The Council has been directed by the Congress, in the Conference Report accompanying the Energy and Water Development Appropriations Act for the Fiscal Year 1998, to review "the major fish mitigation capital construction activities proposed for implementation at the Federal dams in the Columbia River Basin." The Council was directed to conduct this review with the assistance of the Independent Scientific Advisory Board (ISAB).The Council identified the Corps’ Dissolved Gas Abatement Program as one of five specific projects for which it requested assistance from the ISAB (Ruff 1998). Other specific projects included installation of extended-length screens at John Day Dam and at other dams, further development and testing of the surface bypass prototype at Lower Granite Dam and at other dams, Bonneville Dam juvenile fish passage improvements including the relocation of a bypass outfall, and adult fish passage improvements. Reports summarizing the ISAB evaluations of (1) extended-length screens and the Bonneville Dam juvenile bypass outfall (ISAB Report 98-4) and (2) surface bypass development (ISAB Report 98-7) have already been provided (ISAB 1998a,b).The ISAB, as generally directed by the Council (Ruff 1998), has attempted to:evaluate how the Gas Abatement Program at mainstem dams fits in the context of the Columbia River ecosystem; evaluate the effectiveness of the Program to mitigate for the effects of mainstem hydroelectric dams (including both positive and negative aspects); identify major, relevant uncertainties or research questions; and evaluate how the scientific uncertainties affect the use and management of gas abatement measures under several scenarios of hydrosystem reconfiguration and over several time frames. This memorandum presents the outcomes of those evaluations. We were aided in our review by a briefing by the Corps and concerned agencies on July 15, 1998, responses by the Corps to our specific written questions, and numerous reports and reprints of published literature.