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

U.S. Geological Survey Natural Resources Conservation Service

Techniques and Methods 11–A3

Federal Guidelines, Requirements, and Procedures 

for the National Watershed Boundary Dataset

Chapter 3 of

Section A, Federal Standards

Book 11, Collection and Delineation of Spatial Data
Hydrologic Unit Levels 

Name Hydrologic

unit level Digits Average size(square miles)

Approximate 

number of

hydrologic

units

Region 1 2 177,560 21 (actual)

Subregion 2 4 16,800 222

Basin 3 6 10,596 370

Subbasin 4 8 700 2,200

Watershed 5 10 227 

(40,000-250,000 acres) 22,000 

Subwatershed 6 12 40 

(10,000-40,000 acres) 160,000 
Federal Guidelines, Requirements,

and Procedures for the 

National Watershed Boundary Dataset

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

Natural Resources Conservation Service

Chapter 3 of

Section A, Federal Standards 

Book 11, Collection and Delineation of Spatial Data

Techniques and Methods 11–A3

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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Suggested citation:

U.S. Geological Survey and U.S. Department of Agriculture, Natural Resources Conservation Service, 2009, Federal 

guidelines, requirements, and procedures for the national Watershed Boundary Dataset: U.S. Geological Survey 

Techniques and Methods 11–A3, 55 p.
iii

Contents

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

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

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

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

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

2.1 Federal Geographic Data Committee (FGDC) Subcommittee on Spatial Water Data.........4

2.2 National Steering Committee and Technical Support Team....................................................4

2.3 Intrastate and Interstate Coordination........................................................................................5

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

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

3.2 Hydrologic Boundaries ..................................................................................................................7

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

3.4 Size of Hydrologic Units...............................................................................................................10

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

3.5.1 Classic Hydrologic Units..................................................................................................11

3.5.2 Remnant Areas..................................................................................................................11

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

3.5.4 Reservoirs and Natural Lakes ........................................................................................13

3.5.5 Diverted Waters ................................................................................................................13

3.5.6 Coastal Areas, Including Open-Water, Water, and Frontal Units .............................15

3.5.7 Islands.................................................................................................................................16

3.5.8 Karst Areas ........................................................................................................................16

4. Required Geographic Data Sources and Recommended Techniques for Boundary 

Delineation.......................................................................................................................................18

4.1 Map Scale and Map Accuracy ..................................................................................................18

4.2 Source Maps .................................................................................................................................19

4.2.1 Base Maps.........................................................................................................................19

4.2.2 Hydrologic Unit Maps ......................................................................................................19

4.2.3 Reference Maps ...............................................................................................................19

4.2.4 Digital Data.........................................................................................................................20

4.3 Hydrologic Unit Mapping Techniques .......................................................................................20

4.3.1 Manual Techniques to Delineate Hydrologic Unit Boundaries.................................21

4.3.2 Digital Techniques to Delineate Hydrologic Unit Boundaries...................................21

4.3.3 Spatial Modeling Techniques to Delineate Hydrologic Unit Boundaries................22

4.4 Updating and Revising Hydrologic Units ..................................................................................22

5. Coding and Naming .................................................................................................................................23

5.1 Hydrologic Unit Levels ................................................................................................................23

5.2 Coding Watersheds and Subwatersheds .................................................................................24

5.2.1 Coding Coastal Areas That Contain Frontal Units .......................................................25

5.2.2 Coding Islands...................................................................................................................25
iv

5.3 Watershed and Subwatershed Naming Protocol ...................................................................26

5.3.1 Sources of Watershed and Subwatershed Names ....................................................26

5.3.2 Feature Priority for Watershed and Subwatershed Names ......................................26

5.3.3 Structure for Watershed and Subwatershed Names.................................................26

5.4 International Borders...................................................................................................................28

6. Geospatial Data Structure......................................................................................................................28

6.1 Specifications for Geospatial Data Structure..........................................................................28

6.1.1 Hydrologic Unit Geometry ...............................................................................................29

6.1.2 Valid Topological Relations .............................................................................................29

6.2 Database Schema for Attributing Hydrologic Unit Polygons ................................................29

6.2.1 8-Digit Hydrologic Unit Code...........................................................................................29

6.2.2 10-Digit Hydrologic Unit Code.........................................................................................30

6.2.3 12-Digit Hydrologic Unit Code.........................................................................................30

6.2.4 Subwatershed Area (6th-Level, 12-Digit Hydrologic Unit).........................................30

6.2.5 State or Outlying Area .....................................................................................................31

6.2.6 Noncontributing Area ......................................................................................................31

6.2.7 10-Digit Hydrologic Unit Code of Downstream Watershed .......................................32

6.2.8 Watershed Name (5th-Level, 10-Digit Hydrologic Unit) ............................................32

6.2.9 Watershed Modifications (5th-Level, 10-Digit Hydrologic Unit) ..............................33

6.2.10 Watershed Type (5th-Level, 10-Digit Hydrologic Unit) .............................................35

6.2.11 12-Digit Hydrologic Unit Code of Downstream Subwatershed ..............................36

6.2.12 Subwatershed Name (6th-Level, 12-Digit Hydrologic Unit) ....................................36

6.2.13 Subwatershed Modifications (6th-Level, 12-Digit Hydrologic Unit) ......................37

6.2.14 Subwatershed Type (6th-Level, 12-Digit Hydrologic Unit) .......................................38

6.2.15 Watershed GNIS Feature Identification Number (5th-Level, 10-Digit 

Hydrologic Unit)............................................................................................................39

6.2.16 Subwatershed GNIS Feature Identification Number (6th-Level, 12-Digit 

Hydrologic Unit)............................................................................................................39

6.2.17 Metadata Identification .................................................................................................39

6.2.18 Polygon Attribute Summary ..........................................................................................40

6.3 Database Schema for Attributing Hydrologic Unit Lines .......................................................41

6.3.1 Hydrologic Unit Level .......................................................................................................42

6.3.2 Line Spatial Data Source.................................................................................................42

6.3.3 Metadata Identification ...................................................................................................43

6.3.4 Line Attribute Summary ...................................................................................................44

7. Quality Assurance and Quality Control ................................................................................................45

7.1 Quality Assurance.........................................................................................................................45

7.2 Quality Control (Editing Checklist)..............................................................................................45

7.2.1 Delineation.........................................................................................................................45

7.2.2 Codes ..................................................................................................................................46

7.2.3 Attributes............................................................................................................................46

7.2.4 Data Format........................................................................................................................46

8. Preparing Data for Certification ............................................................................................................46

8.1 State Certification Process .........................................................................................................46

8.2 File Names .....................................................................................................................................48
v

8.3 Map Projection and Horizontal Datum......................................................................................48

8.4 Data Delivery Packaging and Compression.............................................................................48

8.5 Data Transfer Protocol.................................................................................................................48

9. Metadata...................................................................................................................................................48

10. References..............................................................................................................................................49

11. Appendixes .............................................................................................................................................51

11.1 Definitions ....................................................................................................................................51

11.1.1 Hydrologic Definitions....................................................................................................51

11.1.2 Geospatial Data Definitions and Standards ...............................................................52

11.1.3 Compliance Definitions..................................................................................................53

11.2 Abbreviations and Acronyms ...................................................................................................54

11.3 Data Type Cross Walk for Data Storage of ESRI Products ..................................................54

11.4 Data History Letter for Certification Submittal.......................................................................55

Figures

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

2. This Subwatershed boundary meets the National Standards for Spatial Data 

Accuracy and the Watershed Boundary Dataset requirements by its georeference to the minimum 1:24,000 scale topographic base map........................................6

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

successively subdivided, the numbering scheme of the units increases by 

two-digits per level.......................................................................................................................8

4. Delineation of hydrologic features must be based on land surface, surfacewater flow, and hydrographic features.....................................................................................9

5. Tributary outlets are delineated correctly at the bank of the receiving stream ..............10

6. In this example of a hydrologically defined Subbasin (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. Isolated noncontributing areas should be 

delineated ....................................................................................................................................12

8. 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 pre-inundation 

contours of underwater features if such a map is available...............................................14

9. Coastal delineation and coding for areas which cannot be delineated to 

Watershed Boundary Dataset standard size criteria and drain into a water 

hydrologic unit.............................................................................................................................15

10. An island that is too small to be its own hydrologic unit can be grouped 

with another island or group of islands or with a nearby land hydrologic 

unit ................................................................................................................................................17

11. 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 Watersheds and 

Subwatersheds. The main-stem hydrologic unit should carry the higher code 

when outlets are adjacent or break at the same place .......................................................24
vi

12. A major lake can be delineated as a separate hydrologic unit ..........................................25

13. Each hydrologic unit of a subdivided main-stem stream should be named with, 

first, the largest tributary name, then a hyphen, then the main-stem stream name, 

for example, Rock Creek-Imnaha River. If a hydrologic unit is not subdivided, 

then the name of the main stream within that unit can be used ........................................27

14. 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.............................................................................................................34

15. The Watershed Boundary Dataset national certification and review procedure 

and roles ......................................................................................................................................47

Conversion Factors

Multiply By To obtain

Length

mile, nautical (nmi) 1.852 kilometer (km)

Area

square meter 0.0002471 acre

acre 0.4047 hectare (ha)

acre 4,047 square meter (m2

)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2)

Elevation, as used in this report is the height above a specific vertical reference datum.
Federal Guidelines, Requirements, and Procedures for the 

National Watershed Boundary Dataset

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 interagency guidelines for creating the WBD as seamless and hierarchical hydrologic unit 

data, based on topographic and hydrologic features across the 

United States. This document provides guidelines, requirements, and procedures for expanding and revising the previous U.S. Geological Survey (USGS)-Natural Resources 

Conservation Service (NRCS) national data published in 

1994. Expansion is accomplished by creating two additional 

levels of detailed hydrologic unit boundaries nested within 

the 1:250,000-scale hydrologic units, and revision occurs 

through increasing the data resolution to 1:24,000-scale in 

the conterminous United States, 1:25,000 scale in the Caribbean, and 1:63,360 scale in Alaska. The guideline contains 

details for compiling the two additional levels, Watersheds 

(5th-level, 10-digit hydrologic units) and Subwatersheds (6thlevel, 12-digit hydrologic units), to be incorporated into the 

WBD. The guidelines are designed to enable local, regional, 

and national partners to consistently and accurately delineate 

watersheds. 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 numeric codes. Detailed requirements and specifications for data are included. The document also includes 

discussion of objectives, communications required to revise 

the 1:250,000-scale source data, 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 requirements. Up-to-date availability of Watersheds 

(5th-level hydrologic units) and Subwatersheds (6th-level 

hydrologic units) is listed at http://www.ncgc.nrcs.usda.gov/

products/datasets/watershed/

This publication has been updated and clarified from the 

previous NRCS guidance document “Federal Standard for 

Delineation of Hydrologic Unit Boundaries, Version 2.0, 

October 1, 2004,” through contributions of the WBD Technical Support Team, as requested by the Subcommittee on 

Spatial Water Data. The USGS online publication is available 

on the Web at http://pubs.usgs.gov/tm/tm11a3/

1. Introduction

1.1 Purpose

This document establishes interagency guidelines, 

requirements, and procedures to create a national, consistent, 

seamless, and hierarchical hydrologic unit dataset based on 

topographic and hydrologic features across the United States. 

This Watershed Boundary Dataset (WBD) at 1:24,000 scale 

in the conterminous United States, 1:25,000 scale in the 

Caribbean, and 1:63,360 scale in Alaska, consists of digital 

geographic data that include two additional levels of detailed 

hydrologic unit boundaries nested within existing or modified 

1:250,000-scale hydrologic units. This document serves as 

interagency guidance for developing digital geographic data 

for Watersheds (5th-level, 10-digit hydrologic units) and Subwatersheds (6th-level, 12-digit hydrologic units) to be incorporated into the WBD. The guidelines are designed 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 numeric codes by the agencies, 

tribes, and other organizations that develop, manage, archive, 

exchange, and analyze data by hydrologic features. The document will enable a variety of 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.

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 
2 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

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 underestimated for potentially 

invaluable 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 detract from the intent of these 

data to reflect surface-water flow.

When the WBD is completed for the Nation, the distribution of GIS layers and associated maps created by use of 

uniform guidelines and procedures will improve quality, consistency, and accessibility to hydrologic unit data nationwide 

and become the framework for continued updates. Up-to-date 

availability of Watersheds (5th-level, 10-digit hydrologic 

units) and Subwatersheds (6th-level, 12-digit hydrologic units) 

is listed at http://datagateway.nrcs.usda.gov/

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. 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 (currently referred to 

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

“Subbasins”). The hierarchical hydrologic unit code consists 

of 2-digit numbers for each of the 4 nested hydrologic unit 

levels. These 4 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 (1st, 

2d, 3d, and 4th levels) are broadly used; however, the geographic area of units is too large to adequately serve many 

water-resource investigations or resource-analysis and 

management needs. For example, the focus of many waterresource issues is pollutant loading and land-surface processes 

and the cumulative effects of pollution over space and time. 

Management of these issues requires 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 land-management-planning and watershedmanagement programs; various programs in the U.S. Environmental Protection Agency (USEPA), Office of Water; and 

programs in the USGS. 

The NRCS is responsible for working with landowners 

to protect, improve, and sustain natural resources on private 

lands. The NRCS completed mapping of Watersheds (5th-level 

hydrologic units) in the early 1980s 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 5th-level 

Watersheds and 6th-level Subwatersheds using methods and 

procedures that result in data that meet national map accuracy 

standards. To promote a standardized criterion for hydrologic 

unit determination and delineation, the NRCS developed 

National Instruction 170-304 in 1992, which served as the 

agency’s policy for delineating and digitizing Watersheds 

and Subwatersheds. 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 geographic data of Watershed 

and Subwatershed boundaries. 

The U.S. Department of Agriculture, Forest Service 

(USDA-FS) and the Bureau of Land Management (BLM) are 

the primary land-management agencies of Federal lands in 

the United States. Both agencies are delineating and digitizing 

Watersheds (5th-level, 10-digit hydrologic units), and Subwatersheds (6th-level, 12-digit hydrologic units) on public lands. 

