APA (7th ed.) Citation

United States. National Park Service. (2018). National park system (Last updated 2017.). National Park Service, U.S. Department of the Interior.

Chicago Style (17th ed.) Citation

United States. National Park Service. National Park System. Last updated 2017. [Washington, D.C.]: National Park Service, U.S. Department of the Interior, 2018.

MLA (9th ed.) Citation

United States. National Park Service. National Park System. Last updated 2017. National Park Service, U.S. Department of the Interior, 2018.