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<br />man's activities. Salt concentrating results when water is <br /> <br /> <br />evaporation or transpiration within the Basin or when water <br /> <br />lost through, <br />,1'\. <br />. ,'. <br />of lower -. :i:., <br /> <br />Nj" <br />00 <br />~,. <br />N <br /> <br />salinity than that of downstream points is diverted from the Basin. <br />The result is an increase in downstream salinity due to the remaining <br />amount of salt being carried in less water. <br />Studies of the effects of salt loading and salt concentrating <br />within the Colorado River Basin have been conducted for about 20 <br /> <br />years. Although adequate information is lacking to accurately identify <br /> <br /> <br />all contributing sources of salinity, the studies have identified the <br /> <br /> <br />major sources of increasing salinity in the river as it flows from <br /> <br /> <br />the headwaters to the Gulf of California. However, present under- <br /> <br /> <br />standing of the cause and effect relationships is limited with <br /> <br /> <br />respect to natural and man-caused salinity in irrigated areas. <br /> <br /> <br />The relative effects of salt loading and salt concentrating <br /> <br /> <br />factors on salinity in the river at Hoover Dam for the period 1942-61, <br /> <br /> <br />as estimated by the Environmental Protection Agency in its 1971 report, <br /> <br /> <br />"The Mineral Quality Problem in the Colorado River Basin", are shown <br /> <br /> <br />in Table 1. Some of the estimates in EPA's report are being <br /> <br /> <br />investigated and brought up to date by USER studies currently <br /> <br /> <br />underway. <br /> <br />-12- <br /> <br />... _ u _ _ .._ _... <br />.. .-. <br />