The delineation is primarily on public lands in the Western 

States; however, often all land ownerships are delineated 

within a Subbasin (4th-level, 8-digit hydrologic unit). Earlier 

delineations of hydrologic units of federally administered 

public lands served administrative purposes but 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), have coordinated and conducted a series 

of regional workshops to develop a nationally consistent 

hydrologic unit coverage. Member agencies of the SSWD 

have assisted 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 concept lines to adjust to 1:24,000 contour lines where 

needed.
Introduction 3

The BLM and USDA-FS are responsible for managing 

Federal lands, and many states have programs within their 

governments that are responsible for managing state-owned 

land. As keepers of the land, states have partnered with Federal agencies within their state to develop a WBD that meets 

map accuracy requirements and reflects local knowledge of 

surface-water resources. 

State agencies are often the intermediary between local 

and Federal programs for land management. A representative example is North Dakota. The North Dakota Department 

of Health administers the nonpoint-source pollution control 

program. This watershed-based program provides USEPA 

grant monies to local organizations for projects that reduce 

nonpoint-source pollution. The North Dakota Game and Fish 

Department uses the 6th-level 12-digit hydrologic unit codes 

for their “Save our Lakes” program. The Private Lands Section of the Wildlife Division uses the 6th-level codes for landuse inventories for their program. 

The NRCS, USDA-FS, and BLM have worked directly 

with the USGS, along with other Federal and state agencies, 

tribes, and the Subcommittee on Spatial Water Data of the 

FGDC to establish these “Federal Guidelines, Requirements, 

and Procedures for the National Watershed Boundary Dataset,” 2009. This document builds upon the original NRCS 

National Instruction 170–304 and establishes the principles 

and procedures that Federal agencies should follow to delineate further subdivisions of the USGS Subbasins (4th-level, 

8-digit hydrologic units). These guidelines are not agency 

specific; they have been agreed upon by all member agencies 

of the Subcommittee on Spatial Water Data.

This USGS Techniques and Methods publication has 

been updated and clarified from the previous guidance document “Federal Standard for Delineation of Hydrologic Unit 

Boundaries, Version 2.0, October 1, 2004,” through contributions of the WBD national Technical Support Team, as 

requested by the Subcommittee on Spatial Water Data. The 

Technical Support Team has first hand knowledge of the use 

of this document by working with a variety of WBD creators 

from different agencies and programs and conducting regional 

workshops to contribute to coordinated watershed management. This new publication would not have been possible 

without the previous work of the USDA, NRCS, and numerous prior contributors and reviewers.
4 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

2. Coordination

2.1 Federal Geographic Data Committee (FGDC) 

Subcommittee on Spatial Water Data

The Subcommittee on Spatial Water Data was chartered 

and sponsored by the Advisory Committee on Water Information (ACWI) and the FGDC. The Spatial Water Data Subcommittee 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, ground water, 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 Subcommittee on Spatial Water Data.

The Subcommittee on Spatial Water Data 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 

requirements and collection of geographic data to minimize 

duplication of efforts.

2.2 National Steering Committee and Technical 

Support Team

To facilitate development of nationally consistent data, 

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

Federal Steering Committee, led by the NRCS and the USGS. 

The second is the WBD Technical Support Team. 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 

certification. The Steering Committee oversees final review 

and acceptance of digital geographic data into the national 

framework by the Technical Support Team, grants certification, and thus approves the data for public access via the Web.

The USGS Water Resources Discipline facilitates the 

WBD effort by maintaining the Guidelines and by providing delineation support and review to all states. The USDANRCS-National Cartography and Geospatial Center reviews, 

integrates, and merges the individual state certified datasets 

into a seamless national layer and processes the data for delivery on the Geospatial Data Gateway.
Coordination 5

Figure 1. Watershed Boundary Dataset coordination participants and relationships are diagrammed above.

FGDC 

Subcommittee

on Spatial 

Water Data 

Watershed 

Boundary Data 

(WBD) Steering 

Committee

WBD Technical 

Support Team

Interagency 

coordinating 

group 

State 1 

Interagency 

coordinating 

group

State 2

Interagency 

coordinating 

group

State 3

Local

participants

State

participants

Federal

participants

Federal

Geographic

Data

Committee

(FGDC)

FGDC

Subcommittee

FGDC

Subcommittee

2.3 Intrastate and Interstate Coordination

The subdivision of previously delineated hydrologic 

units into Watersheds (5th-level, 10-digit hydrologic units) 

and Subwatersheds (6th-level, 12-digit hydrologic units) is an 

opportunity to develop consistent and commonly used nationwide digital geospatial data for interagency sharing and to 

foster Federal, state, and locally consistent uses of hydrologic 

units. States are encouraged to form an interagency hydrologic unit coordinating group composed of Federal, state, 

local, and watershed agencies with an investment in developing watershed data, specifically including state agencies 

identified as responsible for watershed data by state statute. 

Each participating organization would have the responsibility to seek consensus appropriate to all interests and to 

obtain mutual technical approval. Inviting adjoining states or 

regional entities to participate in the coordinating group would 

be advantageous. The intent of the coordinating group would 

be to promote the development, use, and maintenance of the 

WBD and to explore opportunities to obtain financial support. 

The coordinating group should consider identifying a point of 

contact to coordinate with the USGS/NRCS national Technical 

Support Team.

All states within a Subbasin (4th-level, 8-digit hydrologic 

unit) would need to coordinate their delineation work to make 

the digital geographic data match across state borders. This 

coordination would cover the locations of outlet points, the 

sizes of the Watersheds (5th-level, 10-digit hydrologic units) 

and Subwatersheds (6th-level, 12-digit hydrologic units), and 

the coding sequence within each level of the hydrologic unit 

hierarchy, as well as other attribute fields. Mapping and edgematching hydrologic boundaries that cross county and state 

boundaries would need to be coordinated to ensure a nationally consistent dataset.

The interaction between these groups and data originators 

is shown in figure 1.
6 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

3. Criteria and Considerations for 

Delineating Hydrologic Units

3.1 General Criteria

This section describes criteria for determining and 

delineating Watershed (5th-level, 10-digit) and Subwatershed 

(6th-level, 12-digit) 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 scales of 1:24,000 in the conterminous United States, 1:25,000 in the Caribbean, or 1:63,360 

in Alaska. Selecting and delineating Watersheds and Subwatersheds requires good hydrologic judgment and must be based 

solely upon hydrologic principles to ensure a homogeneous, 

national, seamless digital data layer. When the new hydrologic 

units are verified and aggregated into the 2- through 8- digit 

data, the accuracy of the previous 1:250,000-scale hydrologic 

unit delineation work will be increased to 1:24,000 scale.

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 National Standards for 

Spatial Data Accuracy (NSSDA) (fig. 2). Digital Orthophoto 

Quarter Quadrangles (DOQQs) or higher resolution imagery, 

contours of elevation, stream locations, and other relevant 

data are useful in improving the accuracy or currentness of the 

hydrologic unit boundaries. 

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 certified for the national Watershed Boundary Dataset.

Figure 2. This Subwatershed boundary meets the National Standards for Spatial Data Accuracy and the 

Watershed Boundary Dataset requirements by its georeference to the minimum 1:24,000 scale topographic base 

map. The Appalachian Trail south of Enders, in Dauphin County, Pa., is shown.

Hydrologic unit boundary

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
Criteria and Considerations for Delineating Hydrologic Units 7

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 Subbasins (4th-level, 8-digit hydrologic units) is the 

basis for subdividing Watersheds (5th-level, 10-digit hydrologic units), and Watershed boundaries are the basis for further 

subdividing Subwatersheds (6th-level, 12-digit hydrologic 

units). By delineating Watersheds and Subwatersheds at 

1:24,000 scale in the conterminous United States, 1:25,000 

scale in the Caribbean, or 1:63,360 scale in Alaska, line 

precision and accuracy will propagate throughout the dataset 

including Region boundaries. 

Subbasin boundaries should be changed or adjusted 

when the existing mapping does not conform to hydrologic 

and topographic reality. If changes or revisions to Subbasin 

boundaries are needed in addition to resolution improvements 

required for 1:24,000-scale data (section 4.4), before making 

the change, inform the WBD in-state coordinator (interagency 

hydrologic unit group point of contact) of the major change to 

prompt review by the WBD national Technical Support Team.

Watersheds and Subwatersheds, 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 Watersheds and Subwatersheds. A hydrologic 

unit has a single flow outlet except in frontal, lake, braidedstream, or playa (closed basin) hydrologic units (section 3.5). 

Hydrologic units with their outlet at a delta or braided stream 

should be treated similar to frontal hydrologic units. Give priority to delineating Watersheds and Subwatersheds that will be 

“classic” units having only one outlet (section 3.5.1).Because 

Watersheds and Subwatersheds 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. 

Previous versions of Watershed and Subwatershed 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. Avoid use of 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 on the basis of topography, surface-water flow, and 

hydrologic features.

Terminate hydrologic unit delineations at the international boundary of the United States (section 5.4). For units 

including offshore waters, use the National Oceanic and 

Atmospheric Administration (NOAA) Three Nautical Mile 

Line (section 3.5.6) as the offshore limit. If delineation agreements are reached with Canada or Mexico existing hydrologic 

units may be revised.
8 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

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)

Region

1st level, 

2-digit hydrologic unit

(177,560 square miles 

average)

Subregion

2d level, 

4-digit hydrologic unit

(16,800 square miles 

average)

Basin

3d level, 

6-digit hydrologic unit

(10,596 square miles 

average)

Subbasin

4th level, 

8-digit hydrologic unit

(700 square miles

 average)

Watershed

5th level, 

10-digit hydrologic unit

(227 square miles average,

40,000−250,000 acres)

Subwatershed

6th level, 

12-digit hydrologic unit

(40 square miles average, 

10,000−40,000 acres)

Hydrologic unit boundary

Region

Subregion

Basin

Subbasin

Watershed

Subwatershed

EXPLANATION
Criteria and Considerations for Delineating Hydrologic Units 9

Hydrologically based boundary delineations 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 3.5.6). 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. Avoid delineating 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 that outlet also defines 

the wide tributary outlet, the boundary should connect to the 

downstream bank of the stream (fig. 5). Information from 

stream gages, locations of major highway crossings, and 

National Hydrography Dataset (NHD) reach endpoints may be 

considered to help identify dividing points for hydrologic units 

and some braided-stream networks when no other hydrologic 

features are available. In the case of a long, narrow hydrologic 

unit that is typical of a parallel or trellis drainage pattern, the 

hydrologic unit boundary should cross the stream at a significant confluence to divide the hydrologic unit into suitably 

sized Watersheds (5th-level, 10-digit hydrologic units) or 

Subwatersheds (6th-level, 12-digit hydrologic units) (section 

3.4). Tributary catchment area may be useful for determining 

the relative significance of tributaries within an area being 

subdivided. The hydrologic unit boundary may use smaller 

tributaries as the delineation point to divide the hydrologic 

unit into suitably sized Watersheds (5th-level, 10-digit hydrologic units) or Subwatersheds (6th-level, 12-digit hydrologic 

units). Delineating the boundary just below a confluence 

accommodates nesting of smaller units within the hydrologic 

unit for future site-specific planning, assessment, monitoring, 

or inventory activities. 

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. 

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

A. Incorrect—Subwatershed boundaries follow the river and administrative boundaries. B. Correct— Subwatershed 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.

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

Watershed

Subwatershed

EXPLANATION

A. Incorrect

B. Correct
10 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

Figure 5. The two tributary outlets shown above are delineated correctly 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.

Hydrologic unit boundary

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

3.3 Number of Subdivided Hydrologic Units

As a general rule, subdivide each hydrologic unit into 5 to 

15 units. For example, 5 to 15 Watersheds (5th-level, 10-digit 

hydrologic units) will be nested in each Subbasin (4th-level, 

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 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 to 15 rule, just as 

there are existing Subregions that contain fewer than 5 Basins 

(3rd-level, 6-digit hydrologic units), and Basins that contain 

fewer than 5 Subbasins (4th-level, 8-digit hydrologic units). 

The number of Watersheds (5th-level, 10-digit hydrologic 

units) nested within Subbasins or the number of Subwatersheds (6th-level, 12-digit hydrologic units) nested within 

some Watersheds may occasionally be reduced or expanded to 

accommodate special areas. In some places, it is not possible 

to delineate 5th- or 6th-level hydrologic units owing to the 

lack of hydrologic features or insufficient topography. For 

offshore waters, subdivisions within the 3 nautical mile limit 

are acceptable but not required to meet the 5 to 15 rule.

3.4 Size of 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. 

There should not be hydrologic units that are several orders 

of magnitude different in size from the rest of the hydrologic 

units for a given level. 

Nationally, the typical size for a Watershed is 40,000 

to 250,000 acres. Use this acreage range as a guide in subdividing Subbasins (4th-level, 8-digit hydrologic units). Each 

Watershed (5th-level, 10-digit hydrologic unit) must be completely contained within one 8-digit Subbasin. Statewide data 

should have no more than 10 percent of the polygons outside 

the size requirement range.

The typical size for a Subwatershed (6th-level, 12-digit 

hydrologic unit) is 10,000 to 40,000 acres. In unique areas, the 

Subwatersheds may be greater than 40,000 acres or less than 

10,000 acres but never less than 3,000 acres. Statewide data 

should have no more than 10 percent of the hydrologic units 

outside the size requirement range of greater than 40,000 acres 

or less than 10,000 acres.

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 

to total greater than 10,000 acres. In open-water coastal areas 

subdivision to meet size criteria is not required.
Criteria and Considerations for Delineating Hydrologic Units 11

3.5 Geomorphic Considerations for Hydrologic 

Units

This section explains the most common geomorphic 

circumstances and those that require additional considerations 

for developing hydrologic units. 

