The TNMID (short for “The National Map Identification”) is a 40-character field that holds unique identifiers for each element in the database and is common to all feature classes (lines and polygons) and FeaturetoMetadata and ExternalCrosswalk tables in the database. The TNMID is a persistent code that is used to link WBD features to their feature-level metadata. For features that are already present in the WBD, the assigned TNMIDs must be maintained throughout any update or edit process. The TNMIDs for new features, for example new 14- or 16-digit HUs, are automatically assigned when the data are fed back into the national WBD.

LoadDate is an automatically generated field representing the date when the data were loaded into the USGS WBD. The field is the effective date for feature edits.

MetaSourceID is a unique identifier that links a feature to a record or records in the metadata tables. This identifier is both generated and assigned automatically. The MetaSourceID field included in the feature class should represent the last edit performed on a feature so that WBDfeatureClass.MetaSourceID = MetaSourceDetail.MetaSourceID of the latest edit.

SourceDataDesc is completed as part of the metadata form provided in the WBD edit tools. The attribute is included in all feature classes except WBDLine. SourceDataDesc describes the data that are used to delineate or update the WBD boundary or update the WBD attribution.

SourceOriginator contains the description of the agency that created the base data used to improve the WBD. This field is common to all polygon feature classes and is completed as part of the metadata form provided in the WBD edit tools. The attribute is included in all feature classes except WBDLine. The SourceOriginator field included in the feature class should represent the last edit performed on a feature so that WBDfeatureClass.SourceOriginator = MetaSourceDetail.SourceOriginator for the latest edit.

The SourceFeatureID field is intended to hold a partner’s unique permanent identifier and enable crosswalks between the WBD and a partner’s data. The attribute is included in all feature classes except WBDLine.

ReferenceGNIS_ID is common to all HU polygon feature classes. The field holds one or more Geographic Names Information System (GNIS) IDs of the feature or features for which the HU is named. Multiple GNIS IDs in the field are organized by listing the GNIS ID of the primary feature followed by the GNIS ID of additional features, separated by commas without spaces. The field is updated and maintained by data editors.

AreaAcres is common to all HU polygon feature classes. The value in the field represents the area, in acres, of the HU polygon cast on the North America Albers Equal Area Conic projection (North American Datum, or NAD, 1983; Well Known ID, or WKID, 102008). Area calculations made when the data are cast in a different projection produce different area values. The value in this field is calculated by database administrators.

AreaSqKm is common to all HU polygon feature classes. The value in the field represents the area, in square kilometers, of the HU polygon cast on the North America Albers Equal Area Conic projection (NAD 1983; WKID 102008). Area calculations made from data cast in a different projection produce different area values. The value in this field is calculated by database administrators.

The states field is common to all polygon HU feature classes. The field is used to identify the States, territories, or countries that the HU is in or touches. Codes in the states field are based on the two-letter U.S. Postal Service State abbreviations included in the TIGER/Line Shapefiles (U.S. Census Bureau, 2021). The values, calculated by the USGS, are based on the most recent annual release of the U.S. Census Bureau TIGER/Line Shapefiles. The code “CN” identifies units that cross into or reside entirely in Canada. The code “MX” identifies units that cross into or reside entirely in Mexico. Multiple abbreviations are organized in alphabetical order and are separated by commas without spaces. The “domain” or list of accepted values in the states field is shown in table 4.

The HUC “level” holds the unique HU code associated with each HU polygon feature class (table 1) where “level” refers to the hierarchical HU levels represented by even numbers in the range 2–16. For example, in the WBDHU10 feature class this field is “HUC10” and contains the unique 10-digit HU code associated with each polygon in the WBDHU10 feature class.

Name refers to the name or names of the geographic features for which the HU is named. Guidelines for naming the HUs are provided in the “Coding and Naming Hydrologic Units” section of this document.

