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Show extends far to the west in Mexico where the head- waters of the Rio Conchas rise a scant 150 miles from the Pacific. The principal tributaries in Mexico include the Rio Conchas, Rio Salado, and Rio San Juan, which are important suppliers of water to the lower basin area. Physical features vary widely. The basin is mountainous in the upper area, a semidesert in the middle section, and a typical delta in the lower reach near the Gulf of Mexico. Elevations range from 12,000 to 14,000 feet on the peaks of Colorado and New Mexico to sea level on the coast. Divides rise abruptly from 1,000 to 5,000 feet above rela- tively level valleys and plains. Steep slopes are prevalent. Cultivated areas are found at 8,000-feet elevation in the San Luis Valley in Colorado where the frost-free period is only 120 days, and at less than 50 feet at the river's mouth where semitropical conditions prevail. A semiarid to arid climate is characteristic of this southwestern region. Low or erratic rainfall pre- vails over most of the area. Although average an- nual precipitation ranges up to more than 30 inches in the high mountain headwaters and to 26 inches in the low coastal plain, the main trough of the valley is arid with very light rainfall-8 to 15 inches annually. This often comes in the form of infre- quent but extremely heavy downpours which cause flash floods. Effective precipitation is lower than actual be- cause of the high evaporation rate throughout the basin. Evaporation is excessive in desert areas where it may total 135 inches a year as measured by floating pans. Generally, irrigation is essential to crop production. Winters are severe, with heavy snowfall in the mountainous northern area. The milder winters in New Mexico and Texas permit year-round use of range land and make the region attractive for winter resort use. The Colorado and upper New Mexico tributaries are mainly snow fed, but their flow is supplemented with torrential runoff from occasional summer rains. Texas and lower New Mexico tributaries are fed almost entirely by rains that are often flood forming in nature. Character and distribution of storms as well as their location affect water yields and influence soil erosion and sediment production. By far the greater part of stream flow comes from the moun- tain areas, where forest stands predominate. Na- tional forest lands including less than 11 percent of the basin yield 80 percent of the total runoff, much of which comes from melting snow. The pifion- juniper and range areas yield comparatively small amounts of water. Floods in the upper Rio Basin and Pecos Valleys are of two kinds. One arises from rapid melt of heavy snow fields during sudden thaws. The other comes from torrential summer thunderstorms on the intermediate country, with flashy and destructive stages. Floods frequently get out of banks and spread over old alluvial flood plains to inundate and damage cities, railroads and roads, and irrigation canals and diversions. The wide differences in climate, elevation, and topography give rise to a variety of vegetation types. These include: alpine grassland, spruce-fir and pon- derosa pine forests, pirion-juniper woodland at in- termediate elevations, and sagebrush and salt desert shrub, short grass, desert grassland, and southern desert shrub in lowland areas. The Sacramento Mountains near El Paso support a magnificent stand of Douglas fir, the most southerly extension of the species on the continent. Desert-shrub types, grassland, and pinon-juniper woodlands occupy the greater area. Less than 5 percent of the basin con- tains commercial timber. Under the natural conditions of rugged topog- raphy, water scarcity, loose soils and poor vegetative cover, erosion rate is high. Flash floods which result from rains of cloudburst proportions accen- tuate this hazard, and stream sedimentation is one of the serious problems in the upper Rio Grande and Pecos River drainage areas. The waters of the Rio Grande are generally clear as they enter and leave San Luis Valley. As they pass down through New Mexico, they become muddy, and they carry a heavy load of sediment when they reach the upper end of Elephant Butte Reservoir. Below Elephant Butte, the Rio Grande picks up and deposits fine sand and sediment along its course through the desert. It arrives at the Gulf a sediment-laden stream again. The New Mexico tributaries of the Rio Grande and Pecos carry heavy loads of sediment into the main streams, especially during periods of flash flood. The Rio Puerco, which provides only about 6 percent of the water in the upper Rio Grande, contributes some 56 percent of the sediment to Elephant Butte Reservoir. During some storms, the flow of tributary streams may be of almost equal volumes of sediment and water. Livestock grazing, irrigated agriculture, and forestry constitute major land uses. Loose, poor soils prevail over much of the basin. Those culti- 287 |