3.5.1 Classic Hydrologic Units

The classic hydrologic unit is a hydrologically defined 

surface-water drainage area (fig. 6). All the surface drainage 

within the classic hydrologic unit boundary converges at a 

single outlet point. Larger classic hydrologic units may be subdivided into multiple standard hydrologic units that 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 classic hydrologic units are 

subdivisions of higher level hydrologic units based on major 

tributaries. When choosing between tributaries for hydrologic 

unit delineation, higher order streams are typically chosen 

over smaller tributaries. The downstream end of the hydrologic unit will be just upstream from the confluence with the 

main stem of the higher level hydrologic unit or the main stem 

of a same-level hydrologic unit when possible. 

Hydrologic units 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 classic 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; however, where nonclassic areas such 

as remnant, noncontributing, and diverted areas exist, they 

will need to be delineated by use of criteria described in the 

following sections. These nonclassic areas typically need to be 

added to adjacent hydrologic units but occasionally may have 

to exist as small, atypical hydrologic units.

3.5.2 Remnant Areas

Delineating hydrologic units may result in remnant areas 

around the main stem of larger streams, even when good 

hydrologic judgment and standard practices described above 

are used. Remnant areas typically occur as wedge-shaped 

areas along interfluvial regions between adjacent hydrologic 

units or as overbank areas along a stream between junctions 

with tributaries, and they typically need to be included with an 

adjacent hydrologic unit. These remnant areas are also referred 

to as “related contributing drainage areas” or “composite” 

areas.

Coastal remnant areas occur where outlet areas of several 

mainland or island hydrologic units are individually smaller 

than the typical size of a given hydrologic unit level. If the 

combined area of remnants adjacent to each other is in the 

size range of the hydrologic unit level being defined, they 

should be combined into a single hydrologic unit. Treatment 

of coastal remnant areas that are relatively small is described 

in section 5.2.1.

Figure 6. In this example of a hydrologically defined Subbasin 

(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.

Imnaha

Sheep Creek

River

Creek

Little Sheep

OREGON

N

Classic hydrologic unit

Lower Watershed

Upper 

Watershed

Middle 

Watershed

Upper, middle, lower 

 hydrologic unit

Hydrologic unit boundary

Subregion

Subbasin

Watershed

EXPLANATION

(Basin-level boundary 

not represented in this 

example)

0 4 8 12 16 MILES

0 8 16 KILOMETERS
12 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

3.5.3 Noncontributing Areas

Drainage areas that do not 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 

a hydrologic unit at any level of the hierarchy if within the 

size range for a given level. The largest noncontributing area 

in the United States is the Great Basin, which covers parts of 

Nevada, Utah, Oregon, and California. This closed basin is 

large enough to be classified as a Region (1st-level, 2-digit 

hydrologic unit). 

Semiconfined basins that contribute surface water to 

another area in wet years but act as a sink in dry years may 

be a hydrologic unit or a noncontributing area. 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, or if they are scattered throughout a drainage area, they should be considered as 

part of the encompassing delineated hydrologic unit. With the 

diversity of unique and unusual landforms across the country, 

the criteria for delineating noncontributing areas caused by 

these unusual land features cannot be fully covered in this 

section. 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 significant 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 throughout the state.

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.

0 4 8 MILES

NEW MEXICO

0 4 8 KILOMETERS

Noncontributing area

Hydrologic unit boundary

Subregion

Subbasin

Watershed

Subwatershed

EXPLANATION

(Basin-level boundary 

not represented in this 

example)

N
Criteria and Considerations for Delineating Hydrologic Units 13

3.5.4 Reservoirs and Natural Lakes

Delineating hydrologic unit boundaries is often complicated by the presence of reservoirs or large natural lakes. 

Reservoirs and lakes obscure the natural drainage system, 

which makes delineating hydrologic units more difficult. The 

way the pool area and its underlying drainage pattern affects 

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: 

1. If the uninterrupted natural drainage network would 

form a unit larger than one of the typical size at that level, 

consider subdividing the hydrologic unit at a dam or natural 

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

hydrologic units and adjacent slope areas.

2. Avoid delineating boundaries to the reservoir’s normal, 

average, or high pool. To be sure all of a reservoir’s pool area 

is accounted for, subdivide the hydrologic units to the legacy 

channel system underlying the pool area (fig. 8).

3. 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 classic hydrologic units 

with an outlet at the lake edge. Areas that drain into a lake but 

cannot be delineated at a consistent size should be included as 

part of the lake hydrologic unit. 

4. Permanent, large-scale water bodies, such as lakes 

and reservoirs with historically documented permanent pools, 

may require exceptions to these priorities. Some states have 

completed hydrologic unit delineations without using legacy 

channels. Where the boundaries are recognized within state 

law and a legal lake elevation has been established, the delineation should reflect as closely as possible the legal lake elevation. The lake is delineated as a hydrologic unit, and adjacent 

slope areas not included in classic units are delineated as 

frontal hydrologic units. These frontal hydrologic units should 

be subdivided, when necessary, on a hydrographic basis and 

should be consistent in size. Lakes or reservoirs delineated as 

hydrologic units should be listed with their legal lake elevation 

in the detailed overview section of the metadata. An example 

of an all-water hydrologic unit is Great Salt Lake in Utah.

3.5.5 Diverted Waters

Delineate hydrologic units on the basis of natural surfacewater flow and natural topographic land features. Ditches and 

canals should be used to determine surface-water drainage 

areas only if 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 

HU_10_MOD and/or HU_12_MOD field (sections 6.2.9 and 

6.2.13). 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. 

A dam, diversion, or stream confluence may be used to 

divide a hydrologic unit into upper and lower parts. Avoid 

adjusting the location of hydrologic units because of interconnected flow from one hydrologic unit to another during high 

flow stages in streams.
14 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

Figure 8. 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 pre-inundation contours of underwater 

features if such a map is available.

Boundary compiled using 

pre-inundation data currently 

hidden by the reservoir

NEVADA

ARIZONA

Mead Lake

N E VA D A

A R I Z O N A

Hydrologic unit boundary

Region

Subregion

Subbasin

Watershed

EXPLANATION

(Basin-level boundary not 

represented in this example)

0 0.5 1 MILE

0 0.5 1 KILOMETER

SCALE 1: 48,000
39370

25648

22717

22683

22240

19682

15138

12750

13675

12173

7169

3340

1324

649

09

11

03

04

06

02

07 10

05

08

01

01

01

08

Watershed

0406010702

Combined acres = 12822

Combined acres = 11833

M I C H I G A N

0 3 6 9 12 MILES

0 3 6 9 12 KILOMETERS

N

Hydrologic unit boundary

Hydrologic unit code

Subregion

Subbasin

Watershed

Acres

Subwatershed

Watershed

Subwatershed

EXPLANATION

0406010702

040601070208

11833

(Basin-level boundary not 

represented in this example)

Milacokia

 Lake

Millcoquins

 Lake

 McDonald

 Lake

Gulliver

 Lake

L a k e M i c h i g a n

Criteria and Considerations for Delineating Hydrologic Units 15

3.5.6 Coastal Areas, Including Open-Water, 

Water, and Frontal Units

The coastal criteria apply to ocean coastal areas, nonocean coastal areas such as the Great Lakes, and large tidal 

rivers such as parts of the Mississippi, Columbia, and Potomac 

Rivers. The coastal guidelines provide a scientific basis to 

extend gravity-based flow through hydrographically complex 

coastal areas. Coastal hydrologic units drain either to an outlet 

(as in inland hydrologic units) or to water hydrologic units. 

The delineation of hydrologic unit boundaries is often 

complicated in coastal areas by the presence of large estuaries, 

bays, or sounds. Because water levels in coastal water bodies 

can fluctuate significantly, hydrologic-unit mapping should 

be based on submerged morphologic features such as shoals, 

ridges, shore faces, and flow channels (legacy channels).

NOAA nautical charts or local marine bathymetric data are 

available for many coastal areas. Help obtaining bathymetric 

data is available through your WBD Technical Coordinators. 

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 

open-water hydrologic units. 

When delineating from bathymetric information, use 

the best available depth data for closure in estuaries, bays, 

and sounds as agreed upon by state partners. For open-water 

hydrologic units use the 30-foot depth contour or the NOAA 

Three Nautical Mile Line. Delineations must transition 

smoothly between physical features and edge match at state 

boundaries. Where bathymetry is not available, use the best 

available coastline data. If the DRG coastline is used and the 

hydrologic unit does not contain offshore waters it is considered a frontal unit.

For a coastal body of water that is of a size consistent 

with the hydrologic unit level being delineated, it is appropriate to delineate the waterbody as a separate water hydrologic 

unit. After delineating the coastal and classic Watersheds (5thlevel, 10-digit hydrologic units) and Subwatersheds (6th-level, 

12-digit hydrologic units) within the designated size criteria 

(section 3.4), there may be areas remaining that contain 

streams that are too small to break out as individual units 

(section 3.5.2). These remnant areas should be combined into 

a unit that meets the outlined size criteria for a Watershed or 

Subwatershed (fig. 9). These composite areas are called frontal 

units (a unit with more than one outlet).

Figure 9. The above example depicts coastal delineation and coding for areas which cannot 

be delineated to Watershed Boundary Dataset standard size criteria and drain into a water 

hydrologic unit (in this case, in coastal Michigan). Small frontal units are grouped together to 

meet the size criteria. Coding is explained in section 5.2.1.
16 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

3.5.7 Islands

Delineate island hydrologic units on the basis of 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. 

Subdivide island hydrologic units to be consistent with unit 

size criteria. An island located between water hydrologic units 

or between a water hydrologic unit and the ocean is bisected 

to indicate surface water flows to the adjacent hydrologic 

units. This technique applies if there is a single barrier island 

or a string of islands along a shoal or reef that 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. An island too small to 

be its own hydrologic unit can be grouped with a nearby land 

unit or with another island or group of islands in the vicinity, 

such as a barrier island or those shown in figure 10.

3.5.8 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 on the basis of 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, ground-water and 

surface-water interaction is relatively direct and rapid compared to that in nonkarst areas. The conduit networks transport 

most of the storm-water 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 are not expressed on topographic maps in areas with soluble rocks such as limestone, 

dolomite, gypsum, or salt near the land surface, 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 hydrologic-unit modifications 

fields HU_10_MOD and/or HU_12_MOD (section 6.2.9 and 

6.2.13). 

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

in metadata (section 9.0).
Criteria and Considerations for Delineating Hydrologic Units 17

Key Peninsula

McNeil Island

Shaw Island

Stuart Is

Waldron 

Island

San Juan Island

Blakely 

Island

Lopez Island

Orcas Island

Fox Island

Tacoma

C a r r I n l e t

R

o s a r i o S t r a i t

A

B

27136

4301

24460

3202

177

W A S H I N G T O N

N

N

0

0 2 4 6 8 KILOMETERS

2 4 6 8 MILES

0 2 6

0 2 4 6 8 KILOMETERS

4 8 MILES

Hydrologic unit boundary

Subwatershed

Combined 

hydrologic unit

Acres

EXPLANATION

3202

Figure 10. An island that 

is too small to be its own 

hydrologic unit can be 

grouped (A) with another 

island or group of islands 

or (B), with a nearby land 

hydrologic unit as shown 

in this example of island 

delineations in the Puget 

Sound, Wash. McNeil 

Island is grouped with the 

east side of Key Peninsula, 

whereas Fox Island is 

grouped with the mainland 

hydrologic unit east of the 

Carr Inlet.
18 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

4. Required Geographic Data Sources 

and Recommended Techniques for 

Boundary Delineation

There are three recommended techniques for delineating 

hydrologic unit boundaries.

The first technique is to interpret the hypsographic and 

topographic information found on 1:24,000-scale paper or 

mylar source materials and manually draw boundary lines 

based on the interpretation of those materials. The manually 

drawn lines are subsequently digitized using scanning and 

vectorization or tablet digitizing.

The second technique combines delineation and digitization of hydrologic unit boundaries on a computer display using 

GIS software. Instead of interpreting the hypsographic and 

topographic information found on paper or mylar sources as 

used in the first 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 include1:24,000-scale Digital Raster Graphics (DRG) 

and orthoimagery and orthophotographs such as Digital 

Orthophoto Quadrangles (DOQ) or Quarter-Quadrangles 

(DOQQ).

The third technique is to generate intermediate or “draft” 

boundary lines using spatial modeling software. Typical 

GIS software has functions for deriving drainage areas from 

elevation data, such as the National Elevation Dataset (NED) 

and supplemental hydrographic and physiographic data. The 

intermediate boundaries are then verified and refined using 

the second technique. WBD tools are available from NRCS 

(http://www.ncgc.nrcs.usda.gov/products/datasets/watershed/

tools.html) to support delineation using the third technique.

4.1 Map Scale and Map Accuracy

Delineations need to meet 1:24,000-scale map accuracy standards in the conterminous U.S. (1:25,000-scale in 

the Caribbean or 1:63,360-scale in Alaska), described in the 

National Standards for Spatial Data Accuracy (NSSDA) 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 are 

used to generate the hydrologic unit boundaries, they must 

be provided to the WBD national Technical Support Team 

for verification and review. If there is a barrier to providing 

the source maps, document the source in detail. Digital data 

sources include the NRCS 1:24,000-scale County Mosaic 

DRGs, Digital Orthophoto Quarter Quadrangles (DOQQs), 

and similar high-resolution digital orthophotography showing 

topography and surface-water features at scales of 1:24,000 

in the conterminous United States, 1:25,000 in the Caribbean, 

1:63,360 in Alaska, or larger scales. 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 National Elevation Dataset (NED) (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 thorough verification and adjustments 

to 1:24,000-scale or better topographic data such as the NRCS 

1:24,000-scale County Mosaic DRGs. 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 when 1:24,000-scale topographic maps are not available.

Subbasins (4th-level, 8-digit hydrologic units) as well 

as Watersheds (5th-level, 10-digit hydrologic units) and 

Subwatersheds (6th-level, 12-digit hydrologic units) must be 

delineated from and georeferenced to a minimum horizontal 

accuracy of 1:24,000-scale geographic data to meet NSSDA. 

