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<br />Salinity From Natural Causes <br />Natural salinity increases result from accretions from natural <br />sources and water lost to the river system from consumption by <br />phreatophytes and other riparian vegetation and by evaporation from <br />the river water surface and backwater areas. <br />As summarized in Table 1, the Environmental Protection Agency <br />in its report estimated that almost two-thirds of the average annual <br />salt load and one-half the concentration at Hoover Dam for the period <br />1942-6~ was caused by natural sources. AL~ost the same res~lts were <br />shown in the report for the 296)-66 period. Of the portion that is <br />from natural sources, a computation based on Table 2 shows that about <br />S2 percent is from diffuse sources and about 18 percent fro~ point <br />sources. <br />Natural diffuse pickup of mineral salts by surface runoff and <br />ground water inflow takes place throughout the Colorado River Basin. <br />The headwa~ers area, which makes up only a small part of the total <br />Basin, is composed of weather resistant crJs~alline rocks containing <br />constituents of very low solubility; as a result, the runoff from <br />the high mountain ,area has a low salinity. The watersheds of ~he <br />lower elevations are made up of materials generally having a relatively <br />high solubility which results in ~~off from these lands having a <br />higher salinity. <br />Natural discrete, or point, salinity sources also occur through- <br />out the Basin. Many springs and. other natural point sources dis- <br />charge highly saline flows into the Basin's streams. While their <br />combined flow is relatively small, their effect on the river salinity <br />is significant. Table 1 of Appendix E lists the major point sources <br /> <br />., <br /> <br />-~4- <br />