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The population of Alamosa in 1910 was 3013; in 1970, the U.S. Bureau of Census reported a population of 6698. The city government consists of a mayor and an eight-member council. In the study area, the average temperature is 64.60P in July and l6.60P in January. The annual mean temperature is 4l.90P. Average precipitation, including winter snows and summer showers, has been approximately 5.5 inches. The maximum 24-hour precipitation recorded at Alamosa is 1.78 inches. During the winter months, there is heavy snowfall in the upper mountainous area of the Rio Grande Watershed. The average annual snowfall at Alamosa is 22.8 inches. The Rio Grande, one of the principal streams in the southwestern United States, is an interstate and international river. From its source on the eastern slopes of the Rocky Mountains in south-central Colorado, the Rio Grande flows eastward for approximately 150 miles to Alamosa and then southward to the Colorado-New Mexico state line. It flows through New Mexico to El Paso, Texas, where it becomes the international boundary between the United States and Mexico. The river originates along the Continental Divide in the San Juan Mountain Range at elevations generally above 10,000 feet National Geodetic Vertical Datum of 1929 (NGVD). Along the Continental Divide, the river is fed by perpetual snowfields and springs. Between its source and the Alamosa stream gage, the river drains approximately 1710 square miles. The Rio Grande meanders through the San Luis Valley, a gently sloped plain surrounded by high mountain ranges. The San Luis valley is approximately 100 miles long and approximately 60 miles wide. The valley is surrounded by the Sangre de Cristo Mountains on the east, the San Juan Mountains on the west, and the La Garita Mountains on the north. The Rio Grande is a perennial river, with streambed elevations ranging from 7538 feet NGVD at the upper end of the study reach to 7525 feet NGVD at the lower end. The natural vegetation contains only a limited number of dominant species. The open pinon and juniper stands of the surrounding foothills give way to a sagebrush association around the well-drained margins of the region. On large portions of the valley floor, the greasewood association, dominated by the black greasewood and rabbit bush, is found. The groves of trees that line the stream courses are dominated by cottonwoods and willows. The flood plains in Alamosa abound with commercial and residential developments, public facilities, and utilities. Transportation facilities crossing the flood plains include the famous Navajo Trail (D.S. Highway 160), the Trans-American Highway (U.S. Highway 285), numerous city streets, and the Denver and Rio Grande Western Railroad. New developments, which are primarily residential, are being located in the northwestern section of Alamosa. 4