<P> The United States has 60 protected areas known as national parks that are operated by the National Park Service, an agency of the Department of the Interior . National parks must be established by an act of the United States Congress . A bill creating the first national park, Yellowstone, was signed into law by President Ulysses S. Grant in 1872, followed by Mackinac National Park in 1875 (decommissioned in 1895), and then Rock Creek Park (later merged into National Capital Parks), Sequoia and Yosemite in 1890 . The Organic Act of 1916 created the National Park Service "to conserve the scenery and the natural and historic objects and wildlife 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 ." Many current national parks had been previously protected as national monuments by the president under the Antiquities Act before being upgraded by Congress . Seven national parks (including six in Alaska) are paired with a national preserve, areas with different levels of protection that are administered together but considered separate units and whose areas are not included in the figures below . Criteria for the selection of national parks include natural beauty, unique geological features, unusual ecosystems, and recreational opportunities (though these criteria are not always considered together). National monuments, on the other hand, are frequently chosen for their historical or archaeological significance . Fourteen national parks are designated UNESCO World Heritage Sites (WHS), while 21 national parks are designated UNESCO Biosphere Reserves (BR). Eight national parks are designated in both UNESCO programs . </P> <P> Twenty - eight states have national parks, as do the territories of American Samoa and the United States Virgin Islands . California has the most (nine), followed by Alaska (eight), Utah (five), and Colorado (four). The largest national park is Wrangell--St. Elias in Alaska: at over 8 million acres (32,375 km), it is larger than each of the nine smallest states . The next three largest parks are also in Alaska . The smallest park is Gateway Arch National Park, Missouri, at approximately 192.83 acres (0.7804 km). The total area protected by national parks is approximately 52.2 million acres (211,000 km), for an average of 870 thousand acres (3,500 km) but a median of only 229 thousand acres (930 km). </P>

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