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.- .. . ;~ ~"']...i.. ..) Relation of Trend Results for the Colorado and Gunnison Rivers to the Salinity-Control Projects The purpose of the trend analyses for the Colorado and Gunnison Rivers was to determine if the salinity-control projects (Grand Valley Unit and Lower Gunnison Basin Unit) have decreased dissolved solids in the Colorado River. Station 09163500 on the Colorado River near the State line is downstream from both salinity-control projects, and decreases in dissolved-solids loads from irrigated areas could affect the dissolved solids at station 09163500. Trends of decreasing dissolved-solids concentrations or loads at stalion 09]63500 could indicate that the salinity- control projects have removed some dissolved- solids loading to the Colorado River. Trends for station 09 I 63500 need to be compared to trends at stations 09095500 (Colorado River near Cameo) and 09152500 (Gunnison River near Grand Junction) before conclusions regarding effects of salinity control can be stated with confidence. Station 09095500 is upstream from the Grand Valley, and station 09]52500 is the outflow site for the Gunnison River Basin and is downstream from the Lower Gunnison Basin Unit. Trends at the inflow stations 09095500 and 09152500 potentially could be the primary cause of trends at station 09 I 63500. Downward dissolved-solids trends at station 09095500 could not be caused by salinity- control projects in the Grand Valley or in the lower Gunnison River Basin. Trends at station 09152500 could be related to salinity-control work in the Lower Gunnison Basin Unit or to other factors affecting dissolved solids in the Gunnison River. Because the salinity-control project did not start until 1988 in the Lower Gunnison Basin Unit, there was less likelihood that significant trends at station 09152500 would be related to salinity-control projects. A consideration in relaling salinity trends in the Colorado River to salinity-control projects con- cerns the quantity of dissolved solids removed by the salinity-control projects. Have the salinity- control projects removed a sufficient quantity of dissolved-solids load to cause detectable trends in concentrations or loads in the Colorado River? The quantity of dissolved solids that should have been removed by BOR and USDA salinity-control projects in the Grand Valley and Lower Gunnison Basin Units by 1993 was estimated. Various reports (Bureau of Reclamation, 1986, 1987; Hedlund, 1994; U.S. Department of the Interior, 1994, 1995) were used to estimate the quantity of dissolved solids removed through water year] 993. Perhaps as much as ] 90,000 to 210,000 tonslyr of dissolved solids was removed by the salinity-control projects by 1993. About 75 percent of the estimated decrease in dissolved-solids load would have resulted from salinity-contro] projects in the Grand Valley Unit. The annual mean dissolved- solids load at station 09163500 for 1970-80 (table I) was about 3.31 million tons; theoretically, the salinity- control projects would have caused about a 6-percent decrease in pre-project annual dissolved-solids loads in the Colorado River by 1993. However, the disso]ved- solids load removed by ] 993 was cumulative because the salinity-control work began in the Grand Valley Unit in 1979 and in the Lower Gunnison Basin Unit in 1988. Decreases in dissolved solids by ] 993 have been incrementally added since] 979, and the year-to-year salinity decreases probably were variable, as more salinity-control work was done in some years than in other years. Because nearly all of the dissolved-solids loading from irrigation sources is from ground-water discharge, there might be some lag time between con- struction of a particular salinity-control feature and its effect on dissolved-solids loading to the Colorado River. There is the queslion whether incremental decreases in dissolved-solids loading from irrigated areas caused by the salinity-control projects over a 14-year period would produce detectable trends in dissolved-solids and major-ion concentrations or loads that are distinguishable from natural variability, c1imale-induced changes, or human-induced changes in the Upper Colorado River Basin. Some of the decreases in dissolved solids in the Colorado River caused by salinity-control projects could partly be off- set by increases in dissolved solids from natural causes or by increasing development and water use in the Upper Colorado River Basin. Trend tests indicated highly significant down- ward trends for the flow-adjusted dissolved-solids (table 2), magnesium, sodium, and sulfate concentra- lions (table 8) and for monthly and annual dissolved- solids loads (tables 3 and 4) at station 09]63500 during water years 1986-93. If only the trends for station 09163500 on the Colorado River near the State line for 1986-93 were considered, a relation between the salinity-control projects and trends of decreasing dissolved-solids concentrations and loads and decreasing major-ion concentrations at station 09163500 could be inferred. The cumula- tive decreases in dissolved-solids loading to the Colorado River resulting from the salinity-control TREND ANALYSIS FOR THE COLORADO AND GUNNISON RIVERS 29