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-1 percent at Yampa River below Stagecoach Reser- voir (site 1 in fig. 1) to 69 percent at Yampa River near Maybell (site 8) to 115 percent at Yampa River at mouth (site 12). Discharges of tributary rivers increased an average of 62 percent between the sampling periods, showing a substantial relative increase from -25 percent at Elk River near mouth (site 4) to 25 percent at Williams Fork at mouth (site 7) to 240 percent at Little Snake River above Yampa River (site 10). The downstream increase in discharge at Yampa River and tributary sites between the two sampling periods is consistent with late-winter melting of snow and ice at lower altitudes in the Yampa River Basin. This melting during March 2000 sampling caused a reversal of relative contributions of discharge to the Yampa River by tributary rivers compared to August 1999 sampling: the Elk River augmented discharge of the Yampa River by about 47 percent (77 percent during August 1999); the Williams Fork augmented it by about 20 percent (23 percent during August 1999); and the Little Snake River augmented it by about 52 percent (26 percent during August 1999). Average specific conductance at Yampa River sites increased by 58 percent during March 2000 (fig. 13) compared to August 1999 (fig. 3); at tributary sites, average specific conductance increased by only 1,000 LLI ~ Za ~ UZQJ ~wwU Zw_II jw 500 O (n ~_ W U~~(~'} U~U~ aZ N 5 percent. Most of the increase at Yampa River sites between the two sampling periods occurred at site 6. Although average concentrations of all major ions increased at Yampa River sites during March 2000, the largest average increases were with sodium (88 percent), magnesium (93 percent), and sulfate (163 percent). Concentrations of dissolved ammonia, dissolved phosphorus, and dissolved orthophosphorus were less than the reporting limits at all sites downstream from site 3 during both sampling periods. Average concen- tration of dissolved nitrogen as ammonia plus organic nitrogen at all Yampa River sites increased only 4 percent during March 2000 compared to August 1999. However, concentrations of nitrite plus nitrate at all Yampa River sites increased about 900 percent, on average (assuming that concentrations less than the reporting limit were one-half the reporting limit) during March 2000 compared to August 1999; the largest increase (greater than 1,200 percent) occurred at site 6. Concentrations of dissolved organic carbon also increased an average of about 32 percent at site 6, although concentrations decreased or were about equal at sites sampled upstream from site 6. The substantial increases in specific conductance and in concentra- tions of nitrite plus nitrate and dissolved organic . 9 , 6 ~,~ 11 12 8 ^ -----___i.-' ~ ^10 !1 \ 3^~ '~5 ~,,i, 2 ^4 0 230 220 200 180 160 140 120 100 80 60 40 20 DISTANCE FROM MOUTH OF YAMPA RIVER, IN RIVER MILES EXPLANATION -f- Specific conductance at Yampa River site ^ Specific conductance at tributary site Note: number by symbol is site number in tables 1 and 3 Figure 13. Specific conductance at Yampa River and tributary sites, March 13-16, 2000. INTERPRETATION OF DATA COLLECTED FOR THIS STUDY 1g