data available in Newell 1976 or in Montana DNRC files), selected because it 

 was the last month bottom samples were obtained. 



The population was summed for all subsections. At a discharge of 9000 cfs 

 (about mean low summer discharge), the population estimate is approximately 

 209,000 for a bank-to-bank, one-meter-wide strip of river bottom at Intake 

 (table 23). This number decreases to about 190,000 at 8,000 cfs and approximately 

 172,000 at 7,000 cfs, about a ten-percent reduction in population with each 

 1 ,000-cfs reduction in discharge. 



TABLE 23. Invertebrate population estimates utilizing data from Intake 

 station 18, subsections from WSP (Water Surface Profile), and regression 

 equation from November kick samples. 



a Shown in figure 59 



Population estimates at 7,000, 8,000, and 9,000 cfs are graphed in 

 figure 60; a diagramatic representation of loss of habitat due to water with- 

 drawal is shown in figure 59. Stage at 9,000 cfs is 1985.30 ft at cross-section 5 

 (opposite the boat launch at Intake). Stage decreased to 1985.15 ft at 8,000 cfs 

 and 1984.90 ft at 7,000 cfs. Thus the river drops only a few inches as dis- 

 charges decrease by 1000 cfs, and only a small percentage of the river bottom 

 is exposed. All of these calculations apply to transect 5 at Intake; the river 

 bottom figuration changes at other locations, as do current and population. 



98 



