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r, '" ., "'- G'i U;) flows occurred during 1983-86 and below-average flows occurred during 1988-92 (fig. 3). Low dissolved-solids concentrations occurred during the high-flow period, and higher concentrations generally occurred during the low-flow period, which caused an upward trend in concentration. Also, there might have been carryover or persistence of lower dissolved-solids concentrations into 1987 caused by discharge of dilute water stored from the high runoff years in reservoirs and in shallow aquifers. In contrast to unadjusted concentrations for water years 1970-93, the flow-adjusted concentrations for 1970-93 were decreasing and had highly signifi- cant trends for the three stations (table 2). The LOWESS smooth curves for Ihe flow-adjusted dissolved-solids concentrations for 1970-93 are shown in figure 4. The lIow-adjusted concentrations are resid- uals from log-log regressions of concentration to streamflow. All regression relations used for 1I0w adjustment were significant and had p values less than 0.00]. The LOWESS smooth curve for 1970-93 seems to indicate that all of the decrease in flow- adjusted dissolved-solids concentrations at station 09152500 in 1970-93 had occurred before about 1981 (fig. 4). Salinity-control projects did not stan in the Gunnison River Basin until 1988. The smooth curves in figure 4 for stations 09095500 and 09163500 indicate that there were decreasing flow- adjusted dissolved-solids concentrations in the Colorado River during the ] 970's. Flow-adjusted dissolved-solids concentra- tions did not have significant trends (alpha 0.05) for water years 1980-93 (table 2). The LOWESS smooth curves for the flow-adjusted concentration data for 1980-93 are shown in figure 5. The large variations in streamflow during 1980-93 (fig. 3) might have affected the flow adjustment of data for this period. The flow- adjustment regressions tended to underpredict (posi- tive residuals) the dissolved-solids concentrations dur- ing 1983-86, and perhaps the flow adjustment was overcompensating for high streamflow during the mid-] 980's. The LOWESS plots in figure 5 also indicate generally decreasing trends in flow-adjusted concentrations after about 1986, especially for stations 09095500 and 09163500. The trend results (table 2) for 1986-93 indicated highly significant trends of decreasing flow-adjusted concentrations for these two stations. If significant trends had occurred only at station 09163500, those trends could be evi- dence that the salinity-control projects were causing a decrease in dissolved-solids concentrations during 1986-93. However, there were similar trends (table 2) Table 2. Monotonic trends in periodic dissolved-solids concentrations for gaging stations 09095500 Colorado River near Cameo, 09152500 Gunnison River near Grand Junction, and 09163500 Colorado River near the Colorado-Utah State line, water years 1970-93 {Periods are in water years; slopes me in milligrams per liter per year; percent is the slope expressed as percent change per year; p value is the significance level of the tesl; SL, significance levels, which are: HS is highly significant, p is less than or equal to 0.01; S is significant. p is greater than 0.01 and less than or equal to 0.05: MS is marginally significant, p is greater than 0.05 and less lhan or equal to 0.10; and NS is not significDnt, p is greater than 0.10: <.Iess than] Station Period Unadjusted concentration Flow~adJusted concentration Slope Percent p value SL Slope Percent p value SL 09095500 t97(1..93 0.50 0.10 0.669 NS -2.23 -0.44 0.001 HS t98(1..93 6.88 1.42 .Ot4 S -3.20 -.66 .051 MS t986-93 3.60 .71 .539 NS -to -2.82 <.001 HS 09152500 197(1..93 --{j.35 -.96 .055 MS -4.86 -.73 <.OOt HS 198(1..93 6.86 1.14 .385 NS .53 .09 .804 NS 1986-93 42.6 6.69 .045 S -4.67 -.73 .274 NS 09163500 197(1..93 -2.50 -.36 .300 NS -3.37 -.48 .002 HS 198(1..93 5.90 .89 .149 NS -1.14 -.17 .479 NS 1986-93 7.22 1.04 .524 NS -12.5 -1.80 <.001 HS TREND ANALYSIS FOR THE COLORADO AND GUNNISON RtVERS 15