The USGS uses cross-sectional depth-integration 

 techniques to sample suspended sediments during both high and 

 low streamflows. The WQB uses the Equal Width Increment 

 (EWI) depth-integration technique. However, most of the WQB 

 monitoring stations located below Garrison are too deep to 

 wade, as are some of the upper stations during runoff 

 conditions. In those instances, samples are depth-integrated 

 to the limit of wadeability, and as a result, only a portion 

 of the channel cross-section is sampled. In some cases, 

 suspended sediment samples are grab-sampled, but only when 

 streamflows are low and sediment concentrations negligible. 



The emphasis of the WQB Clark Fork water quality 

 assessment has been comparisons with aquatic life criteria 

 because those standards are usually more conservative than 

 the criteria established to protect other water uses. 

 However, it is a difficult proposition to establish aquatic 

 life criteria for suspended sediment concentrations, because 

 impacts are a function of duration of exposure as well as 

 concentration. For example, most Montana streams carry 

 appreciable suspended sediment concentrations during the 

 usually short period of sprin'g runoff. Resident aquatic life 

 forms are adapted to these annual events and are able to 

 tolerate them. The same conditions sustained over a longer 

 period of time could significantly degrade the aquatic 

 habitat. 



Because the periodic sampling programs are limited in 

 their ability to measure the duration of suspended sediment 

 concentrations, the WQB instantaneous data are compared to 

 simple criteria that are not based on duration of exposure. 

 The National Academy of Sciences-National Academy of 

 Engineering (1973) has published the following suspended 

 sediment guidelines for the maintenance of freshwater 

 fisheries. The frequency of distribution of measured values 

 among the various categories is the basis for the WQB 

 assessments in this report. 



Water normally containing suspended sediment concen- 

 trations of: 



<25 mg/1 High level of protection; no harmful 



effects on fisheries. 



25-80 mg/1 Moderate level of protection; good or 



moderate fisheries. 



80-400 mg/1 Low level of protection; unlikely to 



support good fisheries. 



>400 mg/1 Very low level of protection; only poor 



fisheries. 



3-75 