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 can be found 

in Federal Geographic Data Committee (1998b).
Required Geographic Data Sources and Recommended Techniques for Boundary Delineation 19

4.2 Source Maps

This section describes the types of source maps, in both 

hardcopy and digital formats, 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 and in the LINESOURCE field (section 6.3.2). 

4.2.1 Base Maps

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

County Mosaic DRGs 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 draft (concept) lines. Other data sources and 

map products can be used in conjunction with the digital, 

1:24,000-scale DRGs or 1:24,000-scale topographic quadrangles to facilitate the interpretation of hydrologic unit boundaries. (See following sections.) Documentation of all supplemental base and source maps, digital and hardcopy should be 

recorded in the LINESOURCE field (section 6.3.2) as well as 

in the general metadata (section 9.0).

4.2.2 Hydrologic Unit Maps

Existing smaller scale hydrologic unit maps provide a 

framework for delineating Watershed (5th-level, 10 digit) 

and Subwatershed (6th-level, 12 digit) boundaries. USGS 

State Hydrologic Unit Maps at 1:500,000-scale, National 

Atlas Hydrologic Unit Boundary data and online maps at 

1:2,000,000, or the Hydrologic Units of the U.S. data at 

1:250,000-scale are useful to determine the general location 

of the new or redefined hydrologic units. The publication 

“Hydrologic Unit Maps” (Seaber, 1987) documents the existing codes and names. The digital publication is available at 

http://pubs.er.usgs.gov/usgspubs/wsp/wsp2294.

Other hydrologic unit maps produced by NRCS, USDAFS, and state and local entities may be used to determine the 

general location and level of complexity of Watersheds (5thlevel, 10-digit hydrologic units) and Subwatersheds (6th-level, 

12-digit hydrologic units). If other hydrologic unit maps are 

used as sources for compiling Watersheds (5th-level, 10-digit 

hydrologic units) or Subwatersheds (6th-level, 12-digit hydrologic units), be sure to record the title, scale, and accuracy of 

the dataset or map series; refer to section 6.3.2.

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 to determine flow 

patterns based on topography. These supplemental reference 

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

delineations based on permanent features on the landscape and 

(b) 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

Retain as a permanent record all maps, measurement 

data, and other supplemental reference data/maps used in 

delineations (section 6.3.2).
20 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

4.2.4 Digital Data

NRCS 1:24,000-scale County Mosaic DRGs are the 

preferred on-screen digital reference layer for initially delineating and digitizing hydrologic unit boundaries. These DRGs 

are also the preferred layer for clarifying and modifying draft 

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

http://datagateway.nrcs.usda.gov.

The 30-meter and 10-meter National Elevation Dataset 

(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 National Hydrography Dataset (NHD) 

(described in the following paragraph), as well as with local 

reference layers, to clarify challenging delineations. More 

specific guidance regarding spatial modeling techniques to 

generate hydrologic unit boundaries can be found in Jenson 

and others (1987). Both 30- and 10-meter NED data can be 

obtained from http://ned.usgs.gov/.

The 1:24,000-scale or better resolution NHD 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 are available at http://nhd.usgs.gov/. 

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 (section 5.1; 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 3.5). 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 the delineation, mapping, and digitizing differ among 

these options, but all can produce boundaries that meet the 

conterminous U.S. 1:24,000-scale, Caribbean 1:25,000-scale, 

or Alaska 1:63,360-scale NSSDA. 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 Watersheds and Subwatersheds and the recommended distribution of Watersheds and Subwatersheds within 

a Subbasin.
Required Geographic Data Sources and Recommended Techniques for Boundary Delineation 21

4.3.1 Manual Techniques to Delineate 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 in conjunction with the USGS 

1:24,000-scale hardcopy topographic maps to facilitate interpretation of hydrologic units.

The following technique is recommended to manually 

delineate hydrologic unit boundaries:

1. Where hydrologic units have not been previously compiled on USGS 1:24,000-scale topographic quadrangles, 

it may be useful to transfer the 1:250,000-scale Subbasin 

boundaries (4th-level, 8-digit hydrologic units) onto the 

USGS 1:100,000-scale topographic quadrangles. 

2. If the location or position of a Subbasin boundary is found 

to be questionable or require major reinterpretation, notify 

the WBD in-state coordinator (interagency hydrologic 

unit group point of contact), who will then notify the 

WBD national Technical Support Team. 

3. If the Subbasin boundaries were transferred to 1:100,000-

scale USGS topographic quadrangles, repeat the same 

process using 1:24,000-scale USGS topographic quadrangles. 

4. Once the Subbasin boundaries are successfully transferred 

on to 1:24,000-scale topographic quadrangles, begin to 

subdivide the Watersheds (5th-level, 10-digit hydrologic 

units) and the Subwatersheds (6th-level, 12-digit hydrologic units).

5. Once all subdivisions of the Subbasins have been compiled on a 1:24,000-scale topographic quadrangle, digitize 

boundaries and codes directly from the 1:24,000-scale 

maps.

4.3.2 Digital Techniques to Delineate Hydrologic 

Unit Boundaries

When delineating and digitizing hydrologic units on 

the computer screen (for example at 1:24,000-scale), use the 

NRCS 1:24,000-scale County Mosaic DRG contours, elevations, and drainage patterns at a minimum to interpret and 

delineate the hydrologic unit boundaries. In special cases–for 

example, complex or flat topography–the scale of digitizing may decrease depending on the resolution of the base or 

source data being used. 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 5th-level, 10-digit Watersheds and 6th-level, 

12-digit Subwatersheds at a scale of approximately 1:7,500. 

Before a Subbasin’s delineation is complete, the perimeter 

boundary associated with the 4th-level, 8-digit Subbasin must 

also be reviewed and updated to meet NSSDA 1:24,000-scale 

map accuracy standards.

The following technique is recommended to delineate 

hydrologic unit boundaries on the computer display using GIS 

software:

1. Develop a reference plot for each Subbasin (4th-level, 

8-digit hydrologic unit) showing the 1:250,000-scale 

hydrologic units and a stream network (preferably the 

1:100,000-scale, 1:24,000-scale, or higher resolution 

NHD) along with other supplemental base information 

such as shaded relief, DEM data, and/or orthophotos 

(section 4.2).

2. Use the plots as guides to develop the boundaries, codes, 

and names of the Watersheds (5th-level, 10-digit hydrologic units) as they are digitized on the computer display. 

Terminate classic hydrologic units at the outlet of the 

hydrologic feature.

3. Once the Watershed (5th-level, 10-digit hydrologic units) 

boundaries have been delineated and units coded for a 

Subbasin, generate another reference plot showing the 

digitized and coded Watersheds, along with supplemental 

base information (section 5.2).

4. Repeat Step 2 to generate the boundaries and codes for 

the Subwatersheds (6th-level, 12-digit hydrologic unit) 

within the Subbasin.
22 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

4.3.3 Spatial Modeling Techniques to Delineate 

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 and hydrography using GIS to simulate drainage 

networks, stream courses, and direction of flow by applying 

hydrologic models. Maps generated from digital hydrography 

data with flow-direction arrows also are helpful in delineating 

hydrologic units. 

Depending upon the spatial modeling technique used, 

data resolution, data consistency, software applications, 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, DOQs, 

1:24,000-scale NHD, or larger scale data. Adjustments to 

boundaries and confluences at all levels will be required to 

ensure 1:24,000-scale accuracy in the conterminous United 

States, 1:25,000-scale accuracy in the Caribbean, or 1:63,360-

scale accuracy in Alaska. 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.

4.4 Updating and Revising Hydrologic Units

Priority should be to resolve Subbasin boundaries (4thlevel, 8-digit hydrologic units) with significant hydrologic 

inaccuracy. Minor related revisions of existing Subbasin 

boundaries are to be expected to place the boundaries accurately on 1:24,000- 1:25,000- or 1:63,360-scale maps. These 

are not considered major revisions and do not require review 

by the WBD national Technical Support Team.

When hydrologic unit boundaries at 1:250,000 or smaller 

scales are compared to 1:24,000-scale delineations, two categories of inconsistency may be observed. (1) Minor inconsistencies and improvements in detail due to scale differences 

can be incorporated into the new state data. (2) Substantial 

differences between the1:24,000-scale and previous smaller 

scale delineations may prove the smaller-scale boundaries 

hydrologically inaccurate at the 1:24,000 scale. If substantial 

differences are identified, refer to guidance that follows.

The Subbasin boundaries (4th-level, 8-digit hydrologic 

units) have been used and referenced so extensively in waterresource activities nationwide that major changes to them 

should be made only in cases of major delineation error or 

significant 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, construction of 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 a Subbasin. For 

example, major revisions include those that place entire stream 

reaches (not small pieces of headwater reaches) in different 

Subbasins (4th-level, 8-digit hydrologic units) or those that 

recode contiguous areas approximating or exceeding the size 

of Watersheds (6th-level, 12-digit hydrologic units of 10,000 

to 40,000 acres). 

If the locations of 8-digit boundaries need to be changed 

to be hydrologically correct, notify the USGS/NRCS national 

Technical Support Team through the WBD in-state coordinator. Obtain assurance from the Technical Support Team that 

proposed changes are acceptable before making the revision 

to Subbasins. When hydrologic unit boundaries have major 

revisions, update the area measurements, and note significant 

revisions as “revised” when the new data are released. Keep a 

record of all changes to the Subbasins in your office.
Coding and Naming 23

5. Coding and Naming

5.1 Hydrologic Unit Levels 

The six different levels of hydrologic units and their characteristics are shown below and in figure 3.

Name Hydrologic

unit level Digits Average size(square miles)

Approximate 

number of

hydrologic

units

Region 1 2 177,560 21 (actual)

Subregion 2 4 16,800 222

Basin 3 6 10,596 370

Subbasin 4 8 700 2,200

Watershed 5 10 227 

(40,000-250,000 acres) 22,000 

Subwatershed 6 12 40 

(10,000-40,000 acres) 160,000 
24 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

5.2 Coding Watersheds and Subwatersheds

This section provides guidance on coding Watersheds 

(5th-level, 10-digit hydrologic units) and Subwatersheds (6thlevel 12-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 Subbasin 

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 a Subbasin, the WBD in-state coordinator should 

notify the WBD national Technical Support Team before making revisions. Assign a new unique numbered code to each 

Watershed and Subwatershed. Maintain the 2-digit field length 

for successive hydrologic units. Coordinate the coding within 

a Subbasin (4th-level, 8-digit hydrologic unit) across state 

boundaries. 

Avoid complicating the coding of hydrologic units by 

assigning codes based on location within a county or multiple 

counties or states. Number the hydrologic units sequentially, 

beginning upstream at the hydrologic unit with the uppermost 

outlet and proceed downstream. Downstream codes are in 

ascending order (fig. 11) 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 Watershed 

as 0908020301, code the next Watershed downstream as 

0908020302, and so forth. No numbers are skipped. The mainstem hydrologic unit is assigned the highest number when 

outlets are adjacent, or break at the same place, as shown in 

figure 11 (Subwatersheds 170601020104 and 170601020105).

Figure 11. 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 Watersheds and Subwatersheds. The 

main-stem hydrologic unit should carry the higher code when outlets are adjacent or break at the same place. Subwatershed codes

are shown for the Upper Imnaha River Watershed.

OREGON

04

02

06

01 05

03

Hydrologic unit boundary

Hydrologic unit code

Subbasin

Watershed

Watershed

Subwatershed

Subregion

Subwatershed

Direction of flow

Upper 

Imnaha River

Watershed

1706010205

1706010202

1706010201

1706010204

1706010203

1706010201

170601020104

EXPLANATION

(Basin-level boundary not 

represented in this example)

N 0 2 4 6 8 MILES

0 2 4 6 8 KILOMETERS

R i v e r

I m n a h a

Subbasin

17060102
Coding and Naming 25

If there is no scientific justification to subdivide a hydrologic unit, then code the hydrologic unit 00. For example, if a 

Watershed (5th-level, 10-digit hydrologic unit) (for example, 

1020305118) cannot be subdivided on the basis of hydrologic principles, then the Subwatershed (6th-level, 12-digit 

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

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

next higher level (fig.12).

A sample numbering of hydrologic units:

 Level

First 2 are the Region fields 01 1

Next 2 are the Subregion fields 0108 2

Next 2 are the Basin fields 010802 3

Next 2 are the Subbasin fields 01080204 4

Next 2 are the Watershed fields 0108020401 5

Next 2 are the Subwatershed fields 010802040101 6

5.2.1 Coding Coastal Areas That Contain Frontal 

Units

This section provides coding rules for frontal hydrologic 

units. Frontal hydrologic units are areas along coasts where 

several classic watersheds flowing into the water hydrologic 

unit are separated by independent remnant areas. Frontal 

hydrologic units that do not meet the size criteria should be 

grouped together to make larger units (fig. 9). When aggregating these frontal hydrologic units, group within levels that are 

flowing into the same hydrologic feature; for example, bay, 

lake, estuary.

These units will have the same codes and names as the 

other frontal hydrologic units that make up the overall unit. 

Code these coastal areas in a clockwise sequence from north 

or east, depending on the orientation of the hydrologic unit 

(fig. 9).

5.2.2 Coding Islands

Islands large enough to be subdivided should be coded in 

the same pattern for classic and frontal-unit coding, in a clockwise sequence from north or east, depending on the orientation 

of the island.

Island groups containing several islands of the required 

size for the given level should be coded individually in a 

clockwise sequence from north or east, depending on the orientation of the islands.

If an island is too small to be its own unit at any given 

level, it should be grouped with the closest island or land 

hydrologic unit and given the same code and name as the 

larger unit (fig. 10). 

Figure 12. A major lake can be delineated as a separate 

hydrologic unit. Salt Lake and Antelope Island, Utah, are 

Watersheds where there is no scientific justification for 

subdividing the areas into Subwatersheds. The addition of the ‘00’ 

to the hydrologic unit code indicates that these Watersheds were 

not subdivided.

