Rio Grande Wild and Scenic River Extension Act of 2008 - Amends the Wild and Scenic Rivers Act to modify the boundary of the Rio Grande Wild and Scenic River.
Prohibits anything in this Act from: (1) being construed to conflict with specified treaties between the United States and Mexico; and (2) preventing or obstructing the planning, construction, operation, or maintenance of a border fence in the area or affecting the operations or duties in such area of the Department of Homeland Security, including the Border Patrol, or of state or local law enforcement agencies.