)—Techniques and Methods 11–A3