Salt Lake

Subbasin

16020310

Hydrologic Unit

160203100200

Hydrologic Unit

160203100100

0 10 20 MILES

UTAH

0 10 20 KILOMETERS

Hydrologic unit boundary

Subregion

Basin

Subbasin

Watershed

EXPLANATION

N
26 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

5.3 Watershed and Subwatershed Naming 

Protocol

The numeric hydrologic unit code is the primary unique 

identifier for each hydrologic unit; however, the numeric 

identifier alone makes it difficult to relate a hydrologic unit 

to a geographic location. Therefore, naming the Watersheds 

and Subwatersheds 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 significant or prominent water features 

in an area; however, if there are no water features in an area, 

hydrologic units may be named after prominent physical 

features. 

Identify each Watershed and Subwatershed 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 Watersheds (5thlevel, 10-digit hydrologic units) and Subwatersheds (6th-level, 

12-digit hydrologic units). Do not change the names of the 

1st-, 2d-, 3d-, and 4th-level hydrologic units 

5.3.1 Sources of Watershed and Subwatershed 

Names

Watersheds and Subwatersheds should be named using 

feature names officially recognized by the U.S. Board on 

Geographic Names. Options are contained in the Geographic 

Names Information System (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, gaging stations, and political or administrative units.

5.3.2 Feature Priority for Watershed and 

Subwatershed Names

Assign each Watershed and Subwatershed a name from 

the list of federally recognized names in this order of priority, 

advancing to the next level if none of that feature type exists 

or is named: 

First–HYDROLOGIC FEATURE (Examples include rivers, 

lakes, dams, wells, 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, airports, and schools)

If no water or prominent physical feature name is 

assigned, the hydrologic unit code is used as a placeholder.

Exceptions may be made to this priority order on the 

basis of feature significance. For example, a canyon may 

be more significant than a hydrologic feature, such as an 

upstream spring. This frequently occurs in arid areas.

5.3.3 Structure for Watershed and Subwatershed 

Names

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 in its entirety, 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, wells, and springs may be used when 

they are the most important feature; for example, “Cresent 

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 that the unit drains into. 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 this 

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. 13). 
Coding and Naming 27

•	 When a major stream is subdivided into multiple 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.” 

•	 Avoid using Upper, Middle, or Lower individually or 

Middle in combination with either 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.

•	 Upper and Lower or Headwaters and Outlet may be used 

in pairs when a stream is subdivided into two hydrologic 

units. 

•	 The words Headwaters and Outlet may be used with the 

hyphenated naming structure or the Upper, Middle, and 

Lower naming structure.

• When bathymetry was used to delineate to 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 code-primary water feature” 

for the name. For example, 030102051706-Atlantic Ocean.

• Islands

•	 When islands are large enough to be subdivided, use 

the standard naming convention for classic and frontal 

hydrologic units.

•	 Islands large enough to be their own unit use the island 

name.

•	 Hydrologic Units that are made up of a group of islands 

use the name of the major island within that group 

(fig. 10). 

Figure 13. Each hydrologic unit of a subdivided main-stem stream should be named with, first, the largest tributary name, then a 

hyphen, then the main-stem stream name, for example, Rock Creek-Imnaha River. If a hydrologic unit is not subdivided, then the name 

of the main stream within that unit can be used. Further naming criteria are given in section 5.3. Subwatershed names for the Upper 

Imnaha River Watershed in the Imnaha Subbasin are shown.

Hydrologic unit boundary

Hydrologic unit name

Subbasin

Watershed

Subregion

Subwatershed

Direction of flow

EXPLANATION

Rock Creek-Imnaha River

(Basin-level boundary not 

represented in this example)

N

0 2 4 6 8 MILES

0 2 4 6 8 KILOMETERS

Upper 

Imnaha River

Watershed

Middle 

Imnaha River

Watershed

Little Sheep

Creek

Watershed

Lower 

Imnaha River

Watershed Big Sheep

Creek

Watershed

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

Gumboot Creek

South Fork Imnaha River

North Fork

Imnaha River

Crazyman Creek-Imnaha River

Rock Creek-Imnaha River

Imnaha Subbasin
28 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

5.4 International Borders

Avoid delineating or coding across international boundaries. Hydrographic mapping across borders with Canada and 

Mexico are addressed at the Federal level. When the headwaters of a hydrologic unit fall in a neighboring nation, and sufficient data and base maps are readily available, delineate and 

attribute using the current WBD guidance. When Watersheds 

and/or Subwatersheds are bisected by an international boundary, be sure to include the downstream code (HU_10 _DS, 

HU_12 _DS) shown in section 6.2.7 and 6.2.11. If data are not 

readily available, estimates may be necessary to maintain a 

basic level of logical continuity within the coding and delineation structure. Delineations or attributes along international 

borders may require future adjustment.

6. Geospatial Data Structure

6.1 Specifications for Geospatial Data Structure

The structure of the Watershed Boundary Dataset is based 

on well known Geographic Information Systems 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.

Hydrologic unit lines and polygons have well-known 

spatial relations traditionally referred to as “topology.” Topological relations between hydrologic unit lines and polygons 

must be consistent and valid to support the national review 

and certification process and to allow watershed boundary data 

to be integrated in a nationally consistent, seamless geospatial 

dataset.

Each hydrologic unit line and polygon has associated 

attributes stored in tabular format. An attribute, also called 

a field or column, is defined by name, type, size, and valid 

values (or range of valid values). A collection of attributes is 

called a schema. Schemas for hydrologic unit lines and polygons must also be consistent to support the national review 

and certification process and to allow watershed boundary data 

to be integrated in a nationally consistent, seamless geospatial 

dataset.

Several geodatabase tools that may help populate the 

attribute tables, including a semiautomated method to populate the attribute “GNIS feature identification number,” can 

be found at http://www.ncgc.nrcs.usda.gov/products/datasets/

watershed.tools.html.
Geospatial Data Structure 29

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 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 ESRI Coverage 

format, and do not necessarily apply to other formats such as 

ESRI Geodatabase format or ESRI Shapefile format. 

6.1.2 Valid Topological Relations

Because of the variation in delineation techniques and 

spatial data formats used, topological relations are defined 

here in general terms. Not all the topological relations 

described here apply to all spatial data formats. For example, 

polygon layers in a spatial data format such as ESRI Geodatabase format always have an associated attribute record and do 

not require or support the concept of having to attach a label to 

a polygon in order to store attribution, as is necessary with the 

ESRI Coverage format. Therefore, independent of a particular 

data format, valid topological relations are used for several 

important functions in the Watershed Boundary Dataset:

•	 Quality control–to prevent polygons from overlapping or 

to eliminate gaps in the data.

•	 Data Integration–to support integrating hydrologic units 

from diverse sources.

•	 Manage coincident hydrologic units–to determine and 

resolve how hydrologic units share boundaries.

•	 Construct polygons from lines–to support development of 

hydrologic unit polygons from diverse linear delineations.

Hydrologic unit lines should have the following valid 

topology:

• Dangling lines: Not allowed. A dangling line is a line that 

has the same polygon on its left and right sides.

• Overlaps and intersects: Not allowed. A hydrologic unit 

line may not overlap itself or another hydrologic unit line. 

A hydrologic unit line may not intersect itself or another 

hydrologic unit line. 

Hydrologic unit polygons should have the following valid 

topology:

•	 Overlaps and intersects: Not allowed. A polygon must 

share coincident geometry with adjacent polygons, which 

dictates that hydrologic unit polygons cannot overlap or 

intersect with adjacent polygons, nor should there be gaps.

6.2 Database Schema for Attributing Hydrologic 

Unit Polygons

Each hydrologic unit polygon has associated attributes 

stored in tabular format. An attribute, also called a field or column, 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, “HUC_8”, 

has space for eight characters, and a value is required (cannot be left blank). The physical schema is immutable; that is,

attribution must adhere to literal field names, field types, and 

the order of fields in the table.

The following sections (6.2.1 to 6.2.17) describe the 

physical schema for hydrologic unit polygons.

6.2.1 8-Digit Hydrologic Unit Code

The 8-digit hydrologic unit code attribute identifies a 

unique Subbasin (4th-level, 8-digit hydrologic unit). Use the 

8-digit codes documented in Seaber (1987) in most circumstances. This code will be the basis for all hydrologic units 

nested within a Subbasin.

--------------------------------------------------------------------------

Field Name HUC_8

Field Type Character

Field Width 8

Domain 01000000-99999999

Value required? Yes

Example 01080201

--------------------------------------------------------------------------
30 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

6.2.2 10-Digit Hydrologic Unit Code

The 10-digit hydrologic unit code attribute identifies a 

unique Watershed (5th-level, 10-digit hydrologic unit). To 

construct the 10-digit code, add 2 digits to the end of the 

8-digit codes documented in Seaber (1987). This code will be 

the basis for all Subwatershed hydrologic unit codes nested 

within the Watershed.

--------------------------------------------------------------------------

Field Name HUC_10

Field Type Character

Field Width 10

Domain 0100000000-9999999999

Value required? Yes

Example 0108020101

--------------------------------------------------------------------------

6.2.3 12-Digit Hydrologic Unit Code

The 12-digit hydrologic unit code attribute identifies a 

unique Subwatershed (6th-level, 12-digit hydrologic unit). 

To construct the 12-digit code, add 2 digits to the end of the 

10-digit code of the Watershed it subdivides. 

--------------------------------------------------------------------------

Field Name HUC_12

Field Type Character

Field Width 12

Domain 010000000000-999999999999

Value required? Yes

Example 010802010101

--------------------------------------------------------------------------

6.2.4 Subwatershed Area (6th-Level, 12-Digit 

Hydrologic Unit)

The Subwatershed area attribute is the area, in acres, of 

the Subwatershed. This value is calculated from the intrinsic 

area value maintained by the GIS software; therefore, acreage 

values may vary depending on the projection of the data. If 

the units of the area field are stored in square meters, use the 

conversion factor 0.0002471. For example, 40,469,446 square 

meters multiplied by 0.0002471 = 10,000 acres. 

--------------------------------------------------------------------------

Field Name ACRES

Field Type Numeric

Field Width Precision: 12, Scale 0 

 INFO notation: 8,12,F,0

Domain 0-unlimited

Value required? Yes

Example 26739

--------------------------------------------------------------------------
Geospatial Data Structure 31

6.2.5 State or Outlying Area 

The state or outlying area attribute identifies state(s) or 

outlying areas that the Subwatershed falls within. Use the twoletter U.S. postal abbreviation. If a hydrologic unit crosses into 

Canada, use the two-letter postal abbreviation for the Province. If a unit crosses into Mexico, use MX. If using more than 

one abbreviation, separate by a comma with no space and sort 

in alphabetical order.

--------------------------------------------------------------------------

Field Name STATES 

Field Type Character 

Field Width 11

Domain U.S. States: 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

 Caribbean Outlying Areas: PR,VI

Pacific Island Outlying Areas: AS,FM,

 GU,MH,MP,PW,UM

 Canadian Provinces: AB,BC,MB,NB,

 ON,QC,SK,YT

 Mexico: MX

Value required? Yes

Example KS,MO,OK

--------------------------------------------------------------------------

6.2.6 Noncontributing Area

The noncontributing area attribute represents the area, in 

acres, of hydrologic units that do 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, 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. 

--------------------------------------------------------------------------

Field Name NCONTRB_A

Field Type Numeric

Field Width Precision: 12, Scale 0 

INFO notation: 8,12,F,0

Domain 0-unlimited

Value required? No

Example 357

--------------------------------------------------------------------------
32 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

6.2.7 10-Digit Hydrologic Unit Code of 

Downstream Watershed

The 10-digit hydrologic unit code of downstream 

Watershed attribute is the code for the Watershed (5th-level, 

10-digit hydrologic unit) that is downstream from and receives 

the majority of the flow from the Watershed that contains this 

Subwatershed. 

For special cases when a downstream code cannot be 

assigned, identify which of these exceptions applies: 

•	 If a hydrologic unit flows into an ocean or the Gulf of 

Mexico, populate this field with the value “OCEAN.”

•	 If a hydrologic unit drains into either Canada or Mexico, 

use the appropriate value “CANADA” or “MEXICO.” 

•	 If a hydrologic unit is a closed basin, populate this field 

with the value “CLOSED BASIN,” which will be truncated 

to “CLOSED BAS.”

--------------------------------------------------------------------------

Field Name HU_10_DS

Field Type Character

Field Width 10

Domain OCEAN, CANADA, MEXICO, CLOSED 

 BAS; 0100000000-9999999999

Value required? No

Example 1 OCEAN

Example 2 1710020504

--------------------------------------------------------------------------

6.2.8 Watershed Name (5th-Level, 10-Digit 

Hydrologic Unit) 

The Watershed name attribute is a name constructed with 

reference to feature names officially recognized by the U.S. 

Board on Geographic Names. Populate this field by following 

section 5.3, “Watershed and Subwatershed Naming Protocol.” 

The value assigned to the Watershed name should be unique 

within the Subbasin that contains this Watershed. The Watershed name values for all the Subwatersheds nested within a 

Watershed are identical.

--------------------------------------------------------------------------

Field Name HU_10_NAME

Field Type Character

Field Width 80

Domain Official names

Value required? No

Example Upper Blue River

--------------------------------------------------------------------------
Geospatial Data Structure 33

6.2.9 Watershed Modifications (5th-Level, 

10-Digit Hydrologic Unit) 

The Watershed modifications attribute is the twocharacter, uppercase abbreviation(s) for either (1) the type of 

modification to natural overland flow that alters the location of 

a Watershed boundary (5th-level, 10-digit hydrologic unit) or 

(2) the special conditions GF-ground-water flow, GL-glacier, 

IF-ice field, OF-overbank flow, KA-karst, NC-noncontributing 

area, and IT-interbasin transfer. The value of the HU_10_

MOD field helps indicate where the modification to the Watershed boundary (5th-level, 10-digit) occurs. If using more than 

one abbreviation, separate them with a comma in the 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 coordinator for more information.

