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IV-1 ~... eD en Io!:~ PART IV. CAUSES AND IMPACTS OF SALINITY A. Causes of Salinity The natural or background salinity of the Colorado River has been changed by the development of water resources in two major ways, the addition of salts and the depletion of water. One of the original studies by Iorns [2J on the pickup of salts showed that irrigated lands in the Upper Colorado River Basin contribute about 3.4 million tons of salt per year. This source of salt is one of the 2 major factors increasing salinity in the Colorado River. The other major factor is the consumptive use of water. The consumptive use reduces the dilution of both natural sources of salt and the new sources of salt caused by irrigation and other developments. 1. Natural Sources of Salinity Flow and quality records reveal that along certain reaches of the Colorado Rive~ large increases in salt loads occur that cannot be attributed to irrigation or other development related activities. These increases are mainly due to natural diffuse sources and saline springs. Very little information was obtained prior to irrigation, making it difficult to identify the magnitude of specific natural sources of salinity in the Colorado River Basin. Natural diffuse sources are those sources of salt which occur gradually over long reaches of the river system. Salt pickup occurs over large surface areas, from underlying soils, and from stream channels and banks. It is difficult to identify, measure, or control. Diffuse sources contribute the largest overall share of the salts to the Colorado River. The natural salt load for the Colorado River at Lees Ferry, Arizona,was estimated to be about 5.3 million tons per year [2,3]. Natural point sources are mainly saline springs where the contribution of salt and water is easily identified, issuing from single or concentrated sources. 2. Agricultural Sources of Salinity Salt balance conditions exist when the amount of dissolved solids carried off the land is equal to the amount added. Pickup of salt as used in this report represents an unbalanced condition shown by the increase of salt load in the return flow over the total load in the applied water. Salt pickup attributed to irrigation is that additional amount which occurs as a result of irrigation and does not include the amount resulting from natural sources. Irrigation development in the Upper Basin took place gradually from the beginning of settlement in about 1860 but was hastened by the purchase of land from the Indians in 1873. About 800,000 acres were being irrigated by 1905. Between 1905 and 1920, the development of irrigated land increased at a rapid rate, and by 1920, nearly 1.4 million acres were being irrigated. The development then leveled off, and increase since that time has been slow