NPS Organic Act
(16 U.S.C. 1) directs the NPS:
"[T]o conserve the scenery and the natural and historic objects and wild life therein and to provide for the enjoyment of the same in such manner and by such means as will leave them unimpaired for the enjoyment of future generations."
Congress has also told the NPS that:
"[T]he authorization of activities shall be construed and the protection, management, and administration of these areas shall be conducted in light of the high public value and integrity of the National Park System and shall not be exercised in derogation of the values and purposes for which these various areas have been established, except as may have been or shall be directly and specifically provided by Congress." 16 U.S.C. 1a-1.
"The Organic Act and the Stewardship of Resources within Park Boundaries", by Michael Mantel and Philip Metzger.
Enabling legislation for each unit of the National Park System may include additional congressional direction specific to a park, but the Organic Act mandate applies to all units unless Congress specifically directs otherwise.