Hydrologic Units of the United States

What are hydrologic units

Hydrologic units (HUs) represent the area of the landscape that drains to a portion of the stream network. More specifically, a hydrologic unit defines the areal extent of surface water drainage to an outlet point on a dendritic stream network or to multiple outlet points where the stream network is not dendritic. A hydrologic unit may represent all or part of the total drainage area to an outlet point so that multiple hydrologic units may be required to define the entire drainage area upstream of a given outlet.

Hydrologic units provide a mechanism for summarizing landscape characteristics, reporting on water resource information including water availability, water quality, point source or non-point source data within a unit, and a variety of other applications.

History of Hydrologic Units of the United States

Historically, a variety of hydrologic unit maps and other geospatial data provided a framework for hydrologic boundary information for the United States. Recognizing that a standardized system for organizing and collecting hydrologic unit data was needed, the United States Geological Survey (USGS) began a process in the early-1970s to produce hydrologic unit maps for the United States. These maps were produced at a scale of 1:500,000 and included hydrologic unit codes and names for four nested levels of hydrologic units (HU). The four nested levels of hydrologic units were assigned a unique code starting with the largest 1st level units being assigned a 2-digit code and each additional sub-divided level having an additional 2 digits appended to the previous levels code. The United States was subdivided into 19 2-digit HUs (1st level of subdivision), 222 4-digit HUs (second level of subdivision), 352 6-digit HUs (third level of subdivision), and approximately 2,150 8-digit HUs (fourth level of subdivision) (Seaber and others, 1987 ).

While the 8-digit (4th level of subdivision) hydrologic units were broadly used, it became obvious that these units were often too large to serve many users’ needs. In the mid- to late-1990’s the Natural Resources Conservation Service (NRCS), USGS and state partners began a national initiative to develop two additional nested levels of hydrologic units (HUs). The formalization of this nation-wide data effort became the Watershed Boundary Dataset (WBD). For more information on the history of the development of the WBD see Techniques and Methods 11-A3 - Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD) (usgs.gov)

What is the Watershed Boundary Dataset (WBD)

The Watershed Boundary Dataset (WBD) is a seamless, national, hydrologic unit dataset that provides a standardized base for water-resources organizations to locate, store, retrieve, and exchange hydrologic data; to index and inventory hydrologic data and information; to catalog water-data acquisition activities; and to use in a variety of other applications. Hydrologic unit boundaries in the WBD are determined based on topographic, hydrologic, and other relevant landscape characteristics without regard for administrative, political, or jurisdictional boundaries. (Watershed Boundary Dataset website/standards | U.S. Geological Survey )

The HUs in the WBD are arranged in a nested, hierarchical system with each HU in the system identified using a unique hydrologic unit code (HUC). Each HU within a nested level is assigned a two-digit suffix that’s appended to the HUC of the HU in the next coarsest nesting level. Because there are eight nesting levels within WBD, the set of HUCs consists of ranges from two to sixteen digits based on the eight levels of classification in the WBD. The dataset is complete for the United States to the 12-digit hydrologic unit. The 14- and 16-digit hydrologic units have only been published for a subset of the nation. (Watershed Boundary Dataset structure visualization | U.S. Geological Survey )

Although the WBD was completed for the nation in 2012, the data has undergone ongoing data improvement activities that updated the original delineations and revised attribution in many areas. These improvements fall into the categories of attribution changes, geometry changes and schema changes. Specific examples include but are not limited to; revisions to the geometry of the WBD based on higher resolution base data (including Interferometric synthetic aperture radar (IfSAR) and Light Detection and Ranging (lidar)-based Digital Elevation Models (DEMs)), harmonization with and inclusion of full drainage basins for units that span the U.S./Canada and U.S./Mexico borders, review and revision to hydrologic unit names to meet the WBD standards and specifications, and addition of new unit types to reflect different terrains and hydrologic conditions.

For more information about the WBD see Watershed Boundary Dataset | U.S. Geological Survey

Moving forward, drainage areas derived from high-resolution elevation data, roughly equivalent to 12-digit WBD hydrologic units, will be developed as part of 3DHP (3D Hydrography Program 3DHP_all: DrainageArea | U.S. Geological Survey ).

WBD 2024 Snapshot

A map to select HUCs from the 2024 WBD snapshot will be added here as soon as available.