HUType is an abbreviation used to identify the type of drainage area the HU represents. The field is present in and is required for 10- to 16-digit HU polygons. Each HU polygon contains a single HUType code that most closely describes the HU. Acceptable HUType codes and definitions are provided in table 5. More information about HU types can be found in the “Guidelines for Delineating Hydrologic Units for the Watershed Boundary Dataset” section of this document.

The polygon HU modification (HUMod) attribute is a two-character, uppercase abbreviation included in 10- to 16-digit HU polygon feature classes. Polygon HUMod codes are used to describe the presence and type of modification that affects flow in and between hydrologic units. For example, the overbank flow modification is used if natural streamflow surpasses bankfull stage, and the water flows into a channel draining to a different hydrologic unit (fig. 3).

If more than one abbreviation is used, the values are listed in order of most to least predominant modification and the codes are separated by commas without spaces. The set of allowable HUMod abbreviations has changed over time. These changes are documented in the metadata associated with each feature or can be determined by checking with the principal WBD State steward. An additional HUMod field and separate domain are applied to the WBDLine feature class. Allowable polygon HUMod codes are provided in table 6.

The ToHUC attribute is present on the 12- to 16-digit HU polygons. The field stores the code of the HU of the same “level” that naturally receives the majority of the flow from the HU under consideration. One of the exception codes listed in table 7 is used in place of an HU code if there is no downstream HU. For example, if an HU’s outlet is to the ocean, the ToHUC field contains “OCEAN.”

The NonContributingAreaAcres attribute contains the area, in acres, of HUs or parts of HUs that do not contribute to downstream accumulation of streamflow under normal flow conditions. Noncontributing areas near the boundary between two HUs should be included in the HU that would receive flow if the area was filled with water. The value in the field is the total area of all the noncontributing areas within the HU when the HU contains more than one noncontributing area. Area calculations should be based on North America Albers Equal Area Conic projection (NAD 1983; WKID 102008). Conversion factors used in the calculation should be taken from the conversion factor table included in this document.

The NonContributingAreaSqKm attribute contains the area, in square kilometers, of HUs or parts of HUs that do not contribute to downstream accumulation of streamflow under normal flow conditions. Follow guidelines under NonContributingAreaAcres for specifications, calculation requirements, and conversion factors.

HUDigit is a domain-based field that identifies the highest HU “level” the boundary defines, where “level” is represented by even numbers in the range 2 through 16. For example, a boundary defining the divide between the two HUs 170601020106 and 170601020401 would have a value of “10” in the HUDigit field, because this is the highest HU level the boundary represents. The HUDigit is determined by comparing the two hydrologic unit codes from left to right and identifying in which placeholder the codes differ.

A domain on the geodatabase translates the field value to text. For example, the value “10” is translated to “10-digit Hydrologic Unit,” “12” is translated to “12-digit Hydrologic Unit,” and so on. The value “0” is translated to “No Data.” Values contained in this field are populated by the editor and verified by the WBD-NTC.

Like the HUMod polygon attribute, the HUMod line attribute is a two-character, uppercase text code used to identify and track either a modification to natural overland flow that alters the location of the HU boundary or other conditions that affect the HU boundary (table 9). If more than one abbreviation is used, the values are listed in order of most to least predominant modification and the codes are separated by commas without spaces.

The LineSource field contains abbreviations that identify the base-data sources used for delineating HU boundaries (table 10). The field is populated during data creation or updates. If more than one abbreviation is used, the values are listed in order of most to least recently used source with codes separated by commas without spaces. Reference and source maps not listed in table 10 should be described in the metadata.

AreaSqKm is the area, in square kilometers, of the NWISDrainageArea and NonContributingDrainageArea polygons. The field is found on both polygon feature classes. The area calculated represents the area of a polygon cast on North America Albers Equal Area Conic projection (NAD 1983; WKID 102008). Area calculations made or updated while the data are cast in a different projection will result in different area values.

