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Known as the Rio Grande in the United States and as the Río Bravo (or, more formally, the Río Bravo del Norte) in Mexico, the river, 3,034 km (1,885 mi) long, rises in the San Juan Mountains of Colorado, flows through the San Luis Valley, then south into New Mexico through Albuquerque and Las Cruces to El Paso, Texas, on the U.S.–Mexico border. A major tributary, Rio Conchos,enters at Presidio, below El Paso and supplies most of the water in the 2,019 km (1,254 mi) Texas border segment.
The river has, since 1845, marked the boundary between Mexico and the United States from the twin cities of El Paso, Texas, and Ciudad Juárez, Chihuahua, to the Gulf of Mexico. As such, it was across this river that Texan slaves fled when seeking their freedom, aided by Mexico's liberal colonization policies and abolitionist stance.