From the PCTA: 

"Trails for America" formed the basis for the original language of what was to become the National Trails System Act. Passed by Congress on October 2, 1968, the Act called for the creation of"national scenic trails which will be extended trails so located as to provide for maximum outdoor recreation potential and for the conservation and enjoyment of the nationally significant scenic, historic, natural, or cultural qualities of the areas though which such trails may pass."

However, in acknowledging the concerns of resource extractors and land developers, Congress significantly altered the suggested language in relation to the way the trail would influence the management of adjacent lands, ". . in selecting the rights-of-way, full consideration shall be given to minimizing the adverse effects upon the adjacent landowner or user and his operation. Development and management of each segment of the National Trails System shall be designed to harmonize with and complement any established multiple-use plans for the specific area in order to ensure continued maximum benefits from the land."

 The Pacific Crest and Appalachian were the nation's first National Scenic Trails.

The Act gave the USDA Forest Service management responsibility for the Pacific Crest Trail. A Pacific Crest National Scenic Trail Advisory Council, representing a diverse group of trail users, but dominated by land managers and private land owners, developed and recommended for approval the "Pacific Crest Trail Guide for Location, Design and Management" based on the language of the Act.

Read more at PCTA.org