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Generally speaking, geologists and geomorphologists are reluctant to estlmate the number of alluvial fans in areas which extend much farther than the relatively small areas with which they are personally familiar. Even localized information can only be approximate. because a good deal of judgment must be used in deciding which formations are. or are not. fans. Judgments related to aprons and washes tend to be even more complex. The most successful attempt at quantifying the number of fans in large areas of the western United states was performed by the U.s. Army Natick Laboratories and reported by R. L. Anstey (1965). A total of 3876 fans were identified in 19.516 square miles of semi- arid landscape. Anstey cautions that fans with a radial length of less than 1760 feet (one-third of a mile) could not be identified and that low-slope fans may have been overlooked due to the coarseness of available maps. It should be noted that the field surveys (see state-of-the-Art Report) indicate that small fans and low-slope fans can present flood hazards which are equally serious to those on larger and steeper fans. Anstey's article reports that apprOXimately 30 percent of the American Southwest deserts (Basin and Range phYSiographic provinces) are occupied by alluvial fans and aprons. Because of their relatively gentle slopes. good drainage. and sorted composition material. alluvial fans are frequently used for roads. agriculture, and sites for urban development. especially in desert regions where the valley land is too soft and saline. and the mountains are too steep and composed of materials that are too hard for these purposes (Anstey, 1965). Although some alluvial fans are concentrated in areas where development is unlikely (over 70 percent of Death Valley, California is covered by fans). many other fans occur in. and are adjacent to. cities which are currently experiencing rapid growth. 34