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Show PART II PRESENT STATUS OF IRRIGATION inhibit infiltration, and deposition can reduce capacity of storage and conveyance systems. Waters of high sediment content when used in sprin- kler irrigation can cause undue wear of pumps, nozzles, and deposition of films on leaf surfaces affecting plant growth and marketability. The increased use of insecticides, fungicides, and herbicides is be- coming of increasing interest in consideration of water quality. Concen- trations are generally very low and have little effect on irrigation in the region. Suitability of water supply for irrigation In general, the quality of the water supply in the Upper Colorado Region is suitable for existing irrigated agriculture. The residual sodium carbonates, chlorides, and boron concentration in the irrigation supply are usually well within acceptable limits. Total dissolved solids and the sodium adsorption ratios, though acceptable overall, do present some local problems. Generally, water in streams increases in concen- tration of total dissolved solids from natural and man-induced sources, including irrigation uses, as it flows downstream. The pattern of in- creasing mineral concentration downstream is due primarily to the type of geologic formations underlying the drainages. While the headwaters are, for the most part, underlain by igneous or well-leached sedimentary rocks, the lower reaches of the streams in areas of low rainfall fre- quently dissect sedimentary formations of marine origin containing high concentrations of soluble salt. These salt-laden beds are the principal sources of stream contamination. The quality is further reduced in the lower reaches as a result of stream depletion and the leaching of salts from irrigated lands. Details concerning the quality of surface and ground water in the region follow. Maps showing the areal distribution of surface and ground water quality, and a more detailed discussion thereof, are presented in Appendix XV, Water Quality, Pollution Control, and Health Factors. Surface Water Water quality of streams near their headwaters is considered excel- lent for irrigation, as the weighted average concentration of total dis- solved solids is less than 100 milligrams per liter and the sodium ad- sorption ratio as well as the sediment content is very low. During the 19^1-66 period, the salt load of the Colorado River at Lee Ferry, Ariz. ,i/ the outlet of the region, averaged about 8.2 million tons annually. Of this total, about 33 percent originated in the Green 1/ Lee Ferry is the official compact point dividing the Upper and Lower Colorado Regions. It lies immediately below Lees Ferry, which is the location of the stream gage. 33 |