RIO GRANDE, a North American river, which rises in the San Juan Mountains of southern Colorado, flows S.E. and S. in Colorado, S. by W. and S.E. through New Mexico, and S.E. between Texas and Mexico to the Gulf of Mexico. Its length is approximately 2200 m., and for about 1300 m. it forms the international boundary between the United States and Mexico. It presents many features of a complex physiographic type, being first a river of the Rocky Mountains, then of the interior deserts and then of the Atlantic Coastal Plain. It also presents a complicated geological history, as it includes what were originally several distinct streams. The Mexicans call it the Rio del Norte in its upper course, the Rio Bravo in the "Big Bend," from the mouth of the Conchas river to the mouth of the Devils river, and the Rio Grande only in its course through the Coastal Plain. From its headwaters, 12,000 ft. above the sea, it rushes rapidly down a mountain canyon to San Luis Valley, in Colorado. It flows with moderate speed through this broad valley, enters a long canyon with a maximum depth of 400 ft., about 4 m. above the boundary between Colorado and New Mexico, and is hemmed in between canyon walls rising as high as too() ft. or between the sides of narrow mountain valleys throughout its course through New Mexico. It passes through a series of picturesque canyons, some of them 1750 ft. in depth, in the "Big Bend," and becomes a silt-laden stream with a shifting channel in its passage through the Coastal Plain. Except in the flood season of May and June, the quantity of water which, for irrigation and by evaporation, is taken from the Rio Grande between its entrance to the San Luis Valley and the mouth of the Conchas, is greater than that received, and as a consequence it is an intermittent stream in this region. The flow of the Conchas is constant, and in the "Big Bend" the volume of the Rio Grande is enhanced by springs which break out in the bed. The total flow of the Rio Grande is ten times greater in some years than in others, and when its waters have been highest there have been great floods in its lower course and so much shifting of its banks as to cause international complications. Even in its course through the Coastal Plain its channel is so much obstructed by sand bars that it is of little importance for navigation. As the increasing diversion of the water of the Upper Rio Grande for irrigation in Colorado and New Mexico resulted in a scarcity of water for this purpose in Mexico, that country complained, and to remedy the evil the Reclamation Service of the United States proposed the construction by the United States of a storage dam across the river near Engle, New Mexico, which would form a storage reservoir having a capacity of 2,000,000 acre-feet and from which Mexico should be furnished with 60,000 acre-feet of water annually. Mexico agreed to this proposal and a treaty covering the matter was proclaimed in January 1907. The principal towns and cities on the river are: Brownsville, Texas; Matamoros, Mexico; Laredo, Texas; El Paso, Texas; and Cindad Juarez, Mexico.
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