The Blanco River rises from springs three miles south of the Gillespie county line in northeastern Kendall County (at 30°05' N, 98°42' W) and flows southeast for eighty-seven miles, through the Hill Country counties of Blanco and Hays, to its mouth on the San Marcos River, inside the San Marcos city limits (at 29°52' N, 97°55' W). The Blanco is part of the Guadalupe River basin and has a drainage area of over 400 square miles. In 1721 members of the Aguayo expedition named the river for the white limestone along the banks and in the streambed. Other early Spanish expeditions reportedly crossed the Blanco, including those of Pedro Vial in 1786 and José Mares in 1788. Indians, including Comanches and Apaches, inhabited the region along the river well into the 1850s. Bartlett Sims first surveyed the land along the Blanco River in what is now Blanco County in 1835, and land grants were made during the period of the Republic of Texas. By the mid-1840s the first settlers had come to the region. Blanco County was formed in 1858. The terrain features stairstep limestone benches and moderate to high slopes, surfaced by dark, calcareous stony clays and clay loams that support oak, juniper, mesquite, and grasses in the surrounding area and water-tolerant hardwoods and conifers along the riverbed. The countryside is used principally for ranchland and secondarily for residences. Much of the bank is privately owned. The river is generally shallow and is impounded by a series of low-water dams. Springs from Glenrose limestone in the Middle Trinity Aquifer support the Blanco River in its upper reaches. Two major tributaries in Blanco County, Callahan Branch and Flat Creek, join the main (or north) fork (once known as Martin's Fork) of the river. In western Hays County the Little Blanco River adds volume to the Blanco. Cypress Creek, another major tributary, rises from Jacob's Well and flows into the Blanco River at Wimberley.
Some unique features along the Blanco River include the Narrows in western Hays County and the Devil's Backbone near Wimberley. Dinosaur tracks are embedded in the limestone riverbed about three miles downstream from Blanco State Recreation Area in Blanco. Indian mounds and related archeological sites can be found along the river downstream from the Little Arkansas Springs, east of Wimberley. The Blanco River also flows underground along several spots in its course. In its lower reaches it flows beneath moderate slopes surfaced by acid sandy and clay loam that supports pecans, hardwoods, and grasses. During the 1960s a major reservoir was proposed at Cloptin's Crossing, two miles southwest of Wimberley. The dam was never built, however, and in 1990 the Cloptin Reservoir still remained the subject of debate. Excessive pumping of the Trinity and Edwards aquifers has reduced spring-flow in the area and caused some concern for the quality of water in the Blanco River. Several recreational areas, including Blanco State Recreation Area in Blanco, private parks and resorts in Wimberley, and Dudley Johnson (Five-Mile Dam) Park near San Marcos, operate on the Blanco.