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diversion and evaporation, diversion for special use, plugging of wells, and deep well injection. w o l'\) ..... 2. Diffuse source control involves salt loading and/or concentrating effects that are spread over comparatively large areas, such as minor tributary subbasins. A general lack of basic data has limited the studies to tentative plans for control of these sources. The techniques available for control are collection and desalting, collection and evaporation, collection and special use, watershed management, and jJhreatophyte control. 3. Irrigation source control involves reductions in the salt loading and q::mcentrating effect caused by dissolution of salts in the soil and substrata and the consumptive use of water. The techniques applicable to irrigation source control are improved onfarm irrigation water use, improved water conveyance systems, ground-water management, return flow management, and evaporation suppression. Additional measures such as river system management, watershed management, return flow utilization for other beneficial uses, and dilution by additional water sources are also being developed. LONG RANGE OUTLOOK The Colorado River Water Quality Improvement Program is only one facet of an overall basin water management concept which could be implemented to meet the objective of maintaining salinity concentrations at or below their present level. The basin water management concept envisions that existing and proposed structures will be coordinated with the physical features of the basin to optimize the use and development of the water resources for the greatest economic and social benefits of those involved. There are several allied programs which are expected to beneficially affect the Colorado River salinity. WEATHER MODIFICATION Weather modification research now in progress is expected to develop by 1980, a reliable and workable system for increasing precipitation. In a limited water area, such as the Colorado River Basin, producing 1 to 2 million acre-feet of usable new water annually might contribute toward salinity improvement. The highly favorable benefit-cost ratios, the flexibility of use, and the opportunity for obtaining even greater new water yields with advanced techniques point to weather modification as a very desirable tool for water resources management. VEGETATION AND WATERSHED MANAGEMENT Vegetation and watershed management could also contribute to reductions in salt concentrations of the Colorado River. Phreatophytes are large contributors to the salt concentrating effects in the basin. Research is underway to determine the effect of treating phreatophyte-infested areas with anti-transpirants. DESALTING The coast of southern California has been under intensive study to site large-scale sea water desalting plants. Recent reconnaissance studies have evaluated desalting plants at Diablo Canyon (40 million gallons per day [mgd]), Encina-San Diego (40 mgd), San Diego Refuse Incinerator Project (32 mgd), and Orange County Water Factory 21 (3 mgd). Ultimately, large-scale, dual-purpose desalting plants may not only augment local municipal and industrial supplies but also export or exchange water to meet inland demands such as on the Colorado River. As a consequence, desalting is still expected to have a role in providing future water supplies for augmentation purposes. As such, the potential impacts of sea water desalting for exchange water and water quality improvement in the Colorado River should be recognized and monitored. CONTROL OF EFFLUENT FROM POWERPLANTS Since 1970, action has been taken with respect to the effect of cooling tower blowdown water from thermal-electric powerplants on the salinity of the Colorado River. Cooling water in .the towers evaporates, and salt in the remaining water becomes more and more concentrated. The cooling water must be maintained at or below specific levels of concentration, and this is done by discharging, or "blowing down" a portion of the water, and replacing it with fresh water. At the present time, there are five large coal-fired thermal-electric generating plants either in operation or under construction within the Colorado River Basin, and others are planned, that will be disposing of their blowdown water away from the river, thereby removing dissolved salts from the Colorado River system. Water depletions, however, will cause some increase in salinity downstream. 25