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002847 Utilizing actual streamflow and water-quality records for sites on the Yampa River near Maybell and Little Snake River near Lily, time-trend analyses were carried out using a set of annual mean values of specific conductance at each site as a function of stream discharge and adjusting for streamflow ef- fects. This procedure is documented by Steele, Gilroy, and Hawkinson (1974) and utilizes the Kendall's Tau nonparametric statistical test on each ranked annual time series (Conover, 1971). Using data for the 1951-72 water-year period of record, a significant long-term increase in specific conductance of 14 percent was observed for the Yampa River (Steele and others, 1974, table 9, p. 67); however, no significant change was observed for the Little Snake River. The trend in increasing salinity for the Yampa River is attributed to increas- ing demands using surface water for agricultural and municipal purposes. Using a shorter period of record (1951-63 water years), no significant changes at either of these two sites were reported in a study by the Colorado River Basin Water Quality Control Project (U.S. Department of the Interior, 1970, table 1). ~. To demonstrate seasonal variability of stream quality, figure 7 indicates the fluctations in observed specific conductance and stream discharge at site Y-47 (fig. 1) from August 1975 to September 1976. The pronounced increase in discharge with corresponding decrease in specific conductance from April through July is typical year after year and elsewhere in the basin. Stream-reach profiles of specific conductance for the mainstem Yampa and Little Snake Rivers are shown in figure 8. Data for the Yampa River were col- lected only downstream to site Y-O (river mife 46 or km 74) during the August- September 1975 reconnaissance. At this point the river enters a steep-sided canyon in Dinosaur National Monument (fig. 1), and access to the lower reaches of the river is restricted until it enters the Green River. In August 1976, a reconnaissance of the lower Yampa River was conducted during low-flow condi- tions to assess the water quality of that part of the Yampa River in Dinosaur National Monument. Specific-conductance values at river mile 46 (km 74) for the 2 years agreed quite well (460 and 445 ~mhos/cm at 250C for 1975 and 1976, respectively). . ~ Most of the specific conductance increase in the Yampa River occurs near the headwaters area (fig. 8), where values change from 100 ~mhos/cm?r less to between 300 and 400 ~mhos/cm in a 20-mile (32-km) reach. Further increases, resulting from irrigation return flows and inflows of naturally high conduct- ance water, are balanced by dilution downstream to the confluence with the Little Snake River. The Little Snake River near its mouth had a specific con- ductance of more than twice that of the Yampa River just upstream and caused the specific conductance of the Yampa River to increase by about 40 percent. In the 46-mile (74-km) reach within Dinosaur National Monument, specific con- ductance increased by 50 ~mhos/cm during the August 1976 reconnaissance, even though there are no major tributaries entering this reach. The dominant fac- tor causing this increase is probably evaporation, as a net decrease in stream discharge of 18 ft3/s (0.5 m3/s) was observed in this 46-mile (74-km) reach. . Iorns, Hembree, and Oakland (1965) estimated that the annual dissolved- solids load of the Yampa River subbasin was about 1.8 times that of the Little Snake River subbasin. However, the streamflow of the Yampa River is nearly 2.6 times that of the Little Snake River. The dissolved-solids load 13 i.~