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<br />because of physical and economic limitations on the availability of water. <br />About 1.5 million acres were irrigated in 1980. <br /> <br />Irrigation development began in the Lower Basin at ahollt the same <br />time as in the Upper Basin but was slow due to the difficulty of diverting <br />from the Colorado River with its widely fluctuating flows. Development of the <br />~ Gila area began in 1875 and the Palo Verde area in 1879. Construction of the <br />CO Boulder Canyon Project in the 1930's, and other downstream projects since that <br />en time, has provided for a continued expansion of the irrigated area. In 1970, <br />~I an additional 21,800 acres were irrigated by private pumping either directly <br />from the Colorado River or from wells in the flood plain. In 1980, nearly <br />400,000 acres were being irrigated along tho Colorado River mainstream; total <br />irrigated lands for the entire Lower Basin were about 1.5 million acres. <br /> <br />Irrigation in the Colorado River Basin has increased the salinity in <br />the Colorado River. Return flows from the irrigated lands dissolve salts from <br />the soils and underlying aquifer material and tl'ansport them to the river. <br /> <br />3. Municipal and Industrial Sources of Salinity <br /> <br />Salt loads contributed to the Colorado River system by municipal and <br />industrial sources are generally minor, totaling about 1 percellt of the Basin <br />salt load. Iorns(2) found that municipal and industrial uses increased <br />salinity by about 100 tons per 1,000 people in the Basin. Reclamation <br />estimates for the Upper Colorado River Basin show the population increasing at <br />a rate of about 25,000 people per year. The population estimate in 1980 was <br />575,900 people. By 2010, munic:i,pal and industrial sources will be increasing <br />salinity by approximately 133,000 tons per year or more. <br /> <br />. Most municipal wastes are relatively low in salt concentration in <br />comparison with natural, industrial, and agricultural sources. Complete <br />elimination of such waste discharges would be expensive when compared to other <br />salinity control methods. <br /> <br />Development of oil and gas, oU shale, and mineri'll resources in the <br />Basin also has the potential to increase salt loading. Many saline aquifers <br />are static (very little water "Iovement) until they are disturbed hy drilling <br />or mining activities. An example is the Meeker Dome Salinity Control Unit, <br />described in Part VII, which came about as the result of saline ground water <br />flowing to the surface through abandoned oil wells. <br /> <br />4. Increased Salinity from Vater Depletions <br /> <br />Addition of salts to the river system is not the only cause of <br />increased salinity concentrations. The depletion of water of hetter quality <br />reduces the dilution of saline inflow, increasing the salinity of the <br />Colorado River. <br /> <br />With the exception of the Central Arizona Project (CAP), the Lowpr <br />Basin has already developed most of its water supply. CAP will soon be <br />responsible for the last major additional depJ.etion (approximately 1.5 million <br />acre-feet per year) in the Lower Basin. Depletions in the Upper Colorado <br />Basin were estimated at 4.1 million acre-feet in 1987. <br /> <br />IV-2 <br />