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 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 have 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 csv table containing NHD CONUS permanent identifiers and mainstem URIs. - nhd_3dhp_conus_flownetwork.zip Zipped csv table containing from / to NHD permanent identifiers (from flowline to flowline) and upmain / downmain attributes. - nhd_3dhp_conus_vpu_membership.zip Zipped csv table containing NHD permanent identifiers and which VPU they belong in. - nhd_3dhp_oconus_mainstems.zip Zipped csv table containing NHD oconus permanent identifiers and mainstem URIs. - nhd_3dhp_oconus_flownetwork.zip Zipped csv table containing from / to NHD oconus permanent identifiers (from flowline to flowline) and upmain / downmain attributes. -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 ids for features not included in the NHD flowtable. - 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.