constituent transported by a stream or water discharge per 

 unit of time, e.g., pounds of phosphorus per day. 



The key to the relationship between constituent 

 concentrations and loads is the volume of water in the river. 

 As the Clark Fork flows downstream, it is joined by numerous 

 tributaries, and its volume becomes progressively larger. 

 Each tributary contributes X number of pounds per day of 

 material to the Clark Fork, which adds to the load of 

 material carried by the river. However, the tributaries 

 generally have lower concentrations of those materials than 

 the Clark Fork, and their inflows help to reduce concentra- 

 tions in the Clark Fork through dilution. This is how a 

 tributary like the Bitterroot River can be a major source of 

 nitrogen loading to the middle Clark Fork, while at the same 

 time cause a reduction in nitrogen concentration in the 

 middle Clark Fork. 



The WQB monitored water quality constituents and stream- 

 flow at each of a number of mainstem locations along the 

 Clark Fork. Measurements were taken once to twice per month 

 from August to March and twice per month from April to July. 

 Monitoring was carefully timed according to streamflow and 

 other factors that would influence water quality. This 

 increased the probability that the data were representative 

 of the time interval (month or half month) . Monthly average 

 constituent concentrations and streamflows were estimated by 

 averaging the instantaneous measurements that were made 

 during each month. Where USGS gaging stations corresponded 

 with WQB sampling sites (most stations) , monthly average 

 streamflows based on continuous measurement were provided by 

 the USGS. These monthly average flows were used to replace 

 the instantaneous average flows. Monthly constituent loads 

 were then computed and summed to provide approximations of 

 total annual loads at each monitoring location. 



Water quality criteria and federal drinking water 

 standards discussed in this section are provided in Tables 

 3-15 and 3-16, respectively. 



Heavy Metals 



Copper and zinc are potentially the most hazardous 

 metals in the Clark Fork system due to their toxic effects on 

 aquatic life. Except at very high concentrations, the 

 presence of copper and zinc does not preclude other water 

 uses. Copper is more toxic than zinc and is a slightly ,^ , 

 greater problem in the Clark Fork. Zinc concentrations, 

 however, are typically higher than copper concentrations 

 throughout the system. Synergistic effects of both copper 

 and zinc (effects that are greater than the combined 



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