Figure 3-24 depicts the range of suspended sediment con- 

 centrations in the Clark Fork during the WQB monitoring 

 period. There were general increases in concentrations and 

 reduced fisheries protection in the upper Clark Fork from the 

 headwaters downstream to monitoring station 12, the Clark 

 Fork at Bonita. The plots of total and volatile suspended 

 sediment load (Figures 3-25 and 3-26) point to the stream 

 reaches between monitoring stations 9 and 10 and 11 and 12 as 

 possibly containing significant sediment sources in the upper 

 Clark Fork, especially during FY 86. The worst overall reach 

 in the upper Clark Fork from the standpoint of fisheries 

 protection was from station 10 to station 12. Suspended 

 sediment concentrations fell in the moderate to low levels of 

 fisheries protection categories in 27 to 55 percent of the 

 samples. The presence of streamside tailings deposits and 

 unstable streambanks throughout the upper Clark Fork are the 

 probable causes. 



Rock Creek, located between monitoring stations 12 and 

 13, is a large tributary that normally carries low concentra- 

 tions of suspended sediment. Clark Fork median suspended 

 sediment concentrations downstream of the Rock Creek 

 confluence were measurably decreased (Figure 3-24) at all 

 times, except during the February 1986 flood. Concentra- 

 tions were also significantly more favorable from the 

 standpoint of fisheries protection. 



Downstream from station 13, the Blackfoot River joins 

 the Clark Fork. This large stream equals the Clark Fork in 

 size, and its suspended sediment concentrations average a 

 quarter to half those in the Clark Fork above the Blackfoot. 

 Its inflow, plus the Milltown Reservoir which is a large 

 sediment trap, decrease Clark Fork sediment concentrations. 

 However, during high-flow events and during past operational 

 drawdowns and construction activities, the settled sediments 

 in the reservoir were mobilized and transported downstream. 

 The reservoir is a significant sediment source in those 

 instances. 



Organic suspended sediment concentrations were generally 

 low throughout the upper Clark Fork and averaged a small 

 fraction of the total suspended sediment concentration. The 

 Deer Lodge sewage discharge appeared to cause measurable 

 though small increases in Clark Fork organic suspended 

 sediment concentrations for several miles downstream of the 

 discharge. 



3-77 



