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t..). o .- ~ Values for intermediate locations in the river are shown in Table 2, Table 2 HISTORIC AND PRESENT MODIFIED QUALITY OF WATER COLORADO RIVER-AVERAGE VALUES 1941-1970 Location Concentration (mg/ll Historic Modified Glenwood Springs, Colorado Cameo, Colorado Cisco, Utah Lees Ferry, Arizona Grand Canyon, Arizona Hoover Dam, Arizona-Nevada Imperial Dam, Arizona~California 271 310 406 443 613 662 556 609 617 669 690 745 757 851 PHYSICAL INTERPRETATION Current water quality criteria! specify that detrimental effects will usually be noted on crops when concentrations in the irrigation water exceed 500 mg/I of total dissolved solids (TDS). The suggested guidelines for salinity in irrigation water are: Table 3 SUGGESTED GUIDELINES FOR SALINITY IN IRRIGATION WATER! Crop response TDS (mg/l) Water for which no detrimental effects will usually be noticed Water which can have detrimental effects on sensitive crops Water that may have adverse effects on many crops and requiring careful management practices Water that can be used for salt- tolerant plants on permeable soils with careful management practices 0-500 500,1,000 1,000-2,000 2,000-5,000 A very complex relationship is involved in relating salinity of irrigation water to crop yield. Major factors involved are soil conditions, crop type and variety, drainage, climate, and cultural and irrigation practices. Many high-valued crops are grown with Colorado River water. Generally, such crops hJve a low salt tolerance thus yield is repressed by the salinity. With respect to criteria for public water supplies, desirable levels are specified as less than 200 mg/l and the recommended level should not exceed 500 mg/l. From Table 2 it is apparent that some present salinity levels exceed these values. It should be emphasized that when time intervals are reduced to a monthly basis, wide fluctuations can be expected. Under historic conditions at Imperial Dam, the salinity concentration for January 1957 was 1,000 mg/l and for December 1967 it was 992 mg/l. Six other months in the period 1941 to 1970 have had average concentrations above 960 mg/l. Moreover, under present conditions of depletion, the mean monthly concentration of 1,000 mg/l at Imperial Dam would have been exceeded in 40 months during the period 1941-70. Such monthly salinity values have greater significance than long-term means in relation to impacts on land and crops, water quality standards, and water treatment. PROJECTED SALINITY LEVELS The salinity in the Colorado River has been the object of longstanding concern and study. Studies by various agencies converge to one simple fact-salinity will increase with continued use and development of the available water supply unless comprehensive, basinwide water quality management planning is implemented and supported by the installation of structural and nonstructural. measures. Projected estimates by various entities are presented in Table 4. These estimates assume that no measures are undertaken to control salinity. The differences in the values reported by the various agencies arise from assumptions made regarding completion dates for water development projects, estimates of the amount of salt loading or concentration effects produced by these projects, the period of analysis used, and estimates of the time involved for the effects to emerge at Imperial Dam_ It is significant that all studies by the various agencies predicted that proposed developments will cause a considerable increase in the future salinity of the river. Even under current salinity conditions, many irrigators are resorting to special practices in using the water to grow salt-sensitive crops. Some areas have adverse drainage conditions which would be magnified if higher 1 "Water Quality Criteria" Report of the National Technical Advisory Committee to the Secretary of the Interior, USDI, April 1968. 15