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9.5 ~ 9.0 Z ~ 8.5 Q ~ 8.0 Z H (A 7.5 Z = 7.0 Q 6.5 1,000 QZ~ J Z J ~ ~ `~a~ J Z Q W U ~ Z ~U~ 1950-74 1975-99 Colorado water-quality standard • t• s .asp s•~. 0~ 1950 500 ~ • , ffi~~~ .. ~'.!` . . ~ t } ~ ~ ~ iet~~ Sl:• .~~. 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 DATE SAMPLED Figure 18. Temporal distribution of measured pH and dissolved solids at Yampa River near Maybell, 1950-99. the 1950-74 period had greater concentrations of chlo- ride (fig. 19), fluoride, and silica. Alkalinity (fig. 20) and concentration of potassium (fig. 19) were not significantly different between the two periods. The relatively large increase in median magne- sium concentration (25 percent) between the two periods compared to increases in median concentra- tions of calcium (7.9 percent) and sodium (8.3 percent) and unchanged median concentrations of potassium caused significantly greater cation-equiva- lent fractions of magnesium and significantly smaller cation-equivalent fractions of calcium, sodium, and potassium during the later period (table 2). Similarly, the large increase in median sulfate concentration (52.8 percent) between the two periods compared to decreased median concentrations of chloride (22.2 percent) and alkalinity (0.7 percent) caused a significantly greater anion-equivalent fraction of sulfate and significantly smaller anion-equivalent frac- tions of chloride and alkalinity during the later period (table 2). Judging by median equivalent percentages (table 2), the water at Yampa River near Maybell, on average, changed from acalcium-sodium-magnesium- bicarbonate-sulfate type during 1950-74 to a calcium- magnesium-sodium-bicarbonate-sulfate type during 1975-99, with sulfate becoming more dominant. The results of statistical hypothesis testing (table 2) are consistent with previous studies. Lieber- mann and others (1989) determined by monotonic- trend analysis that, at Yampa River near Maybell over the period 1951-83, median annual dissolved-solids concentration increased by 0.9 mg/L per year, a 21-percent increase (29 percent, 1.2 mg/L per year, when flow adjusted). Dissolved-solids load increased by 65 percent over the 33 years of record. In addition, they determined that annual flow-adjusted concentra- tions of magnesium, sodium, and sulfate were deter- mined to have significant upward trends for the period 1951-83. For the Yampa River near Maybell for the period 1951-96, Vaill and Butler (1999) determined significant annual increases in annual, flow-adjusted dissolved-solids concentrations (by 0.61 percent per year) and loads (by 0.57 percent per year). INTERPRETATION OF HISTORICAL DATA FROM YAMPA RIVER NEAR MAYBELL 27