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<br /> <br />Of the land within the Colorado River Basin, about 75 <br />percent is owned and administered by the federal government <br />or held in trust for Indians Tribes. By far the greatest <br />~ portion of natural salt load originates on these federally <br />~ owned and administered lands. Of the salinity resulting <br />~ from human activities, irrigated agriculture accounts for <br />v1 the largest share. Much of this contribution is from <br />federally developed irrigation projects. <br />Evaluations of the salinity of the Colorado River have <br />been made by Reclamation, U.S. Geological Survey (USGS), <br />Environmental Protection Agency (EPA) , and Bureau of Land <br />Management (BLM). Appendix C is a list of selected federal <br />reports prepared on salinity related studies conducted in <br />the Colorado River Basin. ' <br />In order to evaluate changes in salinity, water <br />quality and .treamflow data are obtained on a daily, <br />weekly, monthly, or quarterly basis at various points on <br />streams throughout the Basin by the USGS in cooperation <br />with the states and other federal agencies. Average <br />salinity concentrations and salt loads are determined on a <br />flow weighted basis using the most frequent data <br />available. Gaging stations in the Basin that are of <br />significance to this report and for which streamflow and <br />water quality records are available are listed on Table 1. <br />This table shows the availability of streamflow and water <br />quality data for key stations during the period 1941-1986 <br />and the current frequency of sampling as classified by the <br />USGS. Where the water quality information is not complete, <br />the missing data have been estimated by correlation ~ith <br />data from other stations. <br />Historical Salinity Conditions <br />Historically salinity concentrations of the river have <br />fluctuated significantly over the period of record, <br />1941-1986. Figure 2 depicts the mean annual historical <br />river salinity contentration at Imperial Dam. Salinity <br />concentrations generally decrease in periods of high flows <br />and increase in periods of low flow. Figure 3 shows the <br />mean annual flo~ of the Colorado River at Imperial Dam. <br />Salinity concentrations at Imperial Dam decreased <br />steadily from 1970-79, increased in 1981-82, and decreased <br />significantly from 1983-86. The period 1983 through 1986 <br />was a period of above normal Basin runoff. Each of the <br />four years had a natural flow in excess of 20.0 million <br />acre-feet, with the four year average of 22.6 million <br />acre-feet. Only one other period has had 20 million <br />acre-feet of natural flow for two or more consecutive <br />years--1920 and 1921, and only one period--1920-23 had an <br />average natural flow exceeding 20.0 maf (20.4 million). <br />During 1983-86, the annual calendar year flow to Mexico <br />exceeded scheduled deliveries by 12.6, 13.8, 10.1, and 9.2 <br />million acre-feet, respectively, for a total of 45.7 maf. <br />The record high flows during the period 1983-86 have <br />resulted in a significant reduction in salinity <br />concentrations in the lower main stem by approximately 250 <br /> <br />-7- <br />