The San Gabriel River, which includes the North, Russell, Middle, and South forks, is part of the Brazos River drainage basin. The river flows from Burnet County eastward through Williamson County to its confluence with the Little River in Milam County, for a total length of 120 miles. The North Fork begins twelve miles north of Burnet, in Burnet County (at 30°54' N, 98°14' W). Russell Fork begins six miles north of Burnet (at 30°50' N, 98°14' W) and runs into the North Fork (at 30°49' N, 98°01' W). The Middle Fork starts five miles east of Liberty Hill, Williamson County (at 30°38' N, 97°50' W), and flows eastward to its mouth on the North Fork, one mile west of Interstate Highway 35 and near the western city limit of Georgetown. The South Fork begins four miles east of Burnet (at 30°45' N, 98°11' W) and flows southeast thirty-four miles to join the North Fork and become the San Gabriel River inside the northern city limits of Georgetown (at 30°38' N, 97°41' W). The San Gabriel and its tributaries wind mostly through the heavily wooded and scenic limestone formations of the Balcones Escarpment, thence onto the Blackland Prairie. There are two large dams on the San Gabriel. One, eight miles west of Granger in Williamson County, forms Granger Lake, which has a pool elevation of 512 feet above sea level. The other, four miles upstream from Georgetown, forms Lake Georgetown, which has a pool elevation of 791 feet above sea level. Both lakes provide flood control and feature boating and picnic facilities. Major tributaries of the San Gabriel are Brushy, Alligator, Opossum, Berry, Pecan, Little, and Oatmeal creeks. Rockdale, Thorndale, Granger, Taylor, Georgetown, Bertram, and Burnet are long-established communities on the San Gabriel.
The San Gabriel was named Río de San Francisco Xavier by the Ramón expedition in 1716 and also figured in the journals of the Aguayo expedition of 1721. On his map of 1829 Stephen F. Austin mistakenly labeled the river "San Javriel," a spurious name that evolved into the present one. The San Xavier missions were founded in 1745 along the river just a few miles upstream from the mouth of Brushy Creek. In May 1839 the San Gabriel was the site of the battle of the San Gabriels, one of the state's most important Indian fights. Brushy Creek was the location of the battle of Brushy Creek in February 1839.