| OCR Text |
Show Studies of Arizona precipitation indicate not only great variations in amount but great variations from place to place. For example, under one set of con- ditions the 24-hour precipitation to be expected in the locality with an annual precipitation of 10 inches is 3.85 inches; for a locality with an annual fall of 30 inches, it is 9.47 inches. In another locality, and with the same variation in annual precipitation, the range is from 2.70 to 6.85 inches. Critical erosion is found on lands of all types of ownership, public and private alike. The extent of erosion and its severity on lands in different types of ownership are shown in table 18. Probably the most serious erosion conditions and the greatest sediment source areas are found on the Navajo Indian Reservation in the San Juan and Little Colorado sub-basins in Arizona, New Mexico, and Colorado. The Fort Apache Reservation in Arizona is considered by comparison to be in much better condition. The Indian lands occupy about 19 percent of the basin above Hoover Dam and produce an estimated 30 percent of all the sedi- ment52 entering Lake Mead. The following data give some indication of this situation. Indian land Area Percent of area by erosion class Slight Moderate Severe Critical Navajo Reservation....... Fort Apache Reservation. . . Indian lands above Hoover Dam.................. Indian lands, entire basin... Sq. miles 24,133 2,600 31, 933 41, 902 4 37 6 11 32 36 32 37 34 12 34 29 30 15 28 23 Water samples from some of the main streams draining the Navajo Reservation were found to contain the following volumes of sediment: Chinle, 47.1 percent; Orarbi, 44.1; Puerco, 14.9; Laguna, 9.2; Dinnebito, 7.0; and Polacca, 6.9 percent. The Navajo Reservation, constituting 14 percent of the basin, contributes about 22 percent of the sediment but only 2.5 percent of the water to Lake Mead.53 62 U. S. Geological Survey. 88 Proceedings, Federal Inter-Agency Sedimentation Conference, 1948. TABLE 18.-Erosion in Colorado River Lands [In square miles] Class of land Total area Erosion conditions Slight Moderate Severe Critical Indian........ National forest. Grazing districts National parks.. Other......... Total___ 41, 902 45, 665 97,116 4,490 52,827 4,651 31,985 14, 600 790 14, 817 15, 417 9,500 41, 200 400 24, 250 12,123 3,240 30, 216 2,875 8,450 9,711 940 11,100 425 5,310 242,000 66,843 90, 767 56, 904 27, 486 Source: Proceedings of Sedimentation Conference, Salt Lake City, May 1950. 431 Rainfall stations Lordsburg, N. Mex..... Red Rock, N. Mex..... Rodeo, N. Mex........ Tyrone, N. Mex....... Thatcher, Ariz......... San Carlos, Ariz....... Fort Grant, Ariz....... Globe, Ariz........... San Simon, Ariz....... No. No. Inches Inches Inches 44 3.3 2.86 0.45 1.17 32 4.2 4.33 .58 .99 29 3.8 3.43 .58 2.62 11 5.6 4.45 .49 1.23 28 3.3 2.96 .49 2.01 19 3.0 2.93 .63 4.09 27 4.6 4.46 .86 4.50 33 4.8 4.36 .69 2.96 24 3.0 2.53 .36 .92 Years recorded Average number rains per year Average summer storm rainfall Average runoff Maximum runoff |
