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<br />addition of soluble salts to the river, and salt concen- <br />tration, caused by a reduction in the volume of river <br />water as a result of evaporation, transpiration, or <br />withdrawals of water. Almost one-half of the river's <br />salt loading is estimated to come from interaction of the <br />river with the basin's naturally saline soil and rocks. <br />In addition to precipitation percolating through the soil <br />and dissolving salts, the three million acres of irri- <br />gated farmland in the region add more soluble minerals <br />to the river as irrigation water leaches minerals from <br />cultivated soils. Salt concentrations in the river are <br />further increased by transpiration by plants and <br />evaporation of water from reservoirs. Exports of over <br />five million acre feet from the basin reduce the <br />potential for dilution downstream. <br />salinity levels vary throughout the basin. <br />Generally, the Colorado River, at its headwaters in <br />north-central colorado, has a salinity concentration of <br />about 50 mg/L (milligrams per litre). The salinity <br />concentration progressively increases as the river flows <br />downstream. Recent record high flows have flushed and <br />filled the major reservoirs, resulting in significantly <br />lower salinity levels at Imperial Dam - from an annual <br />average of 826 mg/L in 1982 to 608 mg/L in 1985. Without <br /> <br />-26- <br />