The 1,865 mile long (3,033 km) Rio Grande ('great or big river') is one of the longest rivers in North America. The river flows south from the San Juan Mountains in southwest Colorado to southwest Texas, then flows southeast, with the remaining 1,250 miles of the river forming the border between the United States and Mexico. The river empties into the Gulf of Mexico near Brownsville, Texas, and Matamoros, Mexico.   The river has long been used for irrigation of farmlands.  The National Park Service working under the U.S. Department of the Interior is the government agency responsible for overseeing the river and maintaining the lands around the river.