Agency codes are used to identify the agency responsible for reporting data to NWIS and are assigned by the USGS. The field is present in the NWISDrainageArea and NWISDrainageLine feature classes. The value is derived from and is maintained in the NWIS database (U.S. Geological Survey, 2022). A list of agency codes is provided by NWIS at https://help.waterdata.usgs.gov/code/agency_cd_query?fmt=html.

The SiteNumber is the unique USGS identifier assigned to each site in the NWIS database (U.S. Geological Survey, 2022). SiteNumber is also included in the NWISDrainageLine feature class. The value is derived from and is maintained in the NWIS database in a text field and may contain leading zeros.

StationName is the site or streamgage name with which the NWISDrainageArea is associated. The value is derived from and is maintained in the NWIS database (U.S. Geological Survey, 2022).

TotalAreaSqMi is the total area, in square miles, of the NWISDrainageArea polygon. The area calculated represents the area of a polygon cast on the North America Albers Equal Area Conic projection (NAD 1983; WKID 102008). Area calculations made or updated for data cast in a different projection result in different area values.

NWISTotalAreaSqMi is the total gaged drainage area, in square miles, reported in the NWIS database (U.S. Geological Survey, 2022). This area may differ from the area reported in TotalAreaSqMi for a variety of reasons including errors in area delineation in either the NWISDrainageArea and the related WBD HUs or in the method used to calculate the value reported in the NWIS database, map scale differences, map interpretation differences, or delineation methodology differences. Both TotalAreaSqMi and NWISTotalAreaSqMi are provided to allow the user to independently evaluate the reported gaged drainage areas.

ContributingAreaSqMi is the total contributing area, in square miles, of the NWISDrainageArea polygon. The area calculated represents the area of a polygon cast on the North America Albers Equal Area Conic projection (NAD 1983; WKID 102008). Area calculations made or updated for data cast in a different projection result in different area values.

NWISContributingAreaSqMi is the total noncontributing area, in square miles, reported in the NWIS database (U.S. Geological Survey, 2022). This area may differ from the area reported in ContributingAreaSqMi for a variety of reasons including errors in area delineation in either the NWISDrainageArea and the related WBD HUs or in the method used to calculate the value reported in the NWIS database, map scale differences, map interpretation differences, or delineation methodology differences. Both values are provided to allow the user to independently evaluate the reported gaged areas.

The ReferenceTNMIDNHDPointEvent is the TNMID of the NHD point event representing the streamgage for which the drainage area was delineated. This field allows users to connect the NHD point-event dataset and the NWIS drainage-area dataset.

This field is reserved for comments about the drainage area or delineation process.

The NonContributingAreaSqMi is the area, in square miles, of the noncontributing area contained within the streamgage drainage area. The area calculated represents the area of a polygon cast on the North America Albers Equal Area Conic projection (NAD 1983; WKID 102008). Area calculations made or updated for data cast in a different projection result in different area values.

LengthKm is the length, in kilometers, of the line representing the gaged drainage area or the noncontributing drainage area. The length calculated represents the streamgage drainage area boundary of a polygon cast on the North America Albers Equal Area Conic projection (NAD 1983; WKID 102008). Length calculations made or updated for data cast in a different projection result in different area values.

LineSource is the source of the information used to delineate the line. The LineSource field in the NWISDrainageLine and NonContributingDrainageLine feature classes is based on the source WBDLine listed in table 10 with the exception of the source of the line connecting the streamgage to the next upstream HU.

The ExternalCrosswalk table is intended to crosswalk WBD features to one or multiple “external features” by tying the TNMID to an associated external permanent identifier. The WBD feature classes do not use this table as of 2021.

The FeatureToMetadata table contains HU feature identifiers (TNMID) and metadata processing identifiers (MetaProcessID) used to relate WBD feature classes to the table of edits completed for the HU. The relationship between the tables is included in the WBD, which is in the MetaProcessToFeature relationship class. This relationship class relates the MetaProcessDetail table to the FeatureToMetadata table through the MetaProcessID field.

