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The drainage area for site 2 includes that of site 1, the Colorado River at Glenwood Springs, Colo" and the intervening area between the two sites. lorns and others (1965) computed the mass balance in the Roaring Fork River tv basin, which is part of the intervening drainage area. They estimated that C) 34,540 acre-feet of water would have been consumptively used annually on (~ 31,400 acres of irrigated land and between 94,200 and 121,400 tons per year of CO' dissolved solids would have been added to the Roaring Fork River because of development, The midpoint of this range, 107,800 tons per year, .was used in table 2 as the dissolved-solids discharge contributed by irrigation in the Roaring Fork River basin. For the remaining intervening drainage area, lorns and others (1965) estimated that 77,830 acre-feet of water per year would have been consumptively used on 48,300 acres of irrigated land and 144,910 tons per year of dissolved solids would have been added because of development. . The historical conditions for site 2 did not include the water or dis- solved solids removed in transbasin diversions through the Twin Lakes and the Busk-lvanhoe tunnels in the Roaring Fork River basin (all transbasin diver- sions referred to in the mass-balance analyses are listed in table 3 and their locations are shown in figure 1), The exported streamflow and dissolved- solids discharge was added in the mass balance to estimate natural conditions. lorns and others (1965) reported the mean annual diversion for water ye1\rs 1954-57 to be 37,500 acre-feet and estimated the dissolved-solids concen- tration as 60 milligrams per liter. Based on these estimates, 3,060 tons per year of dissolved solids were exported from the basin. To complete the mass balance, anthropogenic effects in the upstream area had to be considered. From a.previous mass balance for site 1, the Colorado River near Glenwood Springs, Colo., an estimated 395,350 acre-feet per year of water would have been consumptively used because of development in the bllsin and an estfimated 108,260 tons per year of dissolved solids would have been added to the reach, The natural dissolved-solids discharge for site 2, the Colorado Rh:er near Cameo, Colo., then was computed by subtracting the dissolved-solids ~i~- charge contributed by irrigation from the historical dissolved-solids dis-. charge at the site. The dissolved-solids discharge contributed by upst.ream development also was subtracted, and the dissolved-solids discharge removed in transbasin exports was added. Natural streamflow was computed by adding all the water consumptively used by development to the historical streamflow, The resulting mean annual natural streamflow for water years 1914-57 was 3,543,220 acre-feet and the mean annual natural dissolved-solids discharge was 1,220,090 tons. Based on these values, the mean annual dissolved-solids concentration at the site would have been approximately 255 milligrams per liter dur~ng 1914-57 had there been no development in the basin. . Mass-balance computations for the other 15 sites considered in this study are tabulated in the '''Supplemental Informatfion" section at the back of this report, A summary of results is reported in table 4, This table also' includes the mean annual natural streamflow estimated by the D.S, Bureau of Reclamation for water years 1914-57 (D.S, Bureau of Reclamation, written commun, , 1983), which were slightly different from the mass-balance estimates. Because the estimates of natural dissolved-solids discharge from this report were intended to be used in conjunction with the D,S. Bureau of Reclamation's 10