On April 20, 1832, President Andrew Jackson set aside the hot springs. On March 4, 1921, the reservation became Hot Springs National Park, the 18th national park.
The history page says the 1832 law reserved four sections of land, including the springs, for the future disposal of the United States and not for any other purpose. It calls this the oldest national park among current units, forty years before Yellowstone, and it also says Congress passed no administration for years, so people built over the springs anyway.
A centennial release says 47 hot springs emerge at the base of Hot Springs Mountain, and that the water is about 4,400 years old. A brochure puts the forested park at 5,500 acres around the downtown. The homepage says there are 26 miles of hiking trails.