The HUMod table contains HUMod values for line and polygon HUMod attributes.

The MetaProcessDetail table contains descriptions of the edits applied to a feature. Each table row contains information about an edit, including the date of the edit and contact information for the entity making the edits. The MetaProcessHasMetaSource relationship class, which is included in the WBD, relates the MetaProcessDetail table to the MetaSourceDetail table through the MetaProcessID field.

MetaSourceDetail is the metadata template where detailed information is entered about the base data used to make the edits and updates to the WBD. This information includes the type of data, the source and creator of the data, relevant dates associated with the creation of the data, the scale and accuracy of the base data, and a brief description of the source data.

The processing parameters table contains information about the schema version and resolution of the WBD.

This table is available to all National Map themes but as of 2021 is only used by the NHD.

Because 14- and 16-digit HUs are optional and are meant to provide HUs for local-scale applications, three flexible delineation approaches are provided to account for differing landscapes, environments, and partner requirements. In all cases, existing “legacy” delineations from partner agencies should be inventoried, examined, and used as the basis for new 14- and 16-digit delineations where applicable. Adjacent jurisdictions should coordinate delineations to develop consistent methods and to ensure seamless delineations. Regardless of the method used, HUs are assigned codes in accordance with the guidance provided in the “Coding and Naming Hydrologic Units” section and should follow relevant hydrologic unit delineation guidelines. Most importantly, these delineations must be complete within the bounds of the larger hierarchical unit being subdivided. For example, if any 14-digit delineation is made within a 12-digit HU, then all of the 14-digit HUs must be completed within that 12-digit HU. The same follows for 16-digit units delineated in a 14-digit unit.

The surface drainage within a standard HU boundary converges at a single outlet point. Because of the hierarchical structure of the WBD, a single HU may represent all or only part of the total surface-water drainage to the outlet point (fig. 8). When subdividing or revising a 10- to 16-digit HU, standard HUs are identified and delineated first, and larger streams typically are selected for subdivision before smaller streams so that the HUs represent the major tributaries in the hydrologic system depicted in the NHD. Standard HUs are delineated starting from the designated outlet, the point on a single stream channel where the area is drained, then proceeding up-drainage to the highest elevation of land dividing the direction of water flow. The boundary delineation then proceeds down-drainage to connect back to the designated outlet crossing perpendicular to the stream channel to complete the delineation. A standard HU boundary should be as simple as possible while capturing the topographically defined area that contributes to the outlet. Identifying standard HUs and delineating them with the recommended number and size of subdivisions can account for much of the area of the subdivided, higher level HU. Criteria for delineating nonstandard areas, such as remnant, noncontributing, or diverted areas, are described in the sections that follow.

Remnant areas around the main stem of larger streams may exist after standard HUs are delineated, even when sound hydrologic judgment and best practices are used. Remnant areas, sometimes referred to as “related contributing drainage areas” or “composite areas,” typically are wedge-shaped areas along interfluvial regions between adjacent standard HUs or as overbank areas along a stream between junctions with tributaries (fig. 9). For example, in coastal areas where radial or centripetal drainage predominates, such as in Hawaiʻi, individual streams with outlets to the ocean may create remnants that are smaller than 3,000 acres. Where feasible, these remnant areas, along with the main stem, should be aggregated into HUs that meet the size criteria for any given HU level. Additional information about nonstandard areas along shorelines is discussed in the “Hydrologic Units in Coastal Areas” section.

