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.