On May 7, 2007, Idaho Power Company and PacifiCorp (doing business as Rocky Mountain Power), collectively known as the Proponents, applied to the Bureau of Land Management (BLM) for a right-of-way (ROW) grant to use the National System of Public Lands for portions of the Gateway West Transmission Line Project (Gateway West or Project). The original application was revised in October 2007, August 2008, May 2009, and January 2010 to reflect changes and refinements in their proposed Project and in response to feedback from the public regarding routing alternatives. The Plan of Development (POD) has been revised several times in response to Project changes and recommendations from the BLM, other reviewing agencies, and public comment. This supplemental environmental impact statement (SEIS) evaluates the revised proposed action for Segments 8 and 9 as stated in the application including environmental protection measures. It also examines the environmental impacts of four other route alignments and two route variations. The BLM has identified seven action alternatives, two of which have been selected as Co-Preferred Alternatives by the BLM. Granting of the ROW for the Revised Proposed Routes or other route alignments would require amendments to BLM Resource Management Plans and BLM Management Framework Plans. Proposed amendments have been identified. Significant impacts were identified from construction and operations of the transmission line on historical resources (historic trails), visual quality, and cumulative impacts on several resources based on past and present levels of disturbance. A framework for compensatory mitigation has been added.