In 2024, external editing of the WBD was completed and the WBD National Technical team worked to complete outstanding review of partner edits, as well as improvements to attribution, geometry, and topology in the national dataset. The 2024 version of the WBD includes all of the WBD updates that were ingested into the national dataset between 2012-2024. This snapshot of the WBD is the official version from the USGS WBD program. (Access National Hydrography Products | U.S. Geological Survey )

NHDPlus V2 WBD Snapshot

The US Environmental Protection agency (EPA) developed the National Hydrography Dataset Plus (NHDPlus) V2 in partnership with the U.S. Geological Survey (USGS) between 2010 and 2016. Several snapshots of the WBD were used in the production of the NHDPlus V2 data as the process took multiple years to complete. For the conterminous United States, nine different WBD snapshots were used, dated from August 2010-February 2012. Between 2012 and 2016, NHDPlus data was developed for the state of Hawaii and U.S. territories including Puerto Rico, the Virgin Islands, Guam, Saipan, and America Samoa, using WBD snapshots dated from February 2012- February 2016. NHDPlus V2 data is not available for Alaska.

Two key components of the WBD were used in the development of NHDPlus V2. The 4-digit hydrologic units were used as the vector processing units, or outer extent, for production of the data while the 12-digit hydrologic units were used in the pre-processing of the raster datasets as built-in walls forcing the flow across the landscape to be contained within the hydrologic units where appropriate.

https://www.epa.gov/waterdata/get-nhdplus-national-hydrography-dataset-plus-data

NHDPlus HR WBD Snapshot

A map to select HUCs from the NHDPlusHR WBD snapshot will be added here as soon as available.

Starting in 2016 the USGS began developing a NHDPlus High Resolution (NHDPlus HR) dataset that built upon the data model, processes, and tools used in the development of the NHDPlus V2 data. The high-resolution National Hydrography Dataset (NHD) and WBD were used in conjunction with the 10 meter-resolution 3D Elevation Product (3DEP) DEM data in the development of the NHDPlus HR.

The version of the WBD used with the NHDPlus HR initial data release includes WBD snapshots from 2018-2021. Updated NHDPlus HR data was released for regions 01, 02, 06, 14, 15 and 16 in 2022 and 2023. These regions were reprocessed using updated software, NHD, WBD and 3DEP data.

The WBD contained within the NHDPlus HR includes many updates that were not available within the NHDPlus V2 data. Updates include the addition of full 8-digit hydrologic units for international basins that span the U.S.-Canada border and the U.S.-Mexico border. In addition, areas that are fully within Canada and Mexico that contribute flow to the United States were added to the WBD as HU8 boundaries. (National Hydrography Products )

National Water Availability Assessment Snapshot

In 2020, USGS used a snapshot of the latest available version of the WBD in their hydrologic and water use models that were included in the first national water availability assessment (see Integrated Water Availability Assessments ). This was done to take advantage of updates and additions that were not available in the NHDPlusV2 version of the dataset. The national water availability assessments estimate how much water is available for human and ecological needs in the United States and explores where, when, and why the Nation may have challenges meeting its demand for water. The assessments currently use HU12 units as the spatial summary layer.

The snapshot is available as a component of a data release which links HU12 units to the river network:

Blodgett, D.L., 2023, Mainstem Rivers of the Conterminous United States (ver. 2.0, February 2023): U.S. Geological Survey data release, https://doi.org/10.5066/P92U7ZUT .

Major differences between WBD versions

Region 01 Updates, New England Region

• Massachusetts, Rhode Island, and outlet of Long Island Sound 8-digit HUs were extended to the Three Nautical Mile (3NM) limit as defined by the National Oceanic and Atmospheric Administration (NOAA) and are included in the 2020 WBD, 2024 WBD, and NHDPlus HR but are not included in the NHDPlus V2.
• The 2020 WBD, 2024 WBD and NHDPlus HR data include full 8-digit units for the following cross-border HUs; 01010009, 01010010, 01010011 and 01050004. The Canadian portion of these units is not available in the NHDPlus V2 data.

Region 03 (Mid Atlantic Region)

• Coastal areas in Region 03 have all been standardized to the 2015 version of the NOAA3NM in the 2020 WBD, 2024 WBD, and NHDPlus HR but have not been extended in NHDPlus V2.

