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drained by coastal streams in the Lower Rio Grande Valley of Texas. The principal tributaries of the Rio Grande are the Conejos River and Alamosa Creek in Colorado; the Red River and Embudo Creek, the Rio Chama, Jemez River, and Rio Puerco in New Mexico; the Pecos in New Mexico and Texas; and Devils River in Texas; and the Rio Conchos, Salado, Rio Alamo, and Rio San Juan in Mexico. The Pecos, the longest tributary, is separated from the Rio Grande in the upper basin of New Mexico by the Sangre de Cristo Mountain Range, an extension of the southern Rockies. For the purpose of the 1975 Assessment the dis- charge from the Mexican portion of the watershed has not been included. A semiarid to arid climate and low and/or erratic rainfall are characteristic of the Region. Average annual precipitation ranges from 30 inches in the high mountains and lower coastal plains to only 8 inches in the middle valley area, where most of the precipita- tion occurs in the form of heavy summer thunderstorms. Winters are severe in Colorado and in the high mountains of New Mexico, but gen- erally mild throughout the lower areas. Recorded history of the Rio Grande Region began with its ex- ploration by Coronado in 1540. Spanish colonization in the 17th and 18th centuries along with pioneering by American settlers in the early 1850's resulted in rapid development of irrigation. In the early 1890's, however, irrigation water shortages began to oc- cur, restricting the magnitude of further irrigation development. The distribution 'pattern of population closely followed that of irrigated land. In recent years urban areas have grown rapidly as a result of tourism, nuclear research, and governmental and mili- tary activities. Populations of the main New Mexico urban centers-- Albuquerque, Alamogordo, Carlsbad, Las Cruces, Los Alamos, Roswell, and Santa Fe--have increased notably. About one-half of New Mexico's population resides in these cities. The population of the Texas part of the Region is largely urban which is concentrated in the El Paso and Laredo metropolitan areas and the Lower Rio Grande Valley. The regional economy is based chiefly on government, services, wholesale and retail trade, manufacturing and agriculture. The surface water resources of the Rio Grande Region from the headwaters to a point about 80 miles below El Paso are essentially all consumed. The supply in this reach is derived principally from snowmelt with some increase from summer rains. In the lower subregion, the Rio Grande is rejuvenated by the short-period flows of the Pecos River and other tributaries which flow in direct response to rainfall. The Upper Rio Grande, extending from the river's headwaters in Colorado to El Paso, Texas, includes the San Luis Valley closed basin fr ')'1--:;, . ".... _ ~ J 6