Monitoring by MPC in early March 1987 downstream from 

 Milltown Dam and the confluence of the Blackfoot River showed 

 moderate concentrations of acid-soluble zinc. River values 

 on March 5 through 9 were 370, 220, 410, 980, and 50 ug/1, 

 respectively. These findings indicate that a water quality 

 event that may control young fish survival may be triggered 

 by the first rapid increase in river flow after the stable 

 flow period of winter. Additional monitoring needs to be 

 performed during this time of year to determine if early 

 snowmelt events occur regularly and if they are an important 

 element in the Clark Fork fishery problems. 



Lower Clark Fork . The Flathead River more than doubles 

 the volume of the Clark Fork, on the average. The result is 

 a dramatic improvement in the water quality of the Clark Fork 

 below the confluence. During the WQB monitoring period, 

 copper criteria were rarely exceeded in samples from the 

 lower river section and have not been documented below 

 Thompson Falls. As shown in Figure 3-20, copper concentra- 

 tions were stable and quite low at all four stations. 

 Exceedences of zinc criteria have not been documented in the 

 lower river. 



Suspended Sediment 



The amount of sediment in a river is important because 

 of its potential effect on beneficial uses of the water. A 

 large volume of sediment in a system can adversely affect 

 aquatic life and interfere with water treatment and irriga- 

 tion. Other pollutants, such as nutrients and metals, can be 

 adsorbed onto sediment particles and transported by them into 

 and through aquatic systems. 



Suspended sediment transport in running waters is 

 difficult to quantify accurately, especially in a river 

 system as complex and as large as the Clark Fork watershed. 

 Suspended sediment concentrations and loads in the Clark Fork 

 system are strongly influenced by variations in streamflows 

 and intensity of runoff events. Each of the three years 

 monitored was characterized by lower than normal runoff, on 

 the whole. FY 85 and 87 were particularly low streamflow 

 years, and suspended sediment production, transport, and 

 severity of problems were generally low. Conversely, the 

 rapid snowmelt event of February 1986 created unusually high 

 mid-winter streamflows and excessive sediment concentrations. 

 A large percentage of the estimated annual suspended sediment 

 load was transported during this relatively short-duration 

 event. Total annual suspended sediment loads and mean 

 concentrations in FY 86 were well above FY 85 or 87 values, 

 due primarily to the February snowmelt event. 



3-74 



