The primary source for the Blanco is a series of springs in northern Kendall County. The river flows generally eastward for 87 miles (140 kilometers) through Kendall County, Blanco County and Hays County. Near San Marcos, it takes a southerly turn and joins with the San Marcos River. The upper reaches of the Blanco River are hilly, and the river’s slopes are frequently steep. As the river reaches the Balcones Escarpment near San Marcos, it widens and slopes moderately; this is typical of rivers in the central part of the state. The river is usually quite shallow, and it briefly dips below ground in some areas in the Hill Country. As with many of the rivers in the Texas Hill Country, there is great variability in the Blanco River’s flow. The mean flow is 93 feet³/s (3 m³/s), but heavy rains in the river’s watershed can cause flash flooding with little warning. The Blanco River winds through over 200 acres of parkland.
Map of Blanco River (PDF)
USGS Blanco River at San Marcos Gauge