<P> The Colorado River is an 862 - mile (1,387 km) long river in the U.S. state of Texas . It is the 18th longest river in the United States and the longest river with both its source and its mouth within Texas . </P> <P> Its drainage basin and some of its usually dry tributaries extend into New Mexico . It flows generally southeast from Dawson County through Ballinger, Marble Falls, Austin, Bastrop, Smithville, La Grange, Columbus, Wharton, and Bay City before emptying into the Gulf of Mexico at Matagorda Bay . </P> <P> The Colorado River originates south of Lubbock, on the Llano Estacado, near Lamesa . It flows generally southeast, out of the Llano Estacado and through the Texas Hill Country, through several reservoirs including Lake J.B. Thomas, E.V. Spence Reservoir, and Lake O.H. Ivie . The river flows through several more reservoirs before reaching Austin, including Lake Buchanan, Inks Lake, Lake Lyndon B. Johnson, commonly referred to as Lake LBJ and Lake Travis . The Llano River joins the Colorado at Lake LBJ near Kingsland . The Pedernales River joins the Colorado at Lake Travis near Briarcliff . After passing through Austin, the Colorado River continues flowing southeast until emptying into Matagorda Bay on the Gulf of Mexico, near Matagorda . The Texas waterway is not connected to, nor part of, the longer and more well - known Colorado River . </P> <P> The Colorado River, which means "red" or "reddish" river in Spanish, was frequently confused by Spanish explorers with the Brazos River to the north . The European discoverer of these two neighboring rivers called the present Colorado River the Brazos de Dios, and called the present Brazos the Colorado River, and later the names were reversed . </P>

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