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<br />W Table I. - Salinity effects <br />W <br />.~ <br />~ mg/L <br />t..J Effect <br /> Below 500 Good drinking water <br /> Above 500 M&! losses <br /> 700 to 850 Agricultural losses <br /> and above <br /> <br />Development of Basin <br /> <br />Irrigation development in the Upper Basin took place gradually from the <br />beginning of settlement about 1860 but was hastened by the purchase of land <br />from the Jndi ans in 1873. About 800.000 acres were irrigated by 1905. <br />Between 1905 and 1920 the development of irrigated land continued at a <br />rapid pace and by 1920 nearly 1,400.000 acres were irrigated. The develop- <br />ment then leveled off to about 1,600.000 acres in 1965. The slow growth in <br />irrigated acreage in the Upper Basin in the last 50 years is a result of <br />c 11 mati c. phys; c a 1. and econom; c limit at i ens on the av a i1 ab; 1 ity of water. <br />By 1920 most of the lower cost and more easily constructed water develop- <br />ments were in operation. <br /> <br />Irrigation development began in the lower Basin about the same time as in <br />the Upper Basin. Development was slow because of difficult diversions from <br />the Colorado River, with its widely fluctuating flows. Development of the <br />Gila and Palo Verde areas began in 1875-1879. Construction of the Boulder <br />Canyon project in the 1930.s and other downstream projects since that time <br />have provided the means for a continued expansion of the irrigated area. <br />In 1974 there were nearly 849,000 acres in the United States irrigated <br />from Colorado River diversions below Hoover Dam. About 25.500 acres of <br />lower Basin lands in Utah are also now under irrigation. <br /> <br />Although the growth of irrigation development in the Upper and lower Basin <br />has slowed. some projects currently are under construction and others are <br />being planned. Studies have shown that the average annual rate of pickup <br />of dissolved solids from newly irrigated lands is in the range of 0 to 2 <br />tons per acre. <br /> <br />The Basin has been subject to a high degree of immigration. as indicated in <br />table 2. The approximate historical, present. and projected population <br />figures are shown for both the Upper and lower Basins. <br /> <br />7 <br />