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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 />00 <br />~ phreatophytes and other riparian vegetation and by evaporation from <br />~ <br /> <br />the river water surface and backwater areas. <br /> <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 />1942-61 was caused by natural sources. Almost the same results were <br />shown in the report for the 196)-66 period, Of the portion that is <br />from natural sources, a computation based on Table 1 shows that about <br />82 percent is from diffuse sources and about 18 percent from point <br /> <br />sources. <br /> <br />Natural diffuse pic~up of mineral salts by surface runoff and <br /> <br /> <br />ground water inflow takes place throughout the Colorado River Basin. <br /> <br /> <br />The headwaters area, which makes up only a small part of the total <br /> <br /> <br />Basin, is composed of weather resistant crystalline rocks containing <br /> <br /> <br />constituents of very low solubility; as a result, the runoff from <br /> <br /> <br />the high mountain area has a low salinity. The watersheds of the <br />. ( <br />/ <br />lower elevations are made up of materials generally having a relatively <br />high solubility which results in runoff 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 />-14- <br />