Frontal HUs drain to multiple outlet points along an ocean or lake coast or to a closed basin or dry lake (playa), sometimes separated by standard (single outlet) HUs flowing into the water or playa. Individual frontal HUs that do not meet delineation size criteria should be grouped together to make larger units. In ocean coastal waters, size criteria can be met by combining the area of the nearshore buffer and the frontal unit or by grouping the frontal units with nearshore or open-ocean HUs. Frontal HUs should only be aggregated if they flow into the same hydrologic feature, for example, a bay, lake, or estuary. Boundaries that truncate onshore should be extended to meet offshore boundaries, based on underlying topography, as described in detail in the “Hydrologic Units in Coastal Areas” section, provided this does not result in major alterations of the land codes.

Drainage areas that do not contribute flow toward the outlet of an HU are called noncontributing areas (fig. 10). Such areas may be due to closed basins, playas, depression lakes or potholes, dry lakebeds, or similar landforms. A noncontributing area may be designated as a closed basin HU at any level of the hierarchy if it is within the size range for that level. If a noncontributing area is near the boundary between two HUs, the noncontributing area should be included in the HU that would receive flow if the area were filled with water. Semiconfined basins that contribute surface-water flow downgradient in wet years, but act as sinks in dry years, may be defined as either standard or noncontributing HUs after review, coordination, and agreement at the State level. Assistance or consultation with climatologists or NOAA on prevailing precipitation regimes that could have a long-term influence on noncontributing areas should be explored.

A noncontributing area should not be designated as an HU if it is smaller than the general size criteria for that HU level. Multiple, disbursed noncontributing areas within a larger HU also should not be combined and designated as a unique HU. In both instances, the total acreage of noncontributing areas within an HU should be recorded in the 12-, 14-, and 16-digit and NonContributingAreaAcres and NonContributingAreaSqKm polygon attributes. If the entire HU is a noncontributing area, the NonContributingAreaAcres and NonContributingAreaSqKm attribute should be populated with the total area of the HU. Calculations should be based on the North America Albers Equal Area Conic projection (NAD 1983; WKID 102008). Conversion factors used in the calculation should be taken from the conversion factor table included in this document.

Noncontributing areas should be delineated consistently within a State. Because the precise definition of a noncontributing area varies from State to State, the criteria used to determine noncontributing areas should be documented in the metadata file, especially if a substantial number of noncontributing areas are defined.

A multiple outlet HU is similar to a standard HU but with more than one natural outlet. This classification type does not include frontal or water hydrologic units, hydrologic units with artificial interbasin transfers, or drainage outlets through karst or shallow subsurface flow. Multiple outlet HU types are commonly delineated in areas with braided streams or deltas or where tributaries split into multiple channels at the confluence with a receiving stream. For example, hydrologic units should be classified using the multiple-outlet type if a delineation crosses a braided stream or an area of complex channels with multiple water courses.

Historically, hydrologic unit delineations for the WBD in urban areas have generally been based on topographic information and reflect natural drainage conditions. Methods based on high resolution elevation and subsurface stormwater-drainage networks are being developed to allow HUs to more accurately depict both surface and subsurface drainage in built environments. In urban areas, HU12 delineations must follow surface drainage, and 14- and 16-digit delineations may depict “sewershed” or subsurface flow through a stormwater system. In urban “sewershed” 14- and 16-digit HU, downstream units may not be spatially adjacent, so development and review of downstream HU codes in the ToHUC field is complex and requires local knowledge of the drainage system. The progressive 2-digit WBD HU code system allows a 12-digit HU to be subdivided into only 99 14-digit units. The same restriction is placed on the number of 16-digit units contained within any 14-digit unit. These limits can easily be reached in urban sewershed delineations.

Areas of indeterminate flow represent areas where flow within and connections between hydrologic units are uncertain or have the potential to change. These areas may reflect terrain that is complex, such as in landscapes covered by lava flows; flat terrain, such as in large salt flats; or hydrologically complex terrain, such as in bayous or marshes. Clear flowpaths, either defined by the NHD or apparent in either elevation or imagery, may not be evident in areas of indeterminate flow. Areas of indeterminate flow may include multiple, potentially bidirectional flowpaths with no clear, single outlet, as well. Where necessary for reporting or other purposes, hydrologic units in areas of indeterminate flow may be defined using topographic information and size criteria, but these areas may also be left undivided if approved by the WBD-NTC.

