The Brazos River, called the Río de los Brazos de Dios by early Spanish explorers, is the 14th-longest river in the United States at 1,280 miles from its headwater source at the head of Blackwater Draw, Roosevelt County, New Mexico to its mouth at the Gulf of Mexico with a 45,000-square-mile drainage basin. Being one of the largest rivers in Texas, it is sometim…The Brazos River, called the Río de los Brazos de Dios by early Spanish explorers, is the 14th-longest river in the United States at 1,280 miles from its headwater source at the head of Blackwater Draw, Roosevelt County, New Mexico to its mouth at the Gulf of Mexico with a 45,000-square-mile drainage basin. Being one of the largest rivers in Texas, it is sometimes used to mark the boundary between East Texas and West Texas.