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JAN FEB MAR APR MAY JUN JUL AUG SFP OCT NOV DEC 



Month 



Figure 12. — Monthly boxplots of streamflows for samples collected at East 

 Poplar River at International Boundary for period 1981-90. 



A histogram and summary statistics for TDS are shown in Figure 13. The 

 data are not normally distributed (90% confidence level), showing a sig- 

 nificant negative skew. Using the Seasonal Kendall Tau test for trend 

 assessment, TDS statistically increases significantly from 1981 to 1990 

 (90% confidence level) . The Seasonal Kendall Sen Slope estimates the 

 approximate TDS increase to be 3.75 mg/L/year (Figure 14). This posi- 

 tive trend could be explained by the drought conditions that occurred 

 over the later half of the data record. Low flow conditions (when flows 

 are derived largely from groundwater sources) likely increased TDS con- 

 centrations and yielded a positive TDS trend in the 1981-90 data. In 

 addition, increases in TDS come from salt buildup in the reservoir as a 

 result of evaporation. Natural reservoir evaporation is augmented by 

 heat transfer from the generating units to the reservoir as water is used 

 for cooling purposes . 



