What is the process to amend the lateral boundary of a wild and scenic river?

Amending the lateral boundary of a designated river requires analysis and decision under the National Environmental Policy Act (NEPA). The environmental analysis process need not be complicated and may only require the proposed action and a no-action alternative. Amendment of a wild and scenic river boundary may also be timed with revision of the comprehensive river management plan and related decision under the NEPA. In either case, notice of the amended boundary should be published in the Federal Register and the legal description and maps forwarded to Congress and made available for public inspection in conformance with the Wild & Scenic Rivers Act. The 90-day review requirement in Section 3(b) provides a chance for the public and/or members of Congress to express concern, if any, about the boundary proposal.

Source
Interagency Wild & Scenic Rivers Council