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u01922 642 J, ENVIRON, QUAL,. VOL. 27, MAY-JUNE 1998 tion by the movement of water through a subsurface flow system or overland flow. The many mineral springs and seeps present in the VCOL study unit, especially in the Glenwood Springs/Dotsero areas, comprise the largest natural source of dissolved solids in the study area (Apodaca et aI., 1996). Municipal and industrial wastes deposit dissolved solids into streams, and irriga- tion return flow, primarily from subsurface percolation of water through saline materials rather than surface- water runoff, discharges large amounts of dissolved sol- ids in the lower Colorado River and its tributaries in Colorado, downstream from Cameo. In the entire seven-state Colorado River Basin, irrigation is the great- est human-induced source of dissolved solids (Law and Hornsby, 1982). METHODS Dissolved-Solids Data and Time Periods of Analysis Dissolved-solids data used in the analyses of trends in con- centrations and loads at sites in the Upper Colorado River Basin in Colorado were obtained from three sources: (i) the U.S. Geological Survey (USGS) National Water Information System (NWIS); (ii) the Bureau of Reclamation (BOR) (U.S. Department of the Interior, 1995); and (iii) the U.S. Environ- mental Protection Agency (USEP A) STOrage and RETrieval (STORET) system. The BOR data are published dissolved- solids loads that were calculated from NWIS data (U.S, De- partment of the Interior, 1995), Data used from STORET were collected by the Colorado Department of Public Health and Environment (CDPHE), Sources of data used in the anal- yses are listed in Table 1, along with the mean annual dis- solved-solids concentration for each site. Site selection was based on the presence of the water-quality record and daily- value records of stream discharge and specific conductance. Butler (1996) chose three time periods for analysis correspond- ing to completion of major water-storage projects, data avail- ability, and timing of salinity-control work: 1. 1970 to 1993 water years-past completion date of major water-storage projects and concurrent records of periodic ma- jor-ion data available. 2. 1980 to 1993 water years-major period of salinity-control work in the Grand Valley. 3. 1986 to 1993 water years-increase in salinity-control projects in the Grand Valley and implementation of salinity- control projects in the lower Gunnison River Basin. The same analysis periods are used in this report. Dissolved-solids and discharge data for some sites were not complete for the entire 1970 to 1993 period. For the Alva B. Adams Tunnel site, data prior to 1980 were incomplete or missing. Therefore, only data for 1980 to 1993 and 1986 to 1993 were analyzed (Table 1), Data also were incomplete or missing prior to 1986 for the sites Muddy Creek at Kremmling and Rock Creek at Crater. These two sites were analyzed for 1986 to 1993 only. The two Gunnison River Basin headwaters sites, Taylor River at Almont and East River at its confluence with Taylor River, did not have complete data records through 1993. These sites are the only sites with long-term data avail- able in the headwaters area of the Upper Gunnison Basin and, therefore, were included in this analysis using shorter periods. Analysis periods for each site are listed in Table 1. Dissolved-Solids Variables Three dissolved-solids variables are discussed in this report: periodic concentration, monthly load, and annual load, Peri- odic dissolved-solids concentration is based on periodic mea- surements of dissolved solids and discharge. Where data are available, the periodic dissolved-solids concentration is com- puted as the sum of the eight major constituents present in a natural stream: calcium (CaH), magnesium (MgH), sodium Table 1. Mean annual dissolved-solids concentrations in milligrams per liter for sites in the Upper Colorado River Basin in Colorado, water years 1970 to 1993, 1980 to 1993, and 1986 to 1993. Based on estimated annual salt load and measured streamftow. Data used in dissolved-solids concentration computation for Colorado River above Glenwood Springs, Colorado River near Cameo, and Gunnison River at Grand Junction are from NWIS and BOR.t Site no. Source of (Fig. 1) Site name and USGS or STORET station no. data 1970 to 1993 1980 to 1993 1986 to 1993 1 Alva B. Adams Tunnel at East Portal NWIS 28.9 29.7 09013000 2 Colorado River at Hot Sulphur Springs NWIS 75.3 74.6 80.2 09034500 3 Muddy Creek at Kremmling NWIS 367 09041500 4 Rock Creek at Crater NWIS 60A 09060550 5 Eagle River at Gypsum NWIS 288 283 291 090690000 6 Colorado River near Dotsero STORET 202 192 197 000046 (CDPHE) 7 Colorado River above Glenwood Springs NWIS 250 251 278 09071750 BOR 8 Roaring Fork River at mouth STORET 274 267 280 000053 (CDPHE) 9 Colorado River near Cameo NWIS 380 373 413 09095500 BOR 10 Taylor River at Almont STORET 76.9 76.9 78.0 000058 (CDPHE) (1970-1992) (1980-1992) (1986-1992) 11 East River at conftuence with Taylor River STORET 138 139 142 000078 (CDPHE) (1970-1991) (1980-1991) (1986-1991) 12 Uncompaghre River at Delta STORET 1129 1050 1121 000055 (CDPHE) 13 Gunnison River at Grand Junction NWIS 483 442 491 09152500 BOR t NWIS = National Water Information System; BOR = Bureau of Reclamation; USGS = U.S. Geological Survey; STORET = STOrage and RETrieval system; CDPHE = Colorado Department of Public Health and Environment.