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BASIC STUDIES <::> ~ w c.o lowest salinity runoff at Grand Canyon has been changed from April-August to July-December. The change in salinity regimen has not affected the re- lations between the dissolved constituents. However, the annual average dissolved-solids concentrations for 1963-65 are not comparable at the same water discharge with those for preceding years due to storage in and release from Lake Powell. The salinity of water below Hoover Dam varied irregularly as Lake Mead formed. Consequently, weighted averages prior to 1941 do not indi- cate present and future river conditions and only the weighted averages for the period 1941-65 are considered to be indicative of the recent river regimen and the quality of water available in the future for down- stream use. Data for selected years (Table A) which define median concen- tration, concentration during maximum storage in Lake Mead, and extremes of annual concentration illustrate the range for the period and suggest the probable future range, provided storage upstream from Lake Mead is considered. In order to show downstream changes, data for the same years for inflow to Imperial Dam are included in the table. Sources and movement of salts in Colorado River water are shown by comparison of ionic loads at points where they can be computed or esti- mated. As the ionic constituents dissolved in the river water are con- tinually transported downstream, the annual loads in tons-equivalent summed for many years indicate river reaches where salts are gained or lost and give evidence of chemical precipitation in the reservoirs or solution from their beds. The loads are summed in tons-equivalent rather than tons because at any point on the river the sum of the cation loads in tons-equivalent must equal. the sum of the anion loads in tons- equivalent; changes in proportion indicate chemical change by solution and precipitation. A computation of the salt burden of the Colorado River by calendar years for the 24-year period 1941-64, from Lees Ferry to Imperial Dam, with loads in thousands of tons-equivalent, is summarized in Table B. As might be expected the computation shows that the salt burden of the river is less at Lees Ferry than at any point down to Imperial Dam. Between Lees Ferry and Grand Canyon there was an increase in the burden of each ionic constituent. The salt burden at Hoover Dam was computed as the amount of salt in the water released from the dam. This cannot be com- pared directly with the burden at Grand Canyon because of change in the contents of Lake Mead between the beginning and the end of the 24-year period. The discharge and total loads shown for "effective flow to Hoover Dam" in Table B were computed by adjusting Hoover Dam releases for changes in contents of Lake Mead. The quality of the water in storage was assumed to be the same as that of the water released at the beginning and end of the period, an assumption not strictly valid at the particular times, but which introduces negligible error for a long period of years. When ad- justed for this change, it is found that there was a net increase between Grand Canyon and Hoover. Dam in all constituents except bicarbonate, which 30