AD Aqueduct A conduit or artificial channel to transport 

water that does not allow inflow from the surrounding land 

(alters natural boundary location).

BC Barge Canal or Navigation Canal An artificial 

waterway built for navigation; for example, by barges (alters 

natural boundary location).

CD Channel Diversion A stream channel within the 

hydrologic unit has altered to divert surface water from one 

location to another (alters natural boundary location). 

DD Drainage Ditch An artificial waterway used to move 

surface water off the land to a stream or water body (alters 

natural boundary location).

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 natural boundary location). 

GC General Canal/Ditch An artificial open waterway 

constructed to transport water, to irrigate or drain land, 

to connect two or more bodies of water, or to serve as a 

waterway for watercraft (alters natural boundary location).

GF Ground-Water Flow A special condition where most of 

the runoff in a hydrologic unit drains underground, usually in 

locations with sandy soil. 

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.

ID Irrigation Ditch An artificial ditch or canal that supplies 

land with water for crops (alters natural boundary location).

IF Ice Field A special condition where a hydrologic unit 

crosses, or includes a field of ice, formed in regions of 

perennial frost.

IT Interbasin Transfer A special condition where a water 

conveyance system within a hydrologic unit is used to divert 

water from one hydrologic unit to another.

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, and caverns. 

LE Levee An artificial bank to confine a stream channel 

or limit adjacent areas subject to flooding (alters natural 

boundary location). 

MA Mining Activity Heavy topographic modification of 

a hydrologic unit by surface mining (alters natural boundary 

location).

NC Noncontributing Area A natural formed area that does 

not contribute surface-water runoff to a hydrologic unit outlet; 

for example, a playa (special condition).

NM  No Modifications No modifications are present.

OC  Overflow Channel or Flume An artificial channel 

built to control excess high flow from a natural channel (alters 

natural boundary location).
34 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

OF Overbank Flow A natural diversion 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. 14). 

OT Other A modification that has not been identified in this 

list.

PD Pipe Diversion A redirection of surface water by a 

pipeline from one hydrologic unit to another (alters natural 

boundary location).

PS Pumping Station A facility along a stream or other 

waterbody used to move water over a levee or other 

obstruction (alters natural boundary location).

Figure 14. 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, Ariz.

0 0.5 1 MILE

N

0 0.5 1 KILOMETER

Hydrologic unit boundary

Subwatershed

EXPLANATION

C ree k

Overbank flow

Overbank flow

ARIZONA
Geospatial Data Structure 35

RS Reservoir A constructed basin formed to contain and 

store water for future use in an artificial lake (alters natural 

boundary location).

SC Stormwater Canal or Drainage Canal An open, 

artificial waterway (many times named on maps) that drains 

stormwater runoff (alters natural boundary location).

SD Stormwater Ditch An open, artificial waterway 

(smaller than a canal, usually not named on maps) that drains 

stormwater runoff (alters natural boundary location).

SI Siphon An artificial diversion (usually named “Siphon” 

on maps) to move surface water from one stream channel to 

another (alters natural boundary location).

TF Transportation Feature A land transportation feature; 

for example, road, railroad, dock, airport, etc. (alters natural 

boundary location).

UA Urban Area Heavy modification of hydrologic unit 

topography by development beyond that described above in 

“Transportation Feature” (alters natural boundary location). 

--------------------------------------------------------------------------

Field Name HU_10_MOD

Field Type Character

Field Width 20

Domain AD,BC,CD,DD,DM,GC,GF,GL,IF,ID,IT,

 KA,LE,MA,NC,NM,OC,OF,OT,PD,PS,RS,

 SC,SD,SI,TF,UA

Value required? Yes

Example CD,NC,ID

--------------------------------------------------------------------------

6.2.10 Watershed Type (5th-Level, 10-Digit 

Hydrologic Unit) 

The Watershed type attribute is the single-letter abbreviation 

for Watershed type from the list provided. Use the single type 

that most closely describes the Watershed. 

S “Standard” hydrologic unit An area with drainage 

flowing to a single outlet point, excluding noncontributing 

areas. Some examples include “true,” “classic,” “composite,” 

and “remnant” hydrologic units (section 3.5.1). 

C “Closed Basin” hydrologic unit A drainage area that is 

100 percent noncontributing. This means all surface flow is 

internal; 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 

lakes, oceans, bays, etc., that has more than one outlet. These 

hydrologic units are predominantly land with some water areas 

at or near the outlet(s) (section 3.5.6).

M “Multiple Outlet” hydrologic unit An area that has 

more than one natural outlet; for example, an outlet located on 

a stream with multiple channels. This 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. This 

code should be used 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 3.5.6). 

I “Island” hydrologic unit An area that is one or more 

islands and adjacent water (section 3.5.7).

U  “Unclassified” hydrologic unit An area that cannot be 

defined or does not fit into one of the other types listed.

--------------------------------------------------------------------------

Field Name HU_10_TYPE

Field Type Character

Field Width 1

Domain S,C,F,M,W,I,U

Value required? Yes

Example S

--------------------------------------------------------------------------
36 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

6.2.11 12-Digit Hydrologic Unit Code of 

Downstream Subwatershed 

The 12-digit hydrologic unit code of downstream Subwatershed attribute is the code for the Subwatershed (6th-level, 

12-digit hydrologic unit) that is downstream from and receives 

the majority of the flow from this Subwatershed. 

For special cases when a downstream code cannot be 

assigned, identify which of these exceptions applies: 

•	 If a hydrologic unit flows into an ocean or the Gulf of 

Mexico, populate this field with the value “OCEAN.” 

•	 If a hydrologic unit drains into either Canada or Mexico, 

use the appropriate value “CANADA” or “MEXICO.” 

•	 If a hydrologic unit is a closed basin, then populate this 

field with the value “CLOSED BASIN,” which will appear 

“CLOSED BAS.”

--------------------------------------------------------------------------

Field Name HU_12_DS

Field Type Character

Field Width 12

Domain OCEAN, CANADA, MEXICO,

 CLOSED BASIN; 

 010000000000-999999999999

Value required? No

Example 1 OCEAN

Example 2 171002050402

--------------------------------------------------------------------------

6.2.12 Subwatershed Name (6th-Level, 12-Digit 

Hydrologic Unit) 

The Subwatershed name attribute is a name constructed 

with reference to feature names officially recognized by the 

U.S. Board on Geographic Names. Populate this field by 

following section 5.3, “Watershed and Subwatershed Naming Protocol.” The value assigned to the Subwatershed name 

should be unique within the Subbasin and Watershed that 

contain this Subwatershed.

--------------------------------------------------------------------------

Field Name HU_12_NAME

Field Type Character

Field Width 80

Domain Official names

Value required? No

Example Drift Creek-Big Bear River

--------------------------------------------------------------------------
Geospatial Data Structure 37

6.2.13 Subwatershed Modifications (6th-Level, 

12-Digit Hydrologic Unit) 

The Subwatershed modifications attribute is the twocharacter, uppercase abbreviation(s) for either (1) the type of 

modification to natural overland flow that alters the location 

of a Subwatershed boundary (6th-level, 12-digit hydrologic 

unit) or (2) the special conditions GF-ground-water flow, 

GL-glacier, IF-ice field, OF-overbank flow, KA-karst, NCnoncontributing area, and IT-interbasin transfer. The value of 

the HU_12_MOD field helps indicate where the modification 

to the Subwatershed boundary (6th-level, 12-digit) occurs. If 

the modification occurs on a boundary that represents both the 

Watershed and Subwatershed, the same modifications should 

be reflected in both the HU_12_MOD and HU_10_MOD 

fields. If using more than one abbreviation, separate them with 

a comma in the 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 coordinator for more information.

AD Aqueduct A conduit or artificial channel to transport 

water that does not allow inflow from the surrounding land 

(alters natural boundary location).

BC Barge Canal or Navigation Canal An artificial 

waterway built for navigation; for example, by barges (alters 

natural boundary location).

CD Channel Diversion A stream channel within the 

hydrologic unit has altered to divert surface water from one 

location to another (alters natural boundary location). 

DD Drainage Ditch An artificial waterway used to move 

surface water off the land to a stream or water body (alters 

natural boundary location).

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 natural boundary location). 

GC General Canal/Ditch An artificial open waterway 

constructed to transport water, to irrigate or drain land, 

to connect two or more bodies of water, or to serve as a 

waterway for watercraft (alters natural boundary location).

GF Ground-Water Flow A special condition where most of 

the runoff in a hydrologic unit drains underground, usually in 

locations with sandy soil. 

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.

ID Irrigation Ditch An artificial ditch or canal that supplies 

land with water for crops (alters natural boundary location).

IF Ice Field A special condition where a hydrologic 

unit crosses or includes a field of ice, formed in regions of 

perennial frost.

IT Interbasin Transfer A special condition where a water 

conveyance system within a hydrologic unit is used to divert 

water from one hydrologic unit to another.

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, and caverns. 

LE Levee An artificial bank to confine a stream channel 

or limit adjacent areas subject to flooding (alters natural 

boundary location). 

MA Mining Activity Heavy topographic modification of 

a hydrologic unit by surface mining (alters natural boundary 

location).

NC Noncontributing Area A natural formed area that does 

not contribute surface-water runoff to a hydrologic unit outlet; 

for example, a playa (special condition).

NM  No Modifications No modifications are present.

OC  Overflow Channel or Flume An artificial channel 

built to control excess high flow from a natural channel (alters 

natural boundary location).

OF Overbank Flow A natural diversion 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. 14). 

OT Other A modification that has not been identified in this 

list.

PD Pipe Diversion A redirection of surface water by a 

pipeline from one hydrologic unit to another (alters natural 

boundary location).

PS Pumping Station A facility along a stream or other 

waterbody used to move water over a levee or other 

obstruction (alters natural boundary location).

RS Reservoir A constructed basin formed to contain and 

store water for future use in an artificial lake (alters natural 

boundary location).

SC Stormwater Canal or Drainage Canal An open, 

artificial waterway (many times named on maps) that drains 

stormwater runoff (alters natural boundary location).

SD Stormwater Ditch An open, artificial waterway 

(smaller than a canal, usually not named on maps) that drains 

stormwater runoff (alters natural boundary location).
38 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

SI Siphon An artificial diversion (usually named “Siphon” 

on maps) to move surface water from one stream channel to 

another (alters natural boundary location).

TF Transportation Feature A land transportation feature; 

for example, road, railroad, dock, airport, etc. (alters natural 

boundary location).

UA Urban Area Heavy modification of hydrologic unit 

topography by development beyond that described above in 

“Transportation Feature” (alters natural boundary location). 

--------------------------------------------------------------------------

Field Name HU_12_MOD

Field Type Character

Field Width 20

Domain AD,BC,CD,DD,DM,GC,GF,GL,IF,ID, IT,

 KA,LE,MA,NC,NM,OC,OF,OT,PD,PS,

 RS,SC,SD,SI,TF,UA

Value required? Yes

Example SD,KA,PD

--------------------------------------------------------------------------

6.2.14 Subwatershed Type (6th-Level, 12-Digit 

Hydrologic Unit)

The Subwatershed type attribute is the single-letter abbreviation for Subwatershed type from the list provided. Use the 

single type that most closely describes the Subwatershed. 

S “Standard” hydrologic unit An area with drainage 

flowing to a single outlet point, excluding noncontributing 

areas. Some examples include “true,” “classic,” “composite,” 

and “remnant” hydrologic units (section 3.5.1). 

C “Closed Basin” hydrologic unit A drainage area that is 

100 percent noncontributing. This means all surface flow is 

internal; 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 

lakes, oceans, bays, etc., that has more than one outlet. These 

hydrologic units are predominantly land with some water areas 

at or near the outlet(s) (section 3.5.6).

M “Multiple Outlet” hydrologic unit An area that has 

more than one natural outlet; for example, an outlet located on 

a stream with multiple channels. This 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. This 

code should be used 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 3.5.6). 

I “Island” hydrologic unit An area that is one or more 

islands and adjacent water (section 3.5.7).

U  “Unclassified” hydrologic unit An area that cannot be 

defined or does not fit into one of the other types listed.

--------------------------------------------------------------------------

Field Name HU_12_TYPE

Field Type Character

Field Width 1

Domain S,C,F,M,W,I,U

Value required? Yes

Example C

--------------------------------------------------------------------------
Geospatial Data Structure 39

6.2.15 Watershed GNIS Feature Identification 

Number (5th-Level, 10-Digit Hydrologic Unit)

The Watershed GNIS Feature Identification Number 

attribute is the 6- or 7-digit Feature Identification Number 

from the Geographic Names Information System (GNIS) that 

correspond to the official names used in the hydrologic unit 

name. Each officially recognized name in the country has a 

unique Feature Identification Number. If a hydrologic unit 

name includes two or more feature names, this field contains 

two GNIS feature numbers separated by a space. 

--------------------------------------------------------------------------

Field Name HU_10_GNIS#

Field Type Character

Field Width 23

Domain 000001-9999999

Value required? No

Example 102354 238437

--------------------------------------------------------------------------

6.2.16 Subwatershed GNIS Feature Identification 

Number (6th-Level, 12-Digit Hydrologic Unit)

The Subwatershed GNIS Feature Identification Number 

attribute is the 6- or 7-digit Feature Identification Number 

from the Geographic Names Information System (GNIS) that 

corresponds to the official names used in the hydrologic unit 

name. Each officially recognized name in the country has a 

unique Feature Identification Number. If a hydrologic unit 

name includes two or more feature names, this field contains 

two GNIS feature numbers separated by a space. 

--------------------------------------------------------------------------

Field Name HU_12_GNIS#

Field Type Character

Field Width 23

Domain 000001-9999999

Value required? No

Example 102354 238437

--------------------------------------------------------------------------

6.2.17 Metadata Identification

The Metadata Identification attribute is the code used to 

track changes made to a specific boundary (line or arc) segment or polygon attribute. When a state has received provisional certification status, all boundary segments and polygon 

attribute values would be 01. After provisional certification 

each time changes are made to a boundary segment or polygon 

attribute, the Meta_ID value is incrementally updated for 

that feature. The change is documented in the updated FGDC 

metadata with reference to the Meta_ID.

