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conditions that are further complicated by the presence of extensive mountainous areas. The major portion of the precipitation in the watershed is derived from the tropical Gulf source region. Occasional inflow of moist Pacific air occurs during the presence of a low pressure area over Southern California and Arizona; but usually, by the time this air arrives over the Rio Grande Basin, its precipitable moisture has been substantially reduced. Precipitation usually occurs as the result of thunderstorm activity caused by convective or orographic lifting, although frontal activity may produce light to moderate storms. The relative weakness of polar air intrusions limits the occurrence of general storms of major importance during the summer. Major flood-producing storms have been experienced most often during the transitional periods between spring and summer, and summer and fall. No major flood-producing storm has occurred in the San Luis Valley watershed during the winter months because precipitation from winter storms is mostly in the form of snow. Climatically, the San Luis Valley is a high mountain desert. The average temperature is 64.60P in July and l6.60F in January. The mean annual temperature is 4l.90F. Average precipitation, including winter snows and summer showers, has been approximately 5.5 inches. During the winter months, there is heavy snowfall in the upper mountainous area of the Rio Grande Watershed. The average annual snowfall at the City of Alamosa is 22.8 inches. The flood plains in the developed.areas around the City of Alamosa abound with commercial and residential developments, public facilities, and utilities. Transportation facilities crossing the flood plain include the famous Navajo Trail (U.S. Highway 160), the Trans-American Highway (U.S. Highway 285), numerous residential streets, and the Denver and Rio Grande Western Railroad. 2.3 Principal Flood Problems The Rio Grande Basin in Colorado, because of its relatively high elevation and sheltered position, is less subject to destructive floods than any other major basin in Colorado. The earliest flood, that of 1869, is mentioned in newspaper accounts of considerably later date and little is known about it except that some damage did occur in the City of Alamosa. A flood in 1884 was caused by the melting of a very heavy snow cover, augmented by heavy rains. This flood lasted longer than any other known flood (Reference 1). The Rio Grande was out of banks at Del Norte from about May 24 to June 20. The flood peaked on June 14 and 15, and was probably the greatest known flood in the San Luis Valley. The flood of 1905 was the only destructive flood in which rainfall was not a factor. This flood was caused by snowmelt. The flood of 1911 was the only one recorded in the upper Rio Grande Basin that was caused wholly by heavy rains. The flood of 1927 was the highest since stream gage 5