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1989 refers to the FWC of the period December 1984 to December 1989. It 

 should be emphasized that the calculations have been based on the results 

 of all samples collected for the three-month and five-year period, and 

 not restricted to samples collected during March to October. 



Total Dissolved Solids 

 There is an inverse relationship between TDS and streamflow at the 

 International Boundary station. During periods of high runoff, such as 

 spring freshet, TDS drops as the proportion of streamflow derived 

 ultimately from groundwater decreases. Conversely, during times of low 

 streamflow (late summer, winter) the contribution of groundwater to 

 streamflow is proportionally greater. Because the natural groundwater 

 has a higher ionic strength than the surface water entering the river, 

 the TDS of the stream increases markedly during low flow conditions. 



TDS grab sample data collected by Environment Canada and the USGS from 

 January 1981 to December 1990 are shown in Figure 8. In Figure 9, 

 individual years are simplified by annual box plots. For any one year 

 the maximum and minimum values are displayed by lines extending from a 

 box. The box displays the 25th, 50th (median), and 75th percentiles for 

 that year. Except for 1982 and 1983, the data ranges remain somewhat 

 close to the median. In 1982 and 1983, high flow conditions (Figure 10) 

 likely explain the extremely low TDS values. The TDS observations at the 

 higher end of the concentration range are probably a consequence of 

 groundwater contributions to streamflow during low flow conditions. 

 Seasonally, the lowest TDS concentrations occur from March to May 

 coinciding with high flow conditions (Figures 11 and 12) . 



