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because of physical and economic limitations on the availability of water. About 1.5 million acres were irrigated in 1980. . . Irrigation development began in the Lower Basin at about the same time as in the upper Basin but was slow due to the difficulty of diverting from the Colorado River with its widely fluctuating flows. Development of the Gila area. began in 1875 and the Palo Verde area in 1879. Construction of the Boulder Canyon project in the 1930's, and other downstream projects since that time, has provided for a continued expansion of the irrigated area. In 1970, an additional 21,800 acres were irrigated by private pumping either directly from the Colorado River or from wells in the flood plain. In 1980, nearly 400,000 acres were being irrigated from the Colorado River mainstream, total irrigated lands for the entire Lower Basin were about 1.5 million acres. ~ --J c-~ Irrigation in the Colorado River Basin has increased the salinity in the Colorado River. Return flows from the irrigated lands dissolve salts from the soils and underlying aquifer material and transport them to the river. The development of future irrigation projects will further increase the salt load to the river. Studies prior to irrigation would be helpful to determine contribu- tion from irrigation, but they have not been made in most areas. The amount of salt from this source must therefore be estimated or determined by detailed investigations, possibly with the use of simulation models. 3. Municipal and Industrial Sources of Salinity Salt loads contributed to the colorado River system by municipal and industrial sources are generally minor, totaling about 1 percent of the Basin salt load. Future increases in salt loads from these sources are expected to be small relative to the total Basin salt burden and will have only a minor effect on salinity levels. Most municipal and industrial wastes are relatively low in total salt load in comparison with natural and agricultural sources, and complete elimination of such waste discharges would have little effect on salinity concentrations in the main river system. Since these wastes are point sources of salinity, control could be achieved if salinity levels in the waste being discharged (i.e., industrial brines) warrant such control. Development of oil and gas, oil shale, and mineral resources in the Basin also has the potential to increase salt loading. Many saline aquifers are static (very little water movement) until they are disturbed by drilling or mining activities. An example is the Meeker Dome Salinity Control unit, described in Part VII, which came about as the result of deep ground water, high in dissolved salts, flowing to the surface through abandoned oil wells. 4. Increased Salinity from Water Depletions Addition of salts to the river system is not the only cause of increased salinity concentrations. The depletion of water of better quality water reduces the dilution of saline inflow, increasing the salinity of the Colorado River. With the exception of the Central Arizona Project (CAP), the Lower Basin has already developed most of its water supply. CAP will soon be IV-2 -