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2 UPDATING THE HOOVER DAM DOCUMENTS A.2 Water Supply The unregulated flow of the river, uneven and unpredictable, varies widely during the year, from year to year, and over long periods of years. Water supply studies of virgin or undepleted flow at Lee Ferry show a maximum of 24 million acre-feet per year (maflyr) in 1924 and a low flow of 5.5 maf in 1977. The long-term average virgin flow of the river at Lee Ferry, from the turn of the century to the present, averages 14.7 maflyr. However, the bulk of the high flow occurred during the early part of this century so that the average virgin flow from 1896 to 1930, a "wet" period, was about 17 maflyr whereas the average virgin flow from 1930 to the present time, a "dry" period, was about 13 maf/yr. The lO-year wettest period saw an average annual virgin flow of 18.8 maf in 1914-1923. The driest 10 years saw an average annual flow of 11.8 maf. Since more accurate measurements of the flow at Lee Ferry were commenced in 1922: the flows have averaged about 14 maf/yr. This range of flows is significant. For example, the Compact negotiators in 1922 divided what was thought to be a water supply of 16 maflyr between the Upper and Lower Basins on the assumption that the flows were in excess of that amount. Since 1922 estimates of the river's flow have stead- ily been revised downward to approximately 14 mal. The lower average river flows; Le., a shrinking supply coupled with an increasing demand, have contributed greatly to the water problems that arose in later years (see Appendix 1 A. 2 for Bar Chart of Water Supply). A.3 Early River Development In the late 1800's developers in the Imperial Valley of California devised plans to divert water from the Colorado River and to irrigate Imperial Valley lands by gravity flow. Diversion works were completed in 1901 for that purpose as a private undertaking. In 1903 80,000 acres were irrigated and in 1920 there were 400,000 irrigated acres. Today there are 500,000 irrigated acres. Following notices of appropriations filed in 1877 by Thomas Blythe, diversion works were also begun by private developers in the Palo Verde Valley in California. Private canal companies also began irrigation in Arizona as early as 1890 in the Yuma Valley and in 1905 in the North Gila Valley. After the passage of the Reclamation Act of 1902, investigations were started to determine the feasibility of large, Federal irrigation projects. The Yuma Reclamation Project in Arizona and California was authorized in 1904 and the first Colorado River water was delivered to it in 1907. By 1920, irrigation works constructed primarily by private enterprise, especially in the Imperial and Palo Verde Valleys of California, had expanded to such an extent that the unregulated flow of the Colorado River was completely utilized during periods of low flow so that further expansion was dependent upon construction of storage reservoirs on the river. The erratic flows of the river, its tendency to destructive flooding and its high silt load limited its usefulness for a dependable year-round water supply without some flood control and storage facilities, both of which were beyond the means of local entities and the States. Before construction of Hoover Dam, which was com- pleted in 1935, the lower reaches of the Colorado River were subjected to severe annual floods. This menace was fully realized in 1905 when the Colorado River, swollen by floodwaters, broke through a cut several miles below the International Boundary, which had been made by the early developers of the Imperial Valley in California. For 16 months it flowed into the fields of the Imperial Valley enlarging the Salton Sea, approx- imately 490 square miles in area,. and threatened to engulf the entire valley. The break was finally closed largely through the efforts of the Southern Pacific Railroad Company but only after 30,000 acres of arable land had been inundated, farms ruined, homes destroyed, highways washed away, and railroad tracks destroyed. This tragic occurrence, indicating the need for flood control of the lower Colorado River, became a motivating reason for the construction of Hoover Dam. That, plus problems in maintenance of the distribu- tion facilities to Imperial Valley because its diversions of water were through facilities in Mexico, led to demands for a canal within the United States. In 1901 the Davis and Lippincott Report recommended studies of two major projects which actually materialized in the Boulder Canyon Project Act, a storage dam at the Boulder Canyon site and a canal from the Colorado River to the Imperial Valley in California. p:;i ;'r;