Region 04 (Great Lakes Region)

• The Great Lakes and their northern Canadian contributing areas are included in the 2020 WBD (except for part of 0427; see bullet below for additional details), the 2024 WBD, and NHDPlus HR but are not included in NHDPlus V2.
• The majority of Lake Ontario and Niagara Peninsula, 04270102-04270205, are included in the 2024 WBD and NHDPlus HR but are not included in the 2020 WBD or NHDPlus V2

Region 08 (Lower Mississippi Region)

• In 2015, additional 12-digit subdivisions in 08030207 and 08030209 were added using lidar based dem’s as the source. These additional 12-digit HUs are included in the 2020 WBD, 2024 WBD, and NHDPlus HR but are not included in the NHDPlus V2.

Region 09 (Souris-Red-Rainy Region)

• Canadian Contributing areas in 0904 are included in the 2020 and 2024 WBD but are not included in NHDPlus V2 or NHD Plus HR.
• Hydrologic units 09010009, 09020317, 09020318, 09020319, 09040003 and 09040004, which are entirely in Canada, are included in the 2020 WBD, 2024 WBD and NHDPlus HR but are not included in the NhdPlus V2 data.

Region 12 (Texas-Gulf Region)

• Coastal areas in region 12 have all been standardized to the 2015 version of the NOAA3NM in the 2020 WBD, 2024 WBD, and NHDPlus HR but have not been extended in NHDPlus V2.

Region 13 (Rio Grande Region)

• Region 13 stops at the US/Mexico border in NHDPlus V2, while the areas that comprise the cross-border 8-digit HUs are included in the 2020 WBD, 2024 WBD, and NHDPlus HR, but are not included in NHDPlus v2.
• Hydrologic units 13100000, 13110000, and 13120000, which are entirely in Mexico, are included in the 2020 WBD and 2024 WBD but are not included in NHDPlus V2 or NHDPlus HR.

Region 15 (Lower Colorado Region)

• Region 15 stops at the US/ Mexico border in NHDPlus V2. Areas that comprise the rest of the cross-border 8-digit HUs are included in the 2020 WBD, 2024 WBD, and NHDPlus HR.

Region 17 (Pacific Northwest Region)

• Canadian Contributing areas in 1701 and 1702 are included in the 2020 WBD, 2024 WBD, and NHD Plus HR but are not included in NHDPlus V2.
• Hydrologic units 17010107, 17010108, 17010109, 17010110, 17020018, 17020019, 17020020, and 17020021, which are entirely in Canada, are included in the 2020 WBD, 2024 WBD and NHDPlus HR but are not included in the NHDPlus V2 data.

Region 19, Alaska Region

• Region 19 is not included in NHDPlus V2.1902010, 1902020, 1902030, 1902050, 1902060, 1902080, 1905040, 1906050, 1906010, 1907040, and 1908030 are the only HU8s included in the NHDPlus HR.
• Hydrologic units 19010108, 19010109, 19010110, 19010111, 19010112, 19010113, 19010114, 19010115, 19010213, 19010214, 19010215, 19010216, 19010217, 19010218, 19010219, 19010220, 19010221, 19010222, 19010223, 19010224, 19010226, 19010305, 19010306, 19010307, 19010308, 19010401, 19070105, 19070106, 19070107, 19070108, 19070109, 19070110, 19070201, 19070202, 19070203, 19070301, 19070302, 19070303, 19070304, 19070401, 19070404, 19070501, 19080101 and 19080102, which are entirely in Canada, are included in the 2024 WBD but are not included in the 2020 WBD, NHDPlus HR, or NHDPlus V2 data.

Region 20, Hawaii Region

• Additional hydrologic units have been added to encompass all US islands and territories in the 2024 WBD but are not included in the 2020 WBD, NHDPlus V2, or NHDPlus HR.
• Coastal areas in region 20 have all been standardized to the 2015 version of the NOAA3NM in the 2020 WBD, 2024 WBD, and NHDPlus HR but have not been extended in NHDPlus V2.

Region 22(South Pacific Region)

• Additional hydrologic units have been added to encompass all US islands and territories in the 2024 WBD and NHDPlus HR but are not included in the 2020 WBD or NHDPlus V2.
• Coastal areas in region 22 have all been standardized to the 2015 version of the NOAA3NM in the 2020 WBD, 2024 WBD, and NHDPlus HR but have not been extended in NHDPlus V2.