Network Attributes for the High Resolution National Hydrography Dataset (NHD) based initial 3D Hydrography Program Network

This dataset contains the initial 3D Hydrography Program (3DHP) flow network topology, which is based on the final High Resolution National Hydrography Dataset (NHD), and a set of mainstem river identifiers based on the Medium resolution NHDPlus Version 2 (NHDPlusV2). The 3DHP flow network topology and mainstem identifiers contained in this data release add two key pieces of information to the flow table of the National Hydrography Dataset:
1) at every junction, one and only one main upstream and downstream connection has been identified
2) every "in network" flowline has been assigned a mainstem identifier that places it into a hierarchy of rivers and drainage basins.

For description of 1 and 2 and additional background, see supplemental information.

Files included:
- HU6_mainstem_outlets.geojson
Outlet locations of six-digit hydrologic units along mainstem network.

- nhd_3dhp_conus_mainstems.zip
Zipped table in Comma Separated Values (csv) format containing NHD permanent identifiers and mainstem Uniform Resource Identifiers (URIs) for the Contiguous United States (CONUS).

- nhd_3dhp_conus_flownetwork.zip
Zipped csv table containing from / to NHD permanent identifiers (from flowline to flowline) and upmain / downmain attributes for CONUS.

- nhd_3dhp_conus_vpu_membership.zip
Zipped csv table containing NHD permanent identifiers and which Vector Processing Unit (VPU) they belong in.

- nhd_3dhp_oconus_mainstems.zip
Zipped csv table containing NHD permanent identifiers and mainstem URIs for areas Outside of the Contiguous United States (OCONUS).

- nhd_3dhp_oconus_flownetwork.zip
Zipped csv table containing from / to NHD permanent identifiers (from flowline to flowline) and upmain / downmain attributes for OCONUS.

-3dhp_flownetwork.zip
Zipped csv table containing from / to 3DHP identifiers (from flowline to flowline) and upmain / downmain attributes.

- nhd_3dhp_nonnetwork_mainstems.zip
Zipped csv table containing mainstem identifiers for features not included in the NHD flow table.

- nhd_final_registry.zip
Zipped csv table containing all expected flowlines from the final NHD -- excludes geometry.

- nhd_reachcode_mainstem.zip
Zipped csv table containing a mapping between mainstem identifiers and NHD reachcodes.

- nhd_3dhp_region_connections.zip
Zipped csv table containing connections between vector processing unit regions defined by four-digit hydrologic unit.

- nhd_3dhp_mainstem_summary.zip
Zipped csv table containing summary information about mainstems contained in nhd_3dhp_conus_mainstems.zip and nhd_3dhp_oconus_mainstems.zip.

Suplemental Information:
Upstream main and downstream main attributes have been derived from interpretation of NHD flowline names and a physical characteristic known as "arbolate sum". Names are derived from the U.S. Geographic Names Information System (GNIS) and arbolate sum is the sum of the lengths of all upstream flowlines along a dendritic representation of the network. This methodology follows that of NHDPlus closely.

Assignment of mainstem attributes was accomplished in three ways. 1) Rivers that cross four-digit hydrologic unit boundaries (~130 rivers) were identified with manual headwater selection and a manual validation of dendritic network traces. 2) A collection of ~30,000 previously selected "reference mainstems" from NHDPlusV2 were matched to the network using an automated method (Blodgett and others, 2021) and targeted manual validation. 3) All rivers not matched to the previously selected reference mainstems were assigned mainstem identifiers based on their "levelpath" within a network including both selected major mainstems (1) and reference mainstems (2).

An important secondary outcome of this work is a network-based domain decomposition (mutually exclusive subsets based on network connectivity) of the NHD flow table. This domain decomposition identifies rivers that cross major region boundaries separately from flowlines that cross region boundaries and are not along a major river. The second class of connection is diverse--containing true interbasin transfers, flowlines that straddle a divide and have ambiguous or incorrect flow direction, and incidental region-crossing connections that may or may not be driven by a hydrologic gradient. These secondary connections are, for the purposes of mainstem identifiers and ability to scale processing and data production, disconnected in the flownetwork attribute table. All major and minor connections are listed in an auxiliary table in this data release for future reference.

Blodgett, D.L., Johnson, J. M., Sondheim, M., Wieczorek, M.E., and Frazier, N., 2021, Mainstems: A logical data model implementing mainstem and drainage basin feature types based on WaterML2 Part 3: HY Features concepts: Environmental Modelling and Software, v. 135, https://doi.org/10.1016/j.envsoft.2020.104927