The Lavaca River rises in the far northeastern corner of Gonzales County (at 29°38' N, 97°08' W) and flows southeast for 115 miles, crossing Lavaca and Jackson counties, to its mouth on Lavaca Bay in northern Calhoun County, 1½ miles north of Point Comfort (at 28°42' N, 96°35' W). The river, which drains an area of approximately 2,280 square miles, was originally described by René Robert Cavelier, Sieur de La Salle, who named it Rivière de Les Veches, or "Cow River," because of the buffalo in the area. The Spanish translated the name to Lavaca. Even though the stream is classed as intermittent because it depends on rainfall rather than permanent springs for its water, the average annual flow is about 600,000 acre-feet, and heavy rains bring frequent flooding as far upstream as Hallettsville. During the nineteenth century the river was normally navigable to Texana, thirty miles above its mouth. According to legend the Pride, flagship of pirate Jean Laffite's fleet, was scuttled in the Lavaca near its mouth when pressured by an American revenue cutter.
The Lavaca River flows by several towns, including Moulton, Hallettsville, Edna, and Vanderbilt, but its environs are used primarily for ranching and the production of oil and gas from the numerous oilfields that dot its banks. Its primary tributary is the Navidad River, which enters from the east two miles northeast of Vanderbilt (at 28°50' N, 96°35' W). The North Fork of the Lavaca River rises on the Lavaca-Fayette county line (at 29°38' N, 97°06' W) and flows south through Lavaca County for 7½ miles to its mouth on the main Lavaca River, three miles southwest of Komensky (at 29°32' N, 97°05' W). The loamy clay upland soils of the area are easily eroded and are used primarily for rangeland, pastureland, and the production of corn and grain sorghum. Until the second half of the twentieth century this area produced good yields of cotton, but soil erosion and depletion encouraged many farmers to convert their lands to pasture for beef and dairy cattle. The course of the stream is marked with scattered oak, willow, and sycamore, and unimproved pasture reverts to scattered cedar and mesquite.
The West Prong of the Lavaca River rises three miles southwest of Moulton in western Lavaca County (at 29°33' N, 97°12' W) and flows east for 7½ miles to its mouth on the Lavaca River, 1½ miles southeast of Moulton (at 29°33' N, 97°07' N). It borders the south and west sides of Moulton and flows through rolling hills surfaced by well-drained loamy and clayey soils of generally open upland prairie; the soils are used primarily for rangeland, pastureland, and the production of corn and grain sorghum. Occasional outcroppings of sandstone dot the area, and on steeper slopes erosion is apt to be severe. Vegetation consists of scattered oak, willow, and hackberry mottes that provide cover for small game and upland birds. The stream is used for recreation in Moulton.