97 



600 ft from the dam. Since the inverted outlet is only 8.5 ft above the 

 old streambed, discharge into the Beaverhead River comes from the deepest 

 part of the reservoir. Generally, the reservoir is completely ice covered 

 by mid-December and ice-out occurs by mid-April. 



The maximum and minimum storage pools in the reservoir for the 1965 

 through 1976 water years are compared in Figure 2. The severe drawdown 

 in the summer of 1975 is the result of basing water allotments on predicted 

 inflows from watershed snowpack information, rather than the known content 

 of the reservoir. This procedure is no longer practiced. 



Limnology 



Limnological data was taken from a study conducted by Berg (1974) 

 between June, 1971 and November, 1972. 



Clark Canyon Reservoir characteristically develops a thermocline, 

 turning over in spring and fall. Thermocline depth ranged from 6.6 to 

 73.8 ft and euphotic zone depth ranged from 16.4 to 50.9 ft. The minimum 

 and maximum conductivity readings were 420 and 650 uohms, respectively. 

 Dissolved oxygen near the surface ranged from 6.5 to 10.8 ppm and from 

 0.2 to 11.1 ppm on the bottom. Low DO concentrations in water over 50 

 ft in depth limit the use of this portion of the reservoir by fish. The 

 pH ranged from 7.8 to 8.6 and, as with DO, highs occurred during overturn 

 and lows during summer stagnation. Following spring and summer overturn, 

 levels of plant nutrients in the euphotic zone were reduced, probably as 

 a result of uptake by phytoplankton and transfer to the deep water zone 

 by sinking. Ranges of total alkalinity and total hardness were 177 to 

 222 ppm as CaCO., and 157 to 230 ppm as CaCO-,, respectively. 



