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~;';G'7 DISSOLVED-SOLIDS LOADS IN THE COLORADO AND GUNNISON RIVERS Monthly and annual dissolved-solids loads were computed for the four gaging stations in the Grand Valley area (fig. I) for water years 1970-93. Loads were computed for two stations (09095500 and 09 I 63500) on the Colorado Ri ver, one station (09152500) on the Gunnison River, and one station (09105000) on Plateau Creek (fig. I). Stream dis- charge in the Gunnison River has been highly regulated since 1965 after completion of the Aspinall Unit (Blue Mesa and Morrow Point Reservoirs in fig. I), and most of the major water-storage projects in the Colorado River Basin upstream from Cameo were completed prior to ] 965 (Liebem,ann and others, 1988). Major changes in the flow regime of a river can cause trends in water-quality concentrations that might not be related to other anthropogenic effects, such as salinity- control projects. To avoid the possible effects of water- storage projects, the trend analysis of salinity data was limited to periods after 1965. The first water year after ] 965 that had concurrent data for all dissolved-solids and major-ion constituents that were used in the trend analysis was 1970; therefore, dissolved-solids loads were computed for water years 1970-93. The monthly and annual dissolved-solids loads were computed for use in trend analysis. Also, the dissolved-solids loads for the four gaging stations were used to estimate the dissolved-solids load from the Grand Valley. Trend tests also were done on the annual Grand Valley dissolved-solids load. Method of Computation A computer program called SLOAD (Liebermann and others, 1987) was used to esti- mate dissolved-solids loads. The program uses the daily stream-discharge record and periodic water- quality samples to estimate loads. If available, the program can incorporate daily specific conductance into the load determinations, which often improves the estimate of dissolved-solids load. The program is writ- ten in Statistical Analysis System (SAS) language (SAS Institute, 1982). Regression relations are computed by SLOAD using periodic, instantaneous data that relate disso]ved- solids loads as a function of stream discharge or of stream discharge and specific conductance. Equations are of the form: In(dsload) = a + b[ln(Q)] (I) where In dsload a and b = natural logarithm; = dissolved-solids load, in tons; = regression coefficients; and = stream discharge, in cubic feet per second; Q or In(dsload) = c + d[ln(Q)] + e[ln(SC)] (2) where c, d, and e SC = regression coefficients; and = speci fic conductance, in micro- siemens per centimeter at 25 degrees Celsius. Liebermann and others (1987) used a logarith- mic transformation of the data to approximate normal distributions. SLOAD computes 3-year moving regressions for each regression relation. The 3-year moving regression method does not remove existing time trends (Kircher and others, 1984; Liebermann and others, ] 987). Once regression coefficients are com- puted, the program computes daily dissolved-solids loads from equation I using daily mean stream discharge. If daily mean specific conductance also is available, then equation 2 is used. An assumption for use of this method of computing daily loads is that the regression coefficients derived from periodic, instantaneous data are applicable to the daily mean values (Liebermann and others, 1987). The daily loads are summed by month to compute monthly loads, which then are summed to compute annual loads (either by water year or calendar year). The SLOAD program was used to compute dissolved-solids loads for stations 09095500, 09152500, and 09163500. For station 09105000, Plateau Creek near Cameo (fig. I), there were only sufficient data to use SLOAD for ]970-79. For 1980-93, monthly loads for Plateau Creek were esti- mated from regression relations with the monthly load at station 09095500. DISSOLVED-SOLIDS LOADS IN THE COLORADO AND GUNNtSON RIVERS 7