°= 5 

 iij 



h- 

 Ui 



£io 



X 



l- 



Q- 15 

 o 



20 



FiGi'RE 3. — Water temperature, oxygen concentrations, 

 and .silicon dioxide concentrations of Brownlee Reservoir 

 from stations 20 to 22 at the time of formation of a 

 visible convergence line, October 23, 1963. 



sense to the "spring turnover"' in lakes. Tempera- 

 tures at this time ranged from 0.5" to 7.2° C. 



Vertical stratification continued to strengthen 

 after mid-March, and by early Jime a thermo- 

 cline began to develop in the upper resem^oir. Also 

 in .Time, a visible convergence line (Frey, 1963) 

 was observed in the upper reservoir near station 

 20. The cooler, heavier water from the Snake 

 River dipped at this point and flowed under the 

 warmer, lighter water of the reservoir. This con- 

 dition was repeated in the fall (fig. 3). A sharp 

 increase in temperature, higher concentration of 

 oxygen (10 p.p.m.), and a dip in concentration 

 of silicon dioxide were characteristic chemical and 

 physical features obsen^ed in the vicinity of the 

 convergence line in October 1963. 



The temperature ditferenc<? was 2.2° to 2.7° C. 

 from one side of the convergence line to the other 

 most of the time that the line could be identified. 

 The temperature gradient near the visible line 

 usually was extended over 0.8 km. The extreme 

 difference of 2.7° C. during one sampling period, 

 however, was from one side of the boat and the 

 other. Turbidity readings and oxygen concentra- 

 tions were higher upstream from the line, whereas 

 conducti\ities were lower downstream. Wind ac- 

 tion and the extent of discharge at the dam seemed 

 to control the movement of the line upstream or 

 downstream in the reservoir, as well as the sharp- 

 ness of the temperature gradient. 



The tliermocline continued to develop during 

 late spring and early sununer of 1963; it was 

 well defuied from the upper reser\'oir to the dam 

 by July 8. The stratification was then complete 

 with a well-defined epilimnion, tliermocline, and 



hypolimnion. Temperatures ranged from 15° to 

 25.5° C. in the epilimnion, 10° to 15.5° C. in the 

 tliermocline, and 3.9° to 9.5° C. in the hypo- 

 lunnion. 



Stratification continued until mid-October 1963, 

 when the convergence or shear line was again 

 foniied. The heavier, cooler water from the river 

 flowed under the warm e]>ilimnion and evidently 

 eroded the thennocline; distinct separation of iso- 

 therms was gradually eliminated until only a 

 remnant of the thermocline remained in Novem- 

 ber. The reservoir was again in a state of turn- 

 over by December 9 when isothenns aligned ver- 

 tically (fig. 2). Temperatures in the Powder 

 River arm were nearly identical to those in the 

 main reservoir. 



The general patterns of the isotherms in 1962 

 and 1963 were similar except for time of forma- 

 tion of the thermocline. During the sununer of 

 1962 a well-defined thermocline was not formed 

 until early September, whereas in 1963 it was es- 

 tablished by late June. Temperature ranges for 

 tlie various months also differed; the greatest dif- 

 ference was in August. In 1962 the maximum was 

 27.2° C. whereas in 1963 it was 25.5° C. The tem- 

 perature range for the epilimnion in August 1962 

 was 20.5° to 27.2° C. ; in 1963 it was between 16.6° 

 and 25.5° C. The reservoir temperatures gradually 

 and consistently increased in 1962 without abrupt 

 changes until September. In 1963 the trend was the 

 same until the last of May, when a brief reversal 

 occurred. The combination of heavy rain and melt- 

 ing snow in the headwaters caused a large volume 

 of cool water to enter the Snake River above the 

 reservoir (table 1). This mass of water cooled the 

 entire upper end of the reservoir about 3.3° C. 

 ( see fig. 4 ) . 



Although air temperatures in June, July, and 

 August 1962 were warmer than for the same pe- 

 riod in 1963 (table 2), the thermocline developed 

 earlier in 1963 than in 1962. The later fonnation 

 of the thermocline in 1962 was due to the extent 

 of reservoir drawdown and the length of the sub- 

 sequent filling period. The drawdown in the winter 

 of 1961 and ensuing months prolonged fill-up in 

 the spring and early sununer of 1962; it took place 

 over a significant depth of 13.7 m. and caused 

 enough water circulation to delay formation of the 

 thermocline, despite the warmer air temperatures 

 that year. By contrast, drawdown in the winter 



302 



U.S. FISH AND WILDLIFE SERVICE 



