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Show Chapter 2 The River and its Tributaries For consideration of water resources, the Rio Grande Basin in the United States may be divided into three separate drainage areas as follows: 1. Upper Rio Grande sub-basin. 2. Lower Rio Grande sub-basin. 3. Pecos sub-basin. Each of these is, in effect, a separate river since neither of the upstream subbasins contributes any appreciable amount of water to the lower subbasin. The Upper Rio Grande sub-basin extends from the river's source in Colorado to Fort Quitman, Tex. Practically all the river flow above that point is utilized in the upper part of the basin. The Pecos River sub-basin lies in southeastern New Mexico and southwestern Texas. Most of the flow of this major tributary is diverted before it reaches the Rio Grande. The Lower Rio Grande sub-basin in- cludes the area below Fort Quitman. Only about 25 percent of the drainage area in this subbasin lies in the United States in a relatively narrow strip along the river. Major tributaries in this reach rise in Mexico and supply nearly all the waters of the lower river. (See color map, figure 7.) Adjoining the Rio Grande Basin are numerous areas which have no surface drainages to the major streams. These closed basins are: 1. San Luis closed basin in Colorado. 2. Estancia, Tularosa, Jornado Del Muerto, and Sacramento closed basins which lie between the Rio Grande main stem and the Pecos in New Mexico. 3. The St. Augustine Plains and the Mimbres closed basins between the Continental Divide and the Rio Grande main stem in New Mexico. The Upper Rio Grande Sub-basin In the upper reaches the headwaters of the Rio Grande are typical snow- and spring-fed mountain streams draining the heavily forested and rugged southern slopes of the Rocky Mountains. In Colo- rado the river is joined by the Conejos and Alamosa Rivers and La Jara, Culebra, and Costilla Creeks. Several streams flow into the San Luis Lakes and contribute no surface flow to the Rio Grande. Through New Mexico to Fort Quitman, Tex., the river flows through a series of deep canyons and alluvial valleys. The valleys vary in width from a few hundred feet to several miles. The valley floor is only slightly higher than the river bed. In some instances where it is lower, levees have been built and must be elevated from time to time to confine the river to its rising channel. Above Otowi the permanently flowing tributaries include the Rio Colorado, Rio Hondo, Rio Taos, Embudo Creek, and Rio Chama. Tributaries below Otowi are usually ephemeral. The principal ones are Jemez Creek, Rio Puerco, and Rio Salado. They are characterized by flash floods with relatively small total volumes of water and carry heavy loads of sediment. Waters of the Upper Rio Grande sub-basin are derived from melting snow. Consequently stream flows are high in early summer. Irrigated lands are subject to deficiencies of water in late summer. Occasional torrential rain storms in summer may cause floods. Annual stream flow in this sub-basin is subject to wide variations. The National Resources Plan- ning Board estimated that average annual water production in the area is 3,060,000 acre-feet. Es- sentially all the water produced in the Upper Rio 291 |