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. N CD Diversions from the Upper Colorado River Basin into surrounding W 0"1 basins occur at or near the headwaters where the river's water has . a very low salt content. This removal of high quality water, even though some salt is also removed from the Basin, results in . a salt concentrating effect downstream. Most industrial plants in the last few years, particularly large coal-fired thermal electric generating plants, have been designed to eliminate the return of cooling tower blowdown water to the Colorado River. As water is circulated through the cooling towers, a part of it evaporates and the dissolved solids in the remaining water become more and more concentrated. As cooling water must . . be maintained at or below specific levels of salinity, a portion of the concentrated water is removed or "blown down" and replaced with fresh water. When this blowdown is not returned to the river . system, the salt load in the water diverted is removed from the river. Even though these no-return uses remove salt from the river, the river's concentration of dissolved solids usually increases . below the point of use because of the reduced flow resulting from . the beneficial use. The effects on river salinity of a no-return consumptive use are similar to those of an out-of-basin export. Historically, salt loads contributed by municipal and industrial sources have been minor, totaling about 1 percent of the Basin salt load for the period 1942-61. With the exception of concentrated . returns from the Las Vegas, Nevada, area most municipal and industrial wastes in the Basin are relatively low in total salt load. . -16- .