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~ to", r-v CJl 1--'" PART III. CAUSES AND IMPACTS OF SALINITY A. Increased Concentration from Salt Additions 1. Natural Sources of Salinity Flow and quality records reveal that along certain reaches of the Colorado River there are large increases in the dissolved solids load that cannot be attrihuted to irrigation or other man-related sc ti vity. This increase is mal nly due to natural di ffuse sources and saline springs. Very little information was ohtained prior to irriga- tion, making it difficult to identify the magnitude of specific natural sources of salinity in the Colorado River Basin. Natural di ffuse sources are those sources of salt contribu- tion 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. 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 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. Ahout 800,000 acres were heing irrigated hy 1905. Between 1905 and 1920 the development of irri- gated land increased at a rapid rate, and hy 1920 nearly 1.4 million acres were being irrigated. The development then leveled off, and in- crease since that time has been slow because of physical and economic limitations on the availability of water. 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 hegan 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 1974, nearly 849,000 acres were irrigated from Colorado River diversions helow Hoover Dam. . Irrigation in the Colorado River Basin has increased the TDS 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. 12