Salmon-Challis National Forest is located in east central sections of the U.S. state of Idaho. At 4,235,940 acres (6,618.66 sq mi, or 17,142.24 km sq) it is one of the largest national forests in the lower 48 states and also has most of the land area of the Frank Church River of No Return Wilderness, which is the largest wilderness area south of Alaska. Borah Peak, the tallest mountain in Idaho, is also found here. The Wild and Scenic Salmon River weaves through the rugged terrain of the Sawtooth Mountains and Sawtooth National Recreation Area, while it flows for over 75 miles (121 km) through the forest. The Big Lost River has its headwaters within the national forest.