Water HUs are composed predominantly of water. Water HUs are most often in areas of open ocean where the boundary has been taken out to the NOAA 3-nautical mile; in areas with large lakes where the HU boundary is the lake shoreline; or in areas of bays, sounds, and estuaries where the coastline or nearshore buffer forms the HU boundary. Other unit types that contain a large percentage of water, such as a large reservoir with the HU boundary at the dam, should be classified as standard or frontal units.

Development of HUs in coastal areas requires careful consideration because of the complex surface drainage in coastal landscapes. Water levels in coastal areas can fluctuate substantially; therefore, consideration should be given to defining boundaries by using either a nearshore buffer distance or submerged morphologic features, such as stable shoals, ridges, shore faces, and flow channels (legacy channels), if reliable and current bathymetric data are available. Transitory features, such as transient berms created by sediment flow from a tributary, should not be used.

Delineation of HU boundaries often is complicated by the presence of man-made reservoirs because the reservoir pool obscures the pre-inundation, natural drainage system. Historically, the boundaries of HUs that drain to reservoirs were delineated using bathymetric data or historical maps of legacy channels and ignored the reservoir pool (fig. 18). The 2021 delineation guidelines are to delineate outlets to the normal pool elevation if one exists. If normal pool elevation does not exist, hydrologic unit outlets should be delineated at the location where the NHD flowline enters the waterbody polygon. Historical reservoir delineations, particularly for the 2- through 8-digit unit, are to remain in the dataset to avoid substantial changes to those unit boundaries. New or updated delineations at the 10-digit or smaller units can be revised to fit the new guidelines if they do not affect larger unit boundaries.

After tributary HUs are delineated, if the remaining drainage network forms a unit larger than one of the typical size at that level, then the HU can be further subdivided at the dam or impoundment structure if the resulting units meet size criteria. If the dam is near the outlet of the major hydrologic feature, the HU boundary should be at the outlet of the main hydrologic feature and not at the dam. The boundary may also be placed at the dam if the resulting units meet size criteria for that level.

Areas where tributaries flow directly into a natural lake and are of a size consistent with the HU level being delineated should be defined as standard HUs. Nonstandard areas and standard areas that are too small to meet the size criteria and drain into a lake can be combined with the lake (fig. 19) to form a single HU composed of land and water components. If the resulting unit is larger than the size guidelines, then it can be subdivided on the basis of bathymetry or it may be left intact if the subdivision is not defensible.

Exceptions to the previous delineation rules may be made for large lakes and reservoirs that have historically documented, permanent pools. Where a legal lake elevation has been extensively used for water management and is recognized in Federal or State law, the HU delineation may be based on the legal lake elevation and designated as a “water” HU (table 5). Large lakes or reservoirs delineated in this manner, such as the Great Salt Lake in Utah, should be identified with a line modification attribute value of SL (table 9).

A unique coastal process was applied to the Great Lakes Basin. This treatment was developed by the WBD-NTC through months of coordination and collaboration with Canadian Federal and Provincial partners as well as all relevant WBD State stewards. This treatment is consistent with and complements the integration of the WBD and NHD with sister datasets in Canada, and it has been carefully developed to accommodate those datasets. The WBD State stewards should consult with the WBD-NTC prior to editing any coastal areas of the Great Lakes. Documents describing the evolution and agreed-upon method can be obtained through the WBD-NTC.

The boundary of HUs should be determined by ditches and canals only if the ditch or canal has permanently altered natural surface-water flow. Many artificial drainage features in the United States originally were either perennial or intermittent channels that local government and private entities converted to 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 HUs. If the present-day canal or ditch was once a legacy stream channel or has perennial flow, then it may be considered for delineating HUs. Small, local ditch systems constructed for seasonal diversion of water or for irrigation of agricultural fields should not be considered permanent drainages suitable for delineation.

