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Table 3. Monotonic trends in monthly dissolved-solids loads 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 are in tons per month per year; percent is the slope expressed as percent change per year; p value is the significance level of the test; 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 N than or equal to 0.05; MS is marginally significant, p is greater than 0.05 and less than Of equal to 0.10; and NS is not significant, p is greater than 0.10; 1'.' <, less than] tv UnadJuated monthly load N Station Period Flow-adjusted monthly load Slope Percent p value SL Slope Percent p value SL 09095500 1970-93 -686 -{).56 <0.001 HS 333 -{).27 <0.001 HS 1980-93 -1,280 -1.03 <.001 HS -687 -.55 .001 HS 1986-93 -5,440 -4.69 <.001 HS -3,840 -3.31 <.001 HS 09152500 1970-93 -660 -.63 .002 HS -451 -.43 <.001 HS 1980-93 -866 -.83 .074 MS 317 .30 .201 NS 1986-93 -2.210 -2.26 .125 NS 332 .34 .592 NS 09163500 1970-93 -2,180 -.80 <.001 HS -1,680 -.61 <.001 HS 1980-93 -2,930 -1.06 .001 HS -1,080 -.39 .009 HS 1986-93 -12,100 -4.72 <.001 HS -4,480 -1.74 <.001 HS Annual Dissolved-Solids Loads None of the trends in the unadjusted annual dissolved-solids loads were significant (table 4). The flow-adjusted annual loads for the Gunnison River at station 09152500 and for the Colorado River at station 09163500 had significant or highly significant downward trends for water years 1970--93. The LOWESS smooth curve in figure 8 indicates that the flow-adjusted annual loads decreased at station 09163500 during the 1970's and again after 1986. The trend slope of -62,300 tons/yr for 1986-93 for station 09163500 was highly significant (table 4). The flow"adjusted annual loads for 1986-93 at station 09095500 also were decreasing, and the trend was highly significant. The LOWESS smooth curve for station 09152500 (fig. 8) indicates that most of the trend in flow-adjusted annual loads for 1970--93 in the Gunnison River occurred before 1980. The trends in flow-adjusted annual loads for 1980--93 and 1986-93 (table 4) for station 09152500 were not significant. There were fewer significant trends in annual dissolved-solids loads than in monthly dissolved-solids loads. One problem with using annual data is that the sample sizes are decreased compared to data sets com- posed of monthly data. The p value for hypothesis test- ing is affected by sample size. For a given magnitude (trend magnitude for trend tests) and variance, p values tend to increase as the sample size decreases (Helsel and Hirsch, 1992); therefore, it becomes more difficult to reject the null hypothesis (of no trend) as sample size becomes smaller. Although the trend tests on the flow- adjusted annual loads for water years 1986-93 are based on only eight values, the magnitude of the slopes for the two Colorado River stations (09095500 and 09163500 in table 4) were sufficiently large to have highly significant p values. Seasonal Dissolved-Solids Loads Seasonal dissolved-solids loads represent the total load for selected periods, or seasons, within a year. Seasonal loads were computed by summing the monthly loads of the individual months in a season and then were treated as annual values for performing trend. tests using linear-regression analysis. The trend results would reflect only the trend slopes and p values for loads for a particular season and would be independent from the remainder of the year. Flow-adjusted trends were determined by using the average daily stream- flows for each seasonal period of each year in the flow- adjustment models. Trends were analyzed for two sea- sonal periods; August through October (late irrigation- season effects) and November through February (low- flow effects). 18 Trend 'Analysla 01 Selected Water-Quality Data Asaoclated With Sallnlty-Control Prolecta In the Grand Valley, In the 'Lower Gunnison River Basin, and at Meeker Dome, Western Colorado