Mainstem rivers are the backbone of a connected network of hydrologic units that cover the landscape. A mainstem connects a headwater source area to an outlet. This data release identifies the same mainstem paths in hydrographic datasets for the conterminous US. The Mainstems dataset includes cross walks between mainstem identifiers and several hydrographic datasets. These cross-walk tables do not include geometry. It also includes a summary of all mainstems with headwater and outlet identifiers from other datasets for those mainstems that could be matched. The summary table has highly simplified geometry and should be used for visualization purposes only. Any analysis should rely on source dataset geometry. As a convenience, a GeoPackage of 12-digit hydrologic unit code watershed outlets is included as points with attributes that indicate the "event" linear reference along the NHDPlusV2 network as well as how the 12-digit hydrologic units nest within the 10, 8, 6, 4, and 2 digit hydrologic unit system. This file is based on the stable NHDPlusV2 snapshot of the Watershed Boundary Dataset.