Stream acidification is an increasingly serious threat to the survival of brook trout. Streams in GRSM naturally become more acidic at higher elevations. However, research shows that areas in the park above 4,800 feet in elevation are receiving some of the highest amounts of acid rain, acid fog, and dry deposition of any monitored site in North America. Consequently, many high elevation streams are becoming increasingly acidic, some to the point that they can no longer support brook trout. In 1996, genetic research conducted by the University of Tennessee confirmed long-held beliefs that the southern Appalachian brook trout is a distinct subspecies, differing in several ways from the stocked fish native to the northeastern United States and Canada. Only about 15% of park streams are believed to harbor pure strains of truly native fish, undiluted by hybridization with northern brook trout. Since the southern subspecies has such a limited range, and because that range has been diminished drastically in the last 100 years, the southern brook trout is considered a species of special concern. The Park Service is especially watchful of acidification and competition from rainbow trout in streams where the subspecies currently survives. The brook trout (Salvelinus fontinalis) is the only trout native to the southern Appalachian Mountains. It was once widespread in Great Smoky Mountains National Park (GRSM), occupying small, swift, high elevation mountain streams as well as larger rivers down to elevations of 1,600 feet. Since 1900, the brook trout has vanished from about 75% of its historical range in GRSM. The initial loss was due to large-scale mechanized logging operations early in the 20th century. Clear cuts and subsequent fires on steep mountainsides led to soil run-off and stream siltation. In addition, trees were cut from stream banks, allowing more sunlight to reach the waterways and raising water temperatures.