Mammoth Spring, on Arkansas’s northern edge in the Ozark Plateau, is the state’s largest spring and one of the Ozarks’ defining natural features. Fed by rainfall in southern Missouri moving through underground passages, it rises in a 10-acre pool and quickly joins the Warm Fork to form the Spring River. Its cold, steady flow has long supported trout fishing, river recreation, and earlier industry, powering mills from early European settlement and later a hydroelectric plant that operated from 1925 to 1972. Now within Mammoth Spring State Park, it was recognized as a National Natural Landmark in June 1972 for both its scale and significance.