RIO GRANDE. The Rio Grande rises in Rio Grande National Forest, San Juan County, Colorado (at 37°47' N, 107°32' W), as a clear, spring and snow-fed mountain stream 12,000 feet above sea level. Its origin is at the Continental Divide in the San Juan Mountains. The river cuts through the middle of New Mexico to the site of El Paso and Ciudad Juárez, at the junction of Chihuahua and Texas. At that point, because of the Treaty of Guadalupe Hidalgo (1848), which terminated the Mexican War, the Rio Grande becomes the international boundary between the United States and Mexico. It forms the western or southern border of the Texas counties of El Paso, Hudspeth, Presidio, Brewster (where the river's sweeping curve gives Big Bend National Park its name), Terrell, Val Verde, Kinney, Maverick, Webb, Zapata, Starr, Hidalgo, and Cameron. The river empties into the Gulf of Mexico (at 25°57' N, 97°09' W). The Rio Grande has been known by many names over time and in different parts of its course. The Pueblo Indians called it Posoge (sometimes spelled P'Osoge), which meant “big river.” The expedition of Hernando de Alvarado called it Río de Nuestra Señora in 1540. It was called the River of May by three British sailors in 1568; in 1581 the Agustín Rodríguez expedition called it the Río de Nuestra Señora de la Concepción and the Río Guadalquivir. By 1598 the Spanish were calling its lower course the Río Bravo, and in modern Mexico it is still known by that name or as the Río Bravo del Norte (bravo denotes “wild, bold, turbulent, restless”). In 1582 an expedition headed by merchant Antonio de Espejo traveled the Río Conchos to its mouth on a river he called Río del Norte (“River of the North”) and Río Turbio (“Turbulent River”). Juan de Oñate is thought to have been the first to call it the Rio Grande, when in 1598 he reached its banks near the site of future El Paso. These names were later consolidated to Río Grande del Norte. It was the Rio Grande that Fernando del Bosque (see BOSQUE-LARIOS EXPEDITION) named the Río de San Buenaventura del Norte in 1675, and that Father Damián Massanet may have called the Río Ganapetuán in 1691. The river was also called the Río Caudaloso (“carrying much water”), and on a 1700 map it appeared as Río del Norte y de Nuevo Mexico.