On March 1, 1872 President Ulysses S. Grant signed the Yellowstone National Park Protection Act into law. The world now had its first national park! This law was the result of the national interest being piqued by several expeditions to study and explore Yellowstone during the prior decade.
Expeditions in 1869 and 1870 served to drive interest amongst scientists and conservationists regarding thermal activity and the abundant species and natural beauty of the Park. The most impactful expedition perhaps was the 1871 Hayden Expedition, led by Ferdinand Hayden, leader of the US Geological and Geographical Survey of the Territories. Hayden’s team was focused on exploring Yellowstone from a scientific perspective and the group included botanists and scientists specializing in meteorology, ornithology, zoology, mineralogy and agricultural studies.
Photographer William Henry Jackson and artists Henry W. Elliot and Thomas Moran also accompanied Hayden, and their dramatic photographs, paintings and sketches helped many in Congress understand the natural significance of Yellowstone and the importance of protecting it.
Grand Canyon of the Yellowstone, Thomas Moran, 1871. Credit: PicturesNow 
Old Faithful, Yellowstone National Park, Wyoming c1899. By photographer William Henry Jackson 1843-1942. Credit: Universal History Archive
Montana politician and businessman Nathaniel P. Langford (a member of the 1870 expedition) pushed Congress to consider an act protecting Yellowstone in 1871 and Congress was further inspired and persuaded by the Hayden Expedition’s reports to act. The Yellowstone National Park Protection Act was created to reserve the Park as “a public park or pleasuring-ground for the benefit and enjoyment of the people”.
Today, Yellowstone is nearly 3,500 square miles in size and although mainly in Wyoming, parts of the Park lie in neighboring Montana and Idaho. More than 4 million people visit Yellowstone annually to explore its dramatic scenery, marvel at Old Faithful and other geysers and thermal pools, and photograph the hundreds of animal species that call the Park home, including bison, bears and wolves.
Steam vents rise from river in Yellowstone National Park, USA. Credit: Nano Calvo/VW Pics 
Lower Falls, Yellowstone National Park, Wyoming, USA. Credit: Claudio Beduschi/REDA&CO 
Winter landscape in Yellowstone National Park, USA. Credit: Nano Calvo/VW Pics 
Yellowstone National Park, Wyoming, USA. Credit: Claudio Beduschi/REDA&CO 
Lupine, sage, and rabbitbrush in Lamar Valley, Yellowstone National Park, Wyoming, USA. Credit: Greg Vaughn/VW Pics 
USA Yellowstone National Park Wyoming Mammoth Hot Springs. Credit: Gregg Waddell/REDA&CO
For more information about Yellowstone and to plan your trip there, visit:
https://www.nps.gov/yell/index.htm
Source: US National Park Service http://www.nps.gov