Construct the 4-character metadata ID starting with the 

2-letter U.S. postal abbreviation, in uppercase, followed by a 

2-digit sequence number; for example, “OK01,” “ID02.” All 

state polygons will have a Meta_ID of (state abbreviation01)at 

the time of provisional certification.

--------------------------------------------------------------------------

Field Name META_ID

Field Type Character

Field Width 4

Domain 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

Value required? Yes

Example OK01

--------------------------------------------------------------------------
40 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset 6.2.18

Polygon Attribute Summary

Attribute Field name Field type Field width Domain Is value required? Example

8-digit hydrologic 

unit code HUC_8 Character 8 01000000-99999999 Yes 01080201

10-digit hydrologic 

unit code HUC_10 Character 10 0100000000-9999999999 Yes 0108020103

12-digit hydrologic 

unit code HUC_12 Character 12 010000000000-999999999999 Yes 010802010310

Subwatershed area ACRES Numeric

Precision:12, 

Scale 0 

Info notation: 8,12,F,0

0-unlimited Yes 26739

State or outlying area STATES Character 11

U.S. state or Tribal Lands, 

Pacific Islands, Puerto Rico, U.S. V.I.

Yes KS,MO,OK

Noncontributing area NCONTRIB_A Numeric

Precision:12, 

Scale 0 

Info notation: 8,12,F,0

0-unlimited No 357

10-digit hydrologic unit 

code of downstream Watershed

HU_10_DS Character 10

OCEAN, CANADA, MEXICO, 

CLOSED BAS, 0100000000-9999999999

No 1710020504

Watershed name HU_10_NAME Character 80 Official names No Upper Blue River

Watershed 

modifications HU_10_MOD Character 20

AD,BC,CD,DD,DM,GC,GF,GL,

ID,IF,IT,KA,LE,MA,NC,NM,OC,OF,OT,PD,PS,RS,SC,SD,SI,TF, UA

Yes CD,NC,ID

Watershed type HU_10_TYPE Character 1 S,C,F,M,W,I,U Yes S

12-digit hydrologic unit 

code of downstream Subwatershed

HU_12_DS Character 12

OCEAN, CANADA, MEXICO, 

CLOSED BASIN, 010000000000-999999999999

No 171002050401

Subwatershed name HU_12_NAME Character 80 Official names No Drift Creek-Big Bear

Subwatershed 

modification HU_12_MOD Character 20

AD, BC, CD, DD, DM, GC, GF, GL, 

ID, IF, IT, KA, LE, MA, NC, NM, OC, OF, OT, PD, PS, RS, SC, SD, SI, TF, UA

Yes CD,NC,ID
Geospatial Data Structure

41

6.3Database Schema for Attributing 

Hydrologic Unit Lines

Each hydrologic unit line has associated attributes stored 

in tabular format. An attribute, also called a field or column, 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 lines. For example, every hydrologic 

unit line 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 line has the literal name “HU_Level” with space for one 

character, which can only be one of the seven characters 0, 

1, 2, 3, 4, 5, or 6, and a value is required (field cannot be left 

blank). The physical schema is immutable; that is, attribution 

must adhere to literal field names, field types, and the order of 

fields in the table.

The following sections (6.3.1 to 6.3.3) describe the physi-

cal schema for hydrologic unit lines. 

6.2.18 Polygon Attribute Summary—ContinuedAttribute Field name Field type Field width Domain Is value required? Example

Subwatershed type HU_12_TYPE Character 1 S,C,F,M,W,I,U Yes S

Watershed GNIS feature 

identification number HU_10_GNIS# Character 23 000001-9999999 No 102354 238437

Subwatershed GNIS 

feature identification number

HU_12_GNIS# Character 23 000001-9999999 No 102354 238437

Metadata identification META_ID 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
42 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

6.3.1 Hydrologic Unit Level 

The hydrologic unit level attribute indicates the relative 

position of each boundary (line, or arc) segment within the 

hydrologic unit hierarchy. Populate this field with the highest 

hydrologic unit level (smallest number) for each line, or arc, 

as indicated by the record. A level is represented by numbers 0 through 6, with 0 at the top and 6 at the bottom of the 

hierarchy. For example, if a line segment is the boundary of a 

Region, then the value is 1 (Region). If a line segment is the 

boundary of a Subbasin, then the value is 4 (Subbasin) even 

though it is also a Watershed and Subwatershed boundary. If 

a line segment ends at an international border and there is no 

information to complete the hydrologic unit beyond the U.S. 

boundary, then the value is 0. If a line segment along a coastal 

area ends at the shoreline because bathymetry was not applied, 

then the value is 0. Use one of the levels in the list below.

Level Digit

number Name

0 0 no data (includes coastline) 

1 2 Region 

2 4 Subregion 

3 6 Basin 

4 8 Subbasin 

5 10 Watershed 

6 12 Subwatershed 

--------------------------------------------------------------------------

Field Name HU_level

Field Type Character

Field Width 1

Domain 0-6

Value required? Yes

Example 6

--------------------------------------------------------------------------

6.3.2 Line Spatial Data Source

The line spatial data source attribute is the abbreviation 

to identify the map or other geographic data source(s) used to 

delineate hydrologic unit boundaries. Populate the field using 

one or more of the standardized codes listed below in uppercase. If using more than one code separate the values with a 

comma and no space. Other reference and source maps not 

listed should be noted in the metadata (section 4).

TOPO24 Delineated from hardcopy 1:24,000-scale 

topographic maps. 

TOPO25 Delineated from hardcopy 1:25,000-scale 

topographic maps for Alaska and the Caribbean outlying areas.

TOPO63 Delineated from hardcopy 1:63,360-scale 

topographic maps for Alaska and the Caribbean outlying areas.

DRG24 Delineated from 1:24,000-scale Digital Raster 

Graphics.

DRG25M Delineated from 1:25,000-scale Digital Raster 

Graphics for Alaska and the Caribbean outlying areas.

DRG63 Delineated from 1:63,360-scale Digital Raster 

Graphics for Alaska and the Caribbean outlying areas.

DEM10 Derived from 10-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.

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. 

TNML Delineated from NOAA nautical charts' Three 

Nautical Mile Line.

NAIP“year” Delineated from aerial photography produced 

by the National Agriculture Imagery Program; for example, 

NAIP2005
Geospatial Data Structure 43

NHD24 Interpreted from 1:24,000-scale National 

Hydrography Dataset.

NHD100 Interpreted from 1:100,000-scale National 

Hydrography Dataset.

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.

LIDAR Derived from LIDAR (light detection and ranging) 

data.

IFSAR Derived from IFSAR (interferometric synthetic 

aperture radar) data.

OTH Other.

UNK Unknown.

--------------------------------------------------------------------------

Field Name LINESOURCE

Field Type Character

Field Width 20

Domain TOPO24,TOPO25,TOPO63,DRG24,

 DRG25, DRG63,DEM10DEM30,

 NED10,NED30,EDNA30,BATH“scale,” 

 TNML, NAIP”year,”NHD24,NHD100 ,

 HYPSO“scale,”ORTHO“scale,”

 DEDEM10,DEDEM30,GPS,LIDAR,

 IFSAR,OTH

Value required? Yes

Example DEM30,DRG24,GPS

--------------------------------------------------------------------------

6.3.3 Metadata Identification

The Metadata Identification attribute is the code used to track 

changes made to a specific boundary (line or arc) segment 

or polygon attribute. When a state has received provisional 

certification status, all boundary segments and polygon 

attribute values would be 01. After provisional certification 

each time changes are made to a boundary segment or polygon 

attribute, the Meta_ID value is incrementally updated for 

that feature. The change is documented in the updated FGDC 

metadata with reference to the Meta_ID. 

Construct the 4-character metadata ID starting with the 2-letter 

U.S. postal abbreviation, in uppercase, followed by a 2-digit 

sequence number; for example, “OK01,” “ID02.” All state 

polygons will have a Meta_ID of (state abbreviation01) at the 

time of provisional certification.

--------------------------------------------------------------------------

Field Name META_ID

Field Type Character

Field Width 4

Domain 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

Value required? Yes

Example OK01

--------------------------------------------------------------------------
44 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

6.3.4 Line Attribute Summary

Attribute Field name Field 

type

Field 

width Domain Is value required? Example

Hydrologic unit 

number HU_LEVEL Character 1 0-6 Yes 6

Line spatial data 

source

LINE

SOURCE Character 20

TOPO24, TOPO25, TOPO63, DRG24, 

DRG25, DRG63, DEM10, DEM30, 

NED10, NED30, EDNA30, BATH“scale,” 

TNML, NAIP”year,” NHD24, NHD100, 

HYPSO“scale,” ORTHO“scale,” DEDEM10, DEDEM30, GPS, LIDAR, IFSAR, 

OTH, UNK

Yes

DEM30, 

DRG24, 

GPS

Metadata 

identification META_ID 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
Quality Assurance and Quality Control 45

7. Quality Assurance and Quality 

Control

7.1 Quality Assurance

Ideally, hydrologic units are delineated and verified 

through an interagency process. However, it may be necessary 

for one agency to lead in developing and certifying the hydrologic unit boundaries. Before newly delineated hydrologic unit 

boundaries are submitted for the WBD, the boundaries must 

be reviewed for conformance to these guidelines by the 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. The originating agency should invite 

representatives of the interagency hydrologic unit coordinating group and or regional/local parties to participate in the 

development and review of delineations before the data are 

submitted for national certification and released to the public. 

The data should be thoroughly reviewed by the state representatives before submitting for certification.

It is recommended that reviews and edit checks be done 

throughout the delineation process. At a minimum, edit checks 

should be made after the Watersheds and Subwatersheds are 

delineated, mapped, and digitized. 

7.2 Quality Control (Editing Checklist)

Below is a recommended list of quality-control items to 

be checked during the delineation, coding, documentation, 

and digitizing process. This is necessary for maintaining a 

consistent national database of hydrologic units. This is not an 

exhaustive list, but it covers most items to be verified.

7.2.1 Delineation

•	 Do the hydrologic unit boundaries match at state 

boundaries, with 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?

•	 Is each hydrologic unit subdivided into 5 to 15 units? 

(Section 3.3.)

•	 Are the areas of 10-digit Watersheds within the 

recommended range of 40,000–250,000 acres? Are the 

areas of 12-digit Subwatersheds within the recommended 

range of 10,000–40,000 acres, with none below 3,000? 

When a hydrologic unit is divided by a state boundary, 

check the total area including both states’ portion (section 

3.4). A combined total of 10 percent of the polygons 

outside the specified size criteria is allowed.

•	 Are the hydrologic units correctly delineated on the 

1:24,000-scale topographic maps with respect to 

hydrography and elevation contours? (Section 4.)

•	 Do hydrologic units that were difficult to delineate have 

a description of the procedure used? Make notes about 

how certain boundaries were established or field checked 

so they can be documented in the field LINESOURCE 

(section 6.3.2) and in the metadata (section 9.0).

•	 Were local sources used to resolve the location of 

questionable boundaries? Make notes where local 

knowledge supersedes source maps and document this 

information in the field LINESOURCE (section 6.3.2). 
46 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

7.2.2 Codes

•	 Is the coding of hydrologic units consistent within a 

Subbasin covering more than one state? Check the coding 

system for correctness, duplication, or missing codes. Each 

polygon must have a single label (section 5.2).

•	 Does the coding of hydrologic units within a given level 

meet the guidelines for starting upstream and progressing 

downstream? (Section 3.)

7.2.3 Attributes

•	 Are the attribute fields complete, with valid values? 

(Sections 6.2–6.3.)

•	 Do polygons have the required values in these fields? 

HUC_8, HUC_10, HUC_12, ACRES, STATES, HU_10_

MOD, HU_10_TYPE, HU_12_MOD, HU_12_TYPE, 

META_ID (section 6.2).

•	 Do lines have the required values in these fields? HU_

LEVEL, LINESOURCE, META_ID (section 6.3).

•	 Is the ACRES field calculated from the AREA data and 

values without decimal places? (Section 6.2.4.)

•	 Is the STATE field complete, with the value correctly 

formatted using the 2-letter postal abbreviation, a comma 

between multiple states, no spaces between abbreviations, 

and multiple states in alphabetical order? (Section 6.2.5.)

•	 Do artificial modifications or special conditions exist? 

Check and document in the fields HU_10_MOD and/or 

HU_12_MOD if modifications to the natural boundary 

location have been made or special conditions exist 

(sections 6.2.9, 6.2.13).

7.2.4 Data format

•	 Does the file format conform to the guidelines? Before 

submitting for national review and certification, verify that 

the data’s map projection is correct. Data units should be 

decimal degrees, North American 1983 horizontal datum, 

Geodetic Reference System, 1980 spheroid. The data precision should be double (section 8.3).

8. Preparing Data for Certification

This section presents guidelines and procedures to be 

followed for successful certification of state contributions to 

the Watershed Boundary Dataset (WBD). Digital geospatial 

data and attributes must match adjoining states or Subregions. 

The following formats are acceptable: ESRI shapefile, ESRI 

Personal Geodatabase, ESRI File-based Geodatabase, SDTS, 

or ESRI exchange format. Data should be topologically valid. 

8.1 State Certification Process

Agencies involved in the development of the WBD 

should establish organizational responsibility for reviewing the 

data before submitting the data to be formally reviewed and 

certified by the USGS and NRCS National Cartography and 

Geospatial Center (NCGC). 

After the Subwatersheds (6th-level, 12-digit hydrologic 

units) are completed, the originating office will submit the 

data to NCGC/USGS for review and certification. A sample 

certification submittal letter is available in section 11.4. 

