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P ART V HISTORICAL AND PRESENT SALINITY CONDITIONS e Sedimentation in reservoirs may influence both TDS and the ratio of dissolved ions. Suspended sediment which is subject to mechan- ical degradation in a river environment may continue to release salts and exchange ions (sodium exchanged for calcium); however, once settled out in the reservoir, these salts and ion exchange capabilities may be isolated. Sediment stored in reservoirs may contain salts which would have been released with continued mechanical breakdown in a riverine environment. 3. Irrigation and Increased Depletions Most of the irrigation projects that deplete water and increase salt pickup to the river were largely in place before 1965. Moreover, like the newly inundated soils in reservoirs, newly irrigated lands are subject to a leach-out period. In cases where lands with poor drainage stored salt, these areas were taken out of production. In addition, irrigation practices changed significantly during the 1960-80 period, with canal and lateral lining, sprinkling systems, gated pipe, and trickle systems being introduced. These changes should result in re- duced return flows and salt pickup. Projected water depletions through the 1965-80 period were largely unrealized; total depletions increased by approximately 12 percent. Years 1970-79 1960-69 1950-59 1940-49 1930-39 1920-29 1910-19 1900-09 Average annual depletions (1,000 acre-feet) Upper Basin 3,565 2,538 2,043 1,894 1,712 1,998 1,656 1,001 by basin e Lower Basin 6,136 6,119 4,757 3,776 3,676 3,662 3,643 3,373 Previous Reclamation salinity projections have been too high, largely because the depletion projections were also too high. Trans- basin diversions and increased reservoir evaporation account for most of the increased depletions from 1960-80; however, no additional salt pickup or loading occurred with these depletions. The large quantities of water expected to be depleted for steam power generation, coal gasification, oil shale, and mineral de- velopment have not been realized in the past decade. Even where new coal-fired powerplants have been constructed, some of the water has been obtained from existing agricultural rights. While water uses have often changed, the total depletions have increased only slightly. e In cases where powerplant water was obtained from existing agricultural supplies, salt pickup may have been reduced since irri- gated lands in areas of coal deposits are often saline soils of Mancos Shale origin. 26