Known in Mexico as Rio Bravo, the Rio Grande rises in the San Juan Mountains of Colorado flows through New Mexico before becoming the boundary between Texas and Mexico, touching the Mexican states of Chihuahua, Coahuila, Nuevo Leon, and Tamaulipas. The first European known to sight the Rio Grande was Alvar Nunez Cabeza de Vaca in 1536. At the time of the Louisiana Purchase, American expansionists regarded the purchase as including the land up to the Rio Grande. They were disappointed when the Adams-Onis Treaty set the limit at the Sabine River. Nevertheless, the growth of American settlement in the Texas province of Mexico renewed the effort to set the northern limits of Mexico at the Rio Grande. The Texas Republic consistently held that the Rio Grande was its border from it origins in Colorado to the Gulf of Mexico. When the United States annexed Texas, it implicitly inherited the same claim. Mexico was not prepared to accept this claim, and their refusal was one of the causes of the Mexican War. After the Mexican War, the Treaty of Guadalupe Hidalgo set the boundary between Mexico and the United States as the Rio Grande between its mouth and El Paso in the west. In 1944, the Big Bend National Park was created out of scenic land along the river in Texas.