Supporting material such as text documents describing the 

procedure used to delineate and digitize the Watersheds and 

Subwatersheds are documented in the LINESOURCE field 

and metadata. In addition, a summary of decisions for Watersheds and Subwatersheds in complex hydrographic areas 

and explanatory hardcopy maps will benefit and expedite the 

review. 

NCGC/USGS will make the data and metadata available 

electronically to the member agencies of the Subcommittee on 

Spatial Water Data for review and provisional or full certification (fig. 15). The data will be reviewed for the following:

•	 Adherence to the current “Federal Guidelines, 

Requirements, and Procedures for the National Watershed 

Boundary Dataset” interagency guidelines

•	 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

•	 Compliance of metadata with the most recent FGDC 

standard

NCGC/USGS will refer review comments from the 

interagency team to the office that originated the data. After 

discrepancies are resolved, NCGC/USGS will send a formal 

certification letter to the state specifying whether the hydrologic unit data are approved provisionally or approved fully 

and meet the national guidelines. Provisional Certification 

usually indicates that work such as providing metadata or 

matching boundaries across state lines still remains. Provisional Certification is not given when obvious corrections 

need to be made.
Preparing Data for Certification

47

Dash indicates path of significant iterationNumber indicates sequence

EXPLANATION

1

Key certification records needed

5

7

8a

Record edge-matching compliance (verify at 8)

 Collect review documents, comments from 5 and degree standards used Fully certify only if delineations and attributes edge-match

State WBD dataset is distributed electronically

Tech. team checks dataset (delineation, accuracy and data structure) to most current guidelines

State 

coordinator

and team

Finish or concur with interstate edge requirements

Technical team recommends dataset for national WBD inclusion

NRCS, USGS

Technical

support team

Public 

milestones,

status maps

Fix problems, and/or add metadata. Submit for certification

Release state data as provisionally certified WBD 

Steering committee 

grants and notifies state 

agency of certification

Submit WBD data to NRCS/USGS for state data review, include official notice to Steering committee

Preliminary feedback per most current guidelines provides review of interim data; discuss results via interactive Web session with state.

Subcomittee on

Spatial Water

Data or 

Steering

committee

1

2

3

4

6

5

 7

Release state data as certified WBD 

Grant provisional certification 

8b

State WBD coordinator conveys status and workplans for Fed. coordination, provides data to tech. team for interim QC

8a

8a

Tech. team evaluates data compliance Notify NCGC/USGS coordinator, who 

notifies state of dataset status

Tech. team recommend provisional certification 

Tech. team recommends data fornational WBD or

Release National WBD with maintenance and update process

Figure 15. The Watershed Boundary Dataset national certification and review procedure and roles are depicted in the above flowchart.
48 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

If the data comply with the “Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary 

Dataset” and are approved, NCGC/USGS will send a letter to 

the originating office stating that the WBD meets the national 

requirements, are fully Certified, and will be added to the 

national database. 

If the data do not conform to the guidelines, the response 

letter will state the problems with the data, and the national 

Technical Support Team evaluation and recommendations will 

continue.

8.2 File Names 

The dataset naming convention for certification is 2-letter state postal abbreviation_hu12_monthyear submitted; for 

example, oh_hu12_0607. 

8.3 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. A “read me” 

file may also accompany the data or instructions included in 

the state submittal letter; this file may contain other pertinent 

information that would be useful during QA/QC and standard 

delineation review. 

8.4 Data Delivery Packaging and Compression

Use a packaging structure appropriate for the user’s 

spatial data format and a compression format appropriate for 

the user’s operating system. For compression, use ZIP format 

for Windows operating systems and GZIP format for Unix 

operating systems. By convention, ZIP format archives use 

the filename extension .zip, and GZIP format files use the filename extension .gzip. Because procedures vary significantly 

depending on operating system and the user’s spatial data 

format, three scenarios are presented for guidance.

Scenario 1: User’s operating system is Unix, and user’s 

working format is an ESRI Arc/INFO Workstation coverage. 

The delivery format should be an ESRI Export file. ESRI 

Export format is a computer-independent file format designed 

to facilitate the exchange of ESRI Arc/INFO Workstation data. 

By convention, ESRI Export files have the file name extension 

“.e00.” For each coverage, use the export command to produce 

the export file: Arc: EXPORT COVER oh_huc oh_huc.e00 

NONE. Use the tar command to add all the .e00 files to a tar 

archive: tar cvf *.e00 oh.tar. Next, use the gzip utility to compress the tar archive: gzip oh.tar (result is oh.tar.gz).

Scenario 2: User’s operating system is Windows, and 

user’s working format is an ESRI shapefile. An ESRI shapefile 

is not a single file, despite the name. Instead an ESRI shapefile 

is a spatial data format that uses at least three files that share a 

common base filename but different extensions. For example, 

an ESRI shapefile named oh_huc is physically implemented 

as three disk files: oh_huc.shp, oh_huc.dbf, oh_huc.shx. No 

packaging is necessary for shapefiles. Use a software utility 

that supports .zip format and add all the shapefile files to a .zip 

archive.

Scenario 3: User’s operating system is Windows, and 

user’s working format is an ESRI personal geodatabase. An 

ESRI personal database stores geospatial information in a single Microsoft (MS) Access database file. By convention, MS 

Access database files (and by extension ESRI personal geodatabases) have the extension .mdb. The .mdb file functions as a 

container for geospatial data. For example, an ESRI personal 

geodatabase named oh_huc is physically implemented as a 

collection of tables inside a disk file named oh_huc.mdb. No 

packaging is necessary for personal geodatabase. Use a software utility that supports .zip format and add the .mdb file to a 

.zip archive.

8.5 Data Transfer Protocol

Post the compressed data to an ftp site as “binary” type. 

Notify the national contact (refer to section 8) responsible 

for data certification and archiving. The dataset may also be 

written to a CD-ROM and mailed to the national contact for 

certification and archiving. These standards do not cover the 

distribution of data to users after certification.

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.ncgc.nrcs.usda.gov/

products/datasets/watershed/metadata.html.
References 49

10. 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, part 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

Federal Geographic Data Committee, 2001, Shoreline metadata profile of the content standards for digital geospatial 

metadata: Washington, D.C., FGDC–STD–001.2–2001, 72 

p., accessed February 14, 2007, at http://www.csc.noaa.gov/

metadata/sprofile.pdf

Jenson, S.K., and Dominique, J.O., 1988, Extracting topographic structure from digital elevation data for geographic 

information system analysis: Photogrammetric Engineering 

and Remote Sensing, v. 54, p. 1593–1600.

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, U.S. Maritime 

Zones/Boundaries: Accessed Dec. 22, 2008, at http://www.

nauticalcharts.noaa.gov/csdl/mbound.htm

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

University of California, Santa Barbara, 2002, Alexandria digital library feature type thesaurus, accessed March 13, 2007, 

at http://www.alexandria.ucsb.edu/gazetteer/FeatureTypes/

ver070302/index.htm

U.S. Bureau of the Budget, 1947, United States national 

map accuracy standards: Washington, D.C., accessed 

February14, 2007, at http://rockyweb.cr.usgs.gov/nmpstds/

acrodocs/nmas/NMAS647.PDF

U.S. Department of Agriculture, Natural Resources Conservation Service, 1992, Mapping and digitizing watershed and 

subwatershed hydrologic unit boundaries: National Instruction 170–304, 26 p., accessed May 30, 1997, at http://www.

nhq.nrcs.usda.gov/hu/ni170304.html

U.S. Department of Agriculture, Natural Resources Conservation Service, 2002, FGDC proposal, Federal standard for 

delineation of hydrologic unit boundaries, version 1: 59 p.

U.S. Department of Agriculture, Natural Resources Conservation Service, 2004, Federal standard for delineation of 

hydrologic unit boundaries, version 2: 60 p.

U.S. Department of Agriculture, Natural Resources Conservation Service, 2007, Watershed Boundary Dataset (WBD) 1: 

Accessed October 23, 2007, at http://www.ncgc.nrcs.usda.

gov/products/datasets/watershed/

U.S. Department of the Interior, Office of Water Research 

and Technology, 1980, Water resources thesaurus (3d ed.): 

OWRT IT–80/1 [variously paged].

U.S. Geological Survey [n.d.], U.S. geographic names information system (GNIS): Accessed February14, 2007, at 

http://geonames.usgs.gov/pls/gnispublic/

U.S. Geological Survey [n.d.], U.S. 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: Scale 1:250,000, 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 March 12, 2007, at 

http://rockyweb.cr.usgs.gov/nmpstds/acrodocs/draft/dlg-f/

nhd/NHDH0799.PDF
50 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

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
Appendixes 51

11. Appendixes

The hydrologic and compliance terms in the following 

lists are defined with reference to the Watershed Boundary 

Dataset process. The definitions may not be the only valid 

ones for these terms. Boldfaced terms within definitions are 

defined elsewhere in the lists.

11.1 Definitions

11.1.1 Hydrologic Definitions

Basin A subdivision of Subregion. A Basin is the 3d-level, 

6-digit unit of the hydrologic unit hierarchy. Basins were 

formerly named “accounting units” in USGS terminology.

Classic hydrologic unit A topographically defined area 

where all the surface drainage converges to a single point. 

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 classic 

hydrologic units. This includes areas or small triangular 

wedges (remnant areas) between adjacent drainage areas that 

remain after classic 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 near 

shore 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. 

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 U.S. 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 5th and 6th 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 

2-digit sequence for each specific level within the delineation 

hierarchy.

Hydrologic unit name A standardized name assigned to a 

hydrologic unit 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 nonwater 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. 

Noncontributing area A naturally formed area that does not 

contribute to downstream accumulation of streamflow under 

normal flow conditions.

Open-water hydrologic unit An area delineated within an 

ocean. Land is not a major portion of the hydrologic unit but 

may be included, as in the case of islands.

Region A Region is the 1st-level, 2-digit unit and largest in 

the hydrologic unit hierarchy. The hydrologic unit category 

name is retained for Watershed Boundary Dataset.

Remnant area A topographically defined area that is residual 

after delineation of classic hydrologic units. They 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 and connecting 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 which 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 classic, composite, and 

remnant hydrologic units.

Subbasin A subdivision of a Basin. A Subbasin is the 4thlevel, 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 

2d-level, 4-digit unit of the hydrologic unit hierarchy; the 

hydrologic unit category name is retained for the Watershed 

Boundary Dataset. 
52 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

Subwatershed A subdivision of a Watershed. A Subwatershed is the 6th-level, 12-digit unit and smallest of the hydrologic unit hierarchy. Subwatersheds generally range in size 

from 10,000 to 40,000 acres. 

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 classic hydrologic unit.

Water hydrologic unit A body of water that can receive flow 

from adjacent frontal hydrologic units, composite hydrologic units, and classic hydrologic units. Examples include 

inland lakes and nearshore ocean waters. Generally, these units 

are not further delineated to the next lower hydrologic unit

level.

Watershed A subdivision of a Subbasin. A Watershed is 

the 5th-level, 10-digit unit of the hydrologic unit hierarchy. 

Watersheds range in size from 40,000 to 250,000 acres. Also 

used as a generic term representing a drainage basin or combination of hydrologic units of any size.

11.1.2 Geospatial Data Definitions and Standards

Attribute A defined characteristic of a geographic feature or 

entity 

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 where 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.

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.

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 numeric 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, 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.
Appendixes 53

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.

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 A National geospatial database of drainage areas consisting of the 1st through 6th hierarchical hydrologic unit levels. The seamless geographic data 

will be finished by merging the certified versions of watershed 

data available at 1:24,000 scale across the United States. WBD 

includes the required attribute and metadata information. 

WBD components have been incrementally released through 

a formal certification process beginning in 1998. The WBD is 

an ongoing multiagency effort to create hierarchical, and integrated hydrologic units across the Nation. For more information, see U.S. Department of Agriculture, Natural Resources 

Conservation Service (2007)

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 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 (1998).

11.1.3 Compliance Definitions

Certification Formal acknowledgment that hydrologic 

units have been reviewed and meet the criteria as stated in the 

“Federal Guidelines, Requirements, and Procedures for the 

National Watershed Boundary Dataset,” 2009, as agreed to by 

member agencies of the Subcommittee on Spatial Water Data.

Provisionally Certified 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 

Subcommittee on Spatial Water Data.

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 Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset,” agreed to by member agencies of the 

Subcommittee on Spatial Water Data.
54 Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset

11.2 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

NCGC National Cartography and Geospatial Center

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 Informix

Exact 

numeric

Short 

integer I Number, boolean Integer Short integer

Number 

(4) Smallint (2) Smallint (2) Smallint (2)

Exact 

numeric

Long

integer B, I Number Long integer

Long 

integer

Number 

(38) Int (4) Integer (4) Integer (4)

Approximate 

numeric Float F, N Number Single Float Number (38,8) Real Decimal (31,8)

Decimal 

(32)

Approximate 

numeric Double F, I, N Number Double Double Number (38,8) Double Decimal (31,8)

Decimal 

(32)

Character 

string Text C String Text Text Varchar Varchar (n) Varchar Varchar (n)

Date time Date D Date Date/time Date Date Datetime Timestamp Datetime

Bit string Blob – – OLE 

object Blob Blob Image Blob Blob

Exact 

numeric Guid – – Number Guid Char (38)

Unique 

identifier 

(16)

Character 

(38) Char (8)

NED National Elevation Dataset

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

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

WBD Watershed Boundary Dataset

11.3 Data Type Cross Walk for Data Storage of ESRI Products
Data History Letter for Certification Submittal 55

11.4 Data History Letter for Certification Submittal

To help the review and certification process, consider including these categories of information about the history and 

development of your dataset in the transmittal letter. Contact the WBD Technical Support Team for questions or assistance.

Date: 

Subject: Submittal of the Watershed Boundary Dataset for 

To: Director

USDA Natural Resources Conservation Service

National Cartography and Geospatial Center (NCGC)

501 West Felix Street, Building 23

P.O. Box 6567

Fort Worth, Tx 76115-6567

 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 [Technical Support Team 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 [Technical Support Team 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 [Technical Support Team 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 Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset—Techniques and Methods 11–A3