Diverted flow describes where all or part of the flow from one HU is continuously discharged to another HU by constructed transbasin diversions. Diverted flow should be documented in the polygon 12-digit HUMod field for the water-losing HU and the water-receiving HU. Information about the date of the diversion, flow rates, and water rights for the receiving and losing HUs may be included in the metadata. Adjusting the location of HU boundaries because of interconnected flow from one HU to another during high-flow stages in streams should be avoided.

The WBD is primarily a surface-water dataset with boundaries derived from surface topography. It is not intended to accurately account for the pattern of underground seepage or flow in karst areas where flow may cross beneath topographically defined HU boundaries. Karst hydrology may be indicated on topographic maps by the presence of sinkholes, sinking streams, springs, or cave entrances or may be inferred or implied in areas where limestone, dolomite, gypsum, or salt is present near the land surface. Delineation of HU boundaries in karst areas is difficult because the surface-drainage pattern typically is disrupted by sinkholes and sinking streams, making it difficult to choose a valid HU boundary or outlet on the basis of topography alone. An HU containing karst should be identified with the modifier “KA” in the relevant HU polygon HUMod field (table 6). Additional details describing interpretation or identification of karst in the HU should be documented in the metadata.

With limited exceptions, the HUs must be named using official feature names contained in the Geographic Names Information System (GNIS; U.S. Geological Survey, undated). When used as an HU name, the name of the feature must be spelled exactly as listed in the GNIS (http://geonames.usgs.gov/). Exceptions to this rule include when the HU code is used as a placeholder for the name or where the HU crosses the international border. Hydrologic unit names may be based on Canadian or Mexican toponomy where HUs cross the international border or are entirely in neighboring countries. Names for HUs that cross the international border or are completely in neighboring countries should be coordinated with the WBD NTC.

Table 12 shows the order of priority used to assign GNIS names to HUs. Names are chosen in priority order given in the table; if no named features of a type exist in the HU, a name from the next prominent feature type should be selected. If no water or prominent physical feature is present, the HU code is used as the HU name. Exceptions may be made to the priority order shown in table 12. For example, in arid areas, a canyon may be more prominent than a hydrologic feature, such as a spring. Identical names should not be assigned within any 8-digit HU at the same HU level. All 10-digit HU names in an 8-digit unit must be unique, and all 12-digit HU names in an 8-digit unit must be unique. The USGS has developed a toolset included with the WBD desktop edit tools to assist with properly naming HUs for the WBD.

The major water feature is usually the water feature at the outlet of the HU. Stream names are preferred, but sloughs, lakes, reservoirs, dams, bays, inlets, harbors, coves, falls, and springs may be used if they are the most important feature. For example, “Crescent Lake,” “Sequim Bay,” and “Grays Harbor” can all be used as valid HU names. The primary water-feature name (for example, Imnaha River) is appended on to a secondary water-feature name (for example, Rock Creek or Dry Creek) with a hyphen in cases where the same primary-water feature exists in multiple equivalent-level HUs or if a main-stem stream is subdivided into more than one HU—for example, “Rock Creek-Imnaha River” or “Dry Creek-Imnaha River” (fig. 20).

The terms “upper,” “middle,” and “lower” are used to uniquely identify HUs where a major stream is subdivided into three HUs along the main-stem stream (for example, “Upper Imnaha River,” “Middle Imnaha River,” and “Lower Imnaha River,” fig. 20). The terms “upper,” “middle,” and “lower” should not be used individually, and “middle” should not be used with only “upper” or with only “lower.” The terms “headwaters” or “outlet” may be used with or without “upper,” “middle,” or “lower.” If a stream is subdivided into two HUs along the main-stem stream, “upper” and “lower” or “headwaters” and “outlet “may be used in pairs. The terms “upper,” “middle,” and “lower” should not be used with the hyphenated naming structure described previously. The terms “headwaters” and “outlet” may be used with the hyphenated naming structure or with the terms “upper,” “middle,” and “lower,” however. The standard hyphenated naming structure should be used rather than “headwaters,” “upper,” “middle,” “lower,” and “outlet” if a main-stem stream is subdivided into more than five HUs. See table 13 for HU naming subdivision options and examples.

