The San Saba River rises in two branches. The North Valley Prong of the San Saba rises four miles southeast of Eldorado in central Schleicher County (at 30°50' N, 100°35' W) and runs east for thirty-seven miles to join the Middle Valley Prong, which begins twelve miles northeast of Sonora in northern Sutton County (at 30°39' N, 100°30' W) and runs northeast for thirty-five miles. The San Saba River proper begins at the eastern edge of Schleicher County at the confluence of these two branches, about two miles west of Fort McKavett (at 30°50' N, 100°07' W). It flows northeast for about 140 miles and drains an area of 3,150 square miles, passing through parts of Menard, Mason, McCulloch, and San Saba counties before reaching its mouth on the Colorado River at the eastern edge of San Saba County, eight miles northeast of the town of San Saba (at 31°15' N, 98°36' W). Its major tributaries are Terrett Draw and Brady Creek. The terrain through which the river flows is generally flat with local shallow depressions; the soil is made up of clay and sandy loams; and the vegetation consists primarily of water-tolerant hardwoods, conifers, and grasses. In 1732 Juan Antonio Bustillo y Ceballos, governor of Spanish Texas, led a group of soldiers west of the Colorado River and discovered another stream that he named Río de San Sabá de las Nueces. It has been suggested that he chose the name because he and his party reached the river on the day of St. Sabbás, a sixth-century monk. In 1757 on the banks of the river the Spanish established Santa Cruz de San Sabá Mission and, to protect the mission, San Luis de las Amarillas Presidio. The United States War Department built Camp San Saba (later called Fort McKavett) in 1852 in order to protect frontier settlers from Indians. Later communities established along the river included Menard, Camp San Saba, Voca, Harkeyville, and San Saba.