Question: The Chamizal dispute was caused by a shift in which one of the principal rivers in the southwest United States and northern Mexico?
Context: Chamizal National Memorial, located in El Paso, Texas, along the United States–Mexico international border, is a National Park Service site commemorating the peaceful settlement of the Chamizal boundary dispute. Grusonia is a genus of opuntioid cacti (family Cactaceae), originating from the North American Deserts in Southwest United States and northern Mexico, including Baja California.  Authors differ on precise boundaries of the genus, which has been included in "Cylindropuntia". The Greater Southwest has long been occupied by hunter-gatherers and agricultural people.  This area, identified with the current states of Colorado, Arizona, New Mexico, Utah, and Nevada in the western United States, and the states of Sonora and Chihuahua in northern Mexico, has seen successive prehistoric cultural traditions since approximately 12,000 years ago.  An often quoted statement from Erik Reed (1964) defined the Greater Southwest culture area as extending north to south from Durango, Mexico to Durango, Colorado and east to west from Las Vegas, Nevada to Las Vegas, New Mexico.  Other names sometimes used to define the region include "American Southwest", "North Mexico", "Chichimeca", and "Oasisamerica/Aridoamerica". Llico River is one of the principal rivers of Llanquihue Province in southern Chile.  It runs from northeast to southwest draining part of the eastern slopes of Cordillera Pelada.  The local equivalent of the Mindel and Elsterian glaciations is named Río Llico glaciation. The Chamizal dispute was a border conflict over about 600 acres on the Mexico–United States border between El Paso, Texas, and Ciudad Juárez, Chihuahua.  It was caused by a shift in the Rio Grande, as a survey presented in 1852 marked differences between the bed of the Rio Grande (in Spanish: "Río Bravo del Norte" ) and the present channel of the river.  The Chamizal was once the only link between El Paso and Ciudad Juárez and tensions over the territory during the historic Taft–Diaz summit almost resulted in the attempted assassination of both presidents on October 16, 1909. Genízaros were Native American slaves who served as house servants, shepherds, and in other capacities, throughout what is today the Southwest United States well into the 1880s.  By the late 18th century, the Genízaros and their descendants, who were often referred to as "Coyotes", comprised nearly one-third of the entire population of New Mexico.  In 2007 the "Genízaros" and their contemporary descendants were recognized as indigenous people by the New Mexico Legislature.  Today, they comprise much of the population of the South Valley of Albuquerque, and significant portions of the population of Northern New Mexico Including Espanola, Taos, Santa Fe, and Las Vegas in Eastern New Mexico. Festuca arizonica, commonly called Arizona fescue, is a grass found in western North America, in the southwest United States and northern Mexico.  This species also has the common names mountain bunchgrass and pinegrass. The Rio Grande ( or ; Spanish: Río Bravo del Norte , ] or simply Río Bravo) is one of the principal rivers in the southwest United States and northern Mexico (the other being the Colorado River).  The Rio Grande begins in south-central Colorado in the United States and flows to the Gulf of Mexico.  Along the way, it forms part of the Mexico–United States border.  According to the International Boundary and Water Commission, its total length was 1896 mi in the late 1980s, though course shifts occasionally result in length changes.  Depending on how it is measured, the Rio Grande is either the fourth- or fifth-longest river system in North America. The 1970 Boundary Treaty between the United States and Mexico settled all then pending boundary disputes and uncertainties related to the Rio Grande (Río Bravo del Norte) border.  The most significant dispute remaining after the Chamizal Settlement in 1963 involved the location of the boundary in the area of Presidio, Texas, and Ojinaga, Chihuahua.  The river channel was jointly relocated to approximate conditions existing prior to the dispute which arose from changes in the course of the river in 1907.  The International Boundary and Water Commission was charged with its implementation.  The American-Mexican Treaty Act of October 25, 1972 authorized the United States Section's participation.  The project was undertaken in 1975 and completed in 1977. The Colorado River is one of the principal rivers of the Southwestern United States and northern Mexico (the other being the Rio Grande).  The 1450 mi river drains an expansive, arid watershed that encompasses parts of seven U.S. and two Mexican states.  Starting in the central Rocky Mountains in the U.S., the river flows generally southwest across the Colorado Plateau and through the Grand Canyon before reaching Lake Mead on the Arizona–Nevada border, where it turns south toward the international border.  After entering Mexico, the Colorado approaches the mostly dry Colorado River Delta at the tip of the Gulf of California between Baja California and Sonora.
Answer: Rio Grande