The word “frontal” is reserved for coastal and lake areas that include multiple, nonconvergent streams associated with frontal HUs. In these cases, the HU name should take the form “Major hydrologic feature—frontal receiving hydrologic feature.” For example, if Squirrel Creek is the major hydrologic feature in a frontal HU that drains to Chesapeake Bay, the frontal unit is named “Squirrel Creek-Frontal Chesapeake Bay.”

Where bathymetry is used to delineate submerged morphologic features, it is possible that the resulting coastal water unit may not contain any named features other than the main water-body name (for example, Atlantic Ocean). In this instance, the name should take the form “HU code-primary water feature,” for example, 030102051406-Atlantic Ocean.

Where islands are large enough to be subdivided, the resulting HUs can be named using standard naming conventions for standard and frontal HUs. If an island is a single HU, the HU name should be the same as the island name. If islands are grouped to form a single HU, the HU name can be the name of the most prominent or major island in the group or a commonly used name for the island group.

Geometry and attributes are edited in the desktop environment using standard Esri ArcMap editing toolsets. After feature-level metadata have been created, the user creates geodatabase topology in the WBD feature dataset. Topology rules for 10- through 16-digit edits are provided in the WBD desktop edit-tool download package and can be loaded directly into the topology wizard. Once geodatabase topology has been created and validated, existing errors should be reviewed and resolved or ignored as applicable. “Gap” errors on the outer boundary of the “job” and pseudonodes already present in the “job” should be ignored. Geometry edits in the desktop environment are made to the 12-digit units and are reflected in the larger hierarchical units after the data are checked into the national database. If 14- or 16-digit units are present, they need to be updated to stay coincident with the 12-digit unit. When using the desktop tools for geometry updates or to add new HU features, polygons and lines must both be edited, and topology must be verified during and after edit sessions to ensure polygons and lines are coincident. The editor should resolve any pseudonodes introduced during editing.

The WBD can be edited outside of a WBD “job” but must be submitted to National Quality Control (NQC) by using the WBD desktop edit tools. Updates outside of a WBD “job” should be coordinated with the WBD-NTC.

The WBD attributes must be reviewed and updated in accordance with the specifications outlined in previous sections. Editors only have access to the 10-digit and smaller units. The desktop tools have functionality developed to help keep HU names and modification codes (MODs) compliant with the specification. More detail about the editing process is provided during mandatory training and during regularly scheduled WBD technical exchange meetings.

Editor validation tools are accessed through the WBD QA/QC menu in the desktop tools. These validation checks are generally run after the editing process is complete but can be used to validate edits at any time during an edit session. The “validate outer boundary” check flags outer boundary issues that need to be fixed before the job is checked in to ensure the job integrates seamlessly into the national dataset. The “validate attributes” check evaluates attributes for compliance with the WBD specifications. Errors are color coded by severity. Major errors (coded red) need to be fixed by the editor before submitting the job for NQC. Minor errors (coded yellow) need to be reviewed by the editor but are not required to be fixed.

After editing is complete in a desktop editing session, the “job” is submitted to NQC using the tool interface. The editing and QA/QC checklist provided by the WBD-NTC should be referred to before submitting data. The NQC team reviews the edits and submits the “job” for inclusion in the national database or returns the “job” to the editor for revision or modification. Rejected edits are not considered for incorporation to the national database. The process for submittal, review, and acceptance is shown in figure 21.