Middle Clark Fork . Suspended sediment concentrations in 

 the middle Clark Fork from Missoula to the Flathead River 

 (Figure 3-24) , can be described as generally decreasing in a 

 downstream direction as a result of additional dilution from 

 cleaner incoming tributaries, such as the Bitterroot River. 

 Concentrations normally fall within the range that would 

 afford a high level of protection to freshwater fisheries. 



Although Bitterroot River suspended sediment concentra- 

 tions are lower than the mainstem, suspended sediment load 

 plots (Figures 3-25 and 3-26) indicate that the Bitterroot 

 River is the most significant source of sediment loading to 

 the middle Clark Fork. Both the Missoula WWTP and Stone 

 Container Corporation wastewater discharges contributed 

 sizeable, largely organic suspended sediment loads to the 

 middle Clark Fork. However, their influences on river 

 concentrations and load were not measurable. 



Lower Clark Fork . Suspended sediment concentrations in 

 the lower Clark Fork are shown in Figure 3-24. The Flathead 

 River more than doubles the volume of the Clark Fork and 

 routinely carries a lower suspended sediment concentration 

 than the Clark Fork. As a result, suspended sediment 

 concentrations measured in the Clark Fork downstream of the 

 Flathead are reduced and nearly always fall within the 

 highest category for fisheries protection. Farther down- 

 stream, the Noxon Rapids Reservoir acts as a settling basin 

 and is responsible for an even more significant reduction in 

 Clark Fork suspended sediment concentration. The last 

 reservoir in the system. Cabinet Gorge, has no apparent 

 effect, presumably because most of the settleable solids have 

 already been trapped upstream. In general, the lower Clark 

 Fork can be described as excellent from the standpoint of 

 suspended sediment concentrations,' largely as a result of 

 dilution by the Flathead and the influences of the reser- 

 voirs. 



Suspended sediment load plots point to the Flathead 

 River as the only significant additional source of sediment 

 to the lower Clark Fork. The reservoirs are responsible for 

 reducing Clark Fork suspended sediment loads to less than 

 those carried by the Clark Fork above the Flathead River. 



Other Water Quality Parameters c 



A number of parameters or conditions other than metals 

 and sediment cause degradation of surface water quality in 

 the Clark Fork, including ammonia, elevated temperature, 

 dissolved oxygen, toxins, foam, and color. These are 

 discussed in the following sections. 



3-78 



