The United States Entity Supplemental Report is a companion report to the Columbia River Treaty Phase 1 Report. Unlike the Phase 1 Report, which was prepared jointly by the Canadian and United States Treaty Entities, the Supplemental Report was produced by the Bonneville Power Administration (BPA) and the U.S. Army Corps of Engineers (Corps) on behalf of the U.S. Entity. It builds on the information compiled in the joint Phase 1 studies and is intended to be read in conjunction with the Phase 1 Report. The purpose of the Supplemental Report is to provide the results of additional studies (Supplemental studies) conducted by the U.S. Entity in which Endangered Species Act (ESA) Biological Opinions (BiOp) and other fish operations were added to the Phase 1 studies. These are important drivers of U.S. reservoir operations and including them in discussions of post-2024 Treaty scenarios represents a more realistic picture of the operation of U.S. projects. The information in the Supplemental Report will inform regional discussions to help develop additional studies for looking at potential post-2024 Treaty scenarios.