"The data you have secured from the U.S. Geological Survey (USGS) database identified as Mainstem Rivers of the Conterminous United States have not received USGS approval and as such are provisional and subject to revision. The data are released on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from its authorized or unauthorized use." "Although these data have been processed successfully on a computer system at the U.S. Geological Survey (USGS), no warranty expressed or implied is made regarding the display or utility of the data for other purposes, nor on all computer systems, nor shall the act of distribution constitute any such warranty. The USGS or the U.S. Government shall not be held liable for improper or incorrect use of the data described and/or contained herein." 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 geodatabase 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.