Sxi^tuu^ ^ctua^am 



LIMNOLOGY 



The drawdown of Tongue River Reservoir in anticipation of repairs to the 

 discharge tunnel which were scheduled for the fall of 1975 was begun in July 

 1975. Storage in the reservoir reached a minimum of 16.15 hm-^ (13,100 af) on 

 September 30 and increased to 23.99 hm^ (19,460 af) on November 1 when repairs 

 began (figure 30). Discharge was maintained at approximately 0.57 m^/sec (20 

 cfs) for the month of November while repairs were made. Due to reduced dis- 

 charges, reservoir storage increased to 43.40 hm^ (35,200 af) on December 31. 

 The reservoir stage remained constant throughout the winter, increasing to a 

 peak of 73.72 hm3 (59,790 af) on June 30. Discharge was regulated in response 

 to increased downstream demands in late summer, when it exceeded inflow, but 

 was diminished in the fall relative to the inflow to increase storage to about 

 43.16 hm3 (35,000 af) for the winter months. 



The Tongue River Reservoir can be characterized as a bicarbonate water with 

 calcium as its dominant cation (Whalen et al . 1976). Other abundant ions are 

 SO4, Mg, and Na. Winter G2 sampling suggested a low potential for winter fish 

 kill in the reservoir since O2 did not fall below lOmg/1 during ice cover. Win- 

 ter fish kill potential for cool-water fish does not become critical until O9 con- 

 centrations fall below 5 mg/1. Even with increased organic production, little 

 winter kill would probably result because of the short retention time of the re- 

 servoir. It was determined that the mine discharge water is altered ground water. 

 The major alteration is an increase in conductivity as a result of increased Ca, 

 HCO3, Mg, and SO4. Mine water flow was never greater than 0.47 percent of the 

 Tongue River flow and thus had a negligible effect on the river because of dilu- 

 tion. 



Van Voast and Hedges (1975) predict that, with three mines in operation, 

 mine effluent will increase to 0.17 m^/sec (6.0 cfs). Assuming this flow level 

 under hypothetical flow and chemistry conditions, Whalen and Leath (1976) pre- 

 dicted little change in river quality. 



FISH POPULATIONS 

 HISTORY 



The Tongue River Reservoir and a portion of the drainage upstream were chem- 

 ically treated in 1957 to remove undesirable fish species. Following rehabilit- 

 ation, the reservoir was stocked with rainbow trout in an attempt to duplicate 

 fishing which commonly follows the initial impounding of reservoirs. Over two 

 million fingerling rainbow trout were planted during the years 1958-1960. Gill 

 net sampling in November 1959 produced 80 rainbow per net night; sampling in 1960 

 took only seven rainbow per net night. Stocking with trout was stopped because 

 the undesirable fish species had again built up to high population levels. How- 

 ever, correspondence in 1962 suggests that the reservoir was still producing some 



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