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SUMMARY l\:) l\:) Ut o Salinity, or the dissolved-solids concentration, can have adverse effects on crops and on municipal and industrial users in the Colorado River Basin. The Colorado River Basin Salinity Control Act was passed in 1974 and authorized construction and planning of . salinity-control projects to maintain salinity at or below existing levels in the Colorado River. This report pre- sents the trends in salinity data and relates the trend results to three salinity-control projects in western Colorado. 1\\'0 of the salinity projects were in the Grand Valley and in the lower Gunnison River Basin, and their purpose was to decrease dissolved-solids loading to the Colorado River from irrigation-induced sources. The salinity-control projects began in 1979 in the Grand Valley and in 1988 in the lower Gunnison River Basin. The third project concerned dissolved- solids loading to the White River from three abandoned oil wells on Meeker Dome, near the town of Meeker. The wells were plugged by June 1981 by the Bureau of Reclamation. Monthly and annual dissolved-solids loads were estimated for water years 1970--93 for four gaging stations that represent the major inflow and outflow of surface water in the Grand Valley. The load data were used to estimate annual dissolved-solids load from the Grand Valley and also were used in trend analysis. For water years 1970--93, the mean annual dissolved- solids (salinity) load in the Colorado River near the Colorado.Utah State line was about 3.32 million tons, and about 16 percent of that load was from the Grand Valley. To determine if the salinity-control projects in the Grand Valley and the lower Gunnison River Basin have affected salinity in the Colorado River, monotonic trend analysis was done using periodic dissolved-solids and selected major-ion concentrations and monthly, annual, and seasonal dissolved-solids loads for three gaging stations. The stations were 09095500 Colorado River near Cameo, which is upstream from the Grand Valley; station 09152500 Gunnison River near Grand Junction, which is the outflow site for the Gunnison River; and station 09163500 Colorado River near the Colorado-Utah State line, which is downstream from the Gunnison River and the Grand Valley. Trend anal- ysis also was done on the annual dissolved-solids load from the Grand Valley. The period selected for trend analysis was water years 1970--93 and two periods within those years, 1980--93 and 1986-93. The salinity-control projects would not have begun to affect dissolved solids in the Colorado River until 1980, and most of the reductions in dissolved solids probably . . occurred after 1985. Monotonic trends for dissolved- solids and major-ion concentrations and monthly dissolved-solids loads were examined using a comput- erized procedure developed by the U.S. Geological Survey called ESTREND. The program ESTREND uses the nonparametric seasonal Kendall test and deter- mines the magnitude of the trend slope and the associ- ated significance level of the test. Trends in annual and seasonal dissolved-solids loads at the three gaging stations and the annual Grand Valley dissolved-solids load were analyzed using linear regression, which is a parametric method. Because streamflow can affect trend tests, the original concentration and load data and the flow-adjusted data were analyzed for trends. A graphical technique that involved use of a data- smoothing procedure called LOWESS was used to aid in interpretation of the trend results. Except for monthly dissolved-solids loads, many trends for the three time periods in dissolved-solids, calcium, magnesium, sodium, and sulfate concentra- tions and annual and seasonal dissolved-solids loads that were not adjusted for streamflow were not signifi- cant (significance level greater than 0.10) or were mar- ginally significant (significance level between 0.05 and 0.10). After flow adjustment, many trend tests on dissolved-solids and the four major-ion concentrations and on the monthly loads were significant (significance level between 0.01 and 0.05) or were highly significant (significance level less than or equal to 0.01). Most of the trend tests on flow-adjusted annual and seasonal loads and on the annual Grand Valley dissolved-solids load were not significant. Flow-adjusted monthly dissolved. solids loads and many of the flow-adjusted concentrations had significant or highly significant downward trends during water years 1970--93 for all three stations. The LOWESS smoothing indicated downward trends in the pre-1980 period at all three stations and downward trends after 1985 or 1986 at the two Colorado River stations. Many trend tests for water years 1980--93 on flow-adjusted data were not significant. The flow-adjustment method might not have completely adjusted for the highly variable streamflows during 1980--93. Many of the flow- adjusted trends for water years 1980--93 and 1986-93 for the Gunnison River (at station 09152500) were not significant. 36 Trend Analyals of Salected Water-Quality Data Associated With Salinity-Control Prolects In the Grand Valley. In the Lower Gunnison River Basin, and at Meeker Dome, Western Colorado