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. N en w Salinity From Natural Causes W . Natural salinity increases result from accretions from natural sources and water lost to the river system from consumption by phreatophytes and other riparian vegetation and by evaporation from the river water surface and backwater areas. . As summarized in Table 1, the Environmental Protection Agency in its report estimated that almost two-thirds of the average annual . salt load and one-half the concentration at Hoover Dam for the period 1942-61 was caused by natural sources. Almost the same results were shown in the report for the 1963-66 period. Of the portion that is . from natural sources, a computation based on Table 1 shows that about $2 percent is from diffuse sources and about 1$ percent from point sources. . Natural diffuse pickup of mineral salts by surface runoff and ground water inflow takes place throughout the Colorado River Basin. The headwaters area, which makes up only a small part of the total . Basin, is composed of weather resistant crystalline rocks containing constituents of very low solubility; as a result, the runoff from the high mountain area has a low salinity. The watersheds of the lower elevations are made up of materials generally having a relatively . high solubility which results in runoff from these lands having a higher salinity. Natural discrete, or point, salinity sources also occur through- out the Basin. Many springs and other natural point sources dis- . charge highly saline flows into the Basin's streams. While their combined flow is relatively small, their effect on the river salinity is significant. Table 1 of Appendix E lists the major point sources . -14- . ;k(f'~