FISHERY BULLETIN: VOL- 69, NO. 3 



to occur when the proposed power plant is in 

 operation. 



The daily maximum and minimum temper- 

 atures for Grace Creek (U.S. Geological Survey, 

 1966: 15; 1967: 17; 1968: 16) for the first and 

 second half of each month between April 16, 

 1965, and March 31, 1967, were averaged to de- 

 termine the annual temperature pattern under 

 normal conditions (Figure 1 ) . The highest tem- 

 perature for this period was 17.8° C and the low- 

 est was 0.6° C. Normally, Grace Creek temper- 

 atures reach a maximum of about 15° C at the 

 start of the spawning season in mid-August, 

 and decline to 10° C at the end of the spawning 

 season the first week of October. After the 

 spawning season, stream temperatures continue 

 to fall, reaching about 6° C by late November 

 and about 1.5° C in midwinter, when ice cover 

 forms on Lake Grace during most years. The 

 average stream temperature is about 1.5° C from 

 the end of December until the end of March. 



If the proposed power development on Lake 

 Grace is completed, it would significantly change 

 the physical dimensions and waterflow of the 

 lake. The dam at the outlet of Lake Grace would 

 raise the lake surface from the present eleva- 

 tion of 131.1 m (mean sea level) to a maximum 

 elevation of 152.4 m and increase its surface 

 area from 668 to 1,046 ha. Water would be di- 

 verted through a pressure tunnel and exposed 

 penstock to a powerhouse on Grace Creek, 1.21 



a 7 



BEFORE INSTALLATION 



AFTER INSTALLATION, LOW RAINFALL 



AFTER INSTALLATION. HIGH RAINFALL 



I I I I I I I I I 1 I I I I I I I I I M I I I I I I I I I I 11 I I I I I I I I I I I I I I I I I M I I 



JLT ' AUG ' SEPT ' OCT ' NOW ' OEC ' JAN ' FEB ' MAR ' APR ' MAY JUNE 



Figure 1. — Average annual temperature pattern of 

 Grace Creek, based on temperature records from April 

 16, 1965, through March "1, 1!)67, and temperature pat- 

 terns that might prevail during year.s of extreme high 

 and low rainfall if water is drawn from a reservoir at 

 the 125.0-m elevation. 



km downstream from the dam. The intake ele- 

 vation of the diversion tunnel would be constant 

 at 125.0 m, but the elevation of the reservoir 

 surface would change from 131.4 to 152.4 m 

 as the active reservoir capacity of 1.84 x 10' m' 

 is used. The maximum depth of the natural lake 

 is 129.5 m. 



Most of the water in the stream below the 

 powerhouse would come through the powerhouse 

 and would therefore be similar in temperature 

 to the water at the tunnel intake level of the 

 lake. To predict the temperature of this water, 

 we obtained temperature-depth jirofiles from 

 Lake Grace during the freshwater phase of the 

 reproductive cycle of pink salmon. Profiles were 

 taken on July 27, 1961, August 12, 1965, Sep- 

 tember 16, 1965, October 8, 1965, November 18, 

 1965, and March 25, 1965 (Figure 2). In addi- 

 tion to the temperature-depth profiles, we ob- 

 tained thermograph records which indicated 

 surface water temperatures attained 4° C No- 

 vember 28, 1965, and again May 28, 1966. These 

 thermograph records provide our best estimate 

 of the dates of autumn and spring overturn of 

 Lake Grace. Although these data were not all 

 taken during a single reproductive cycle of pink 

 salmon, we feel they are representative. These 

 pi-edicted estimated temjieratures of the lake 

 waters that would enter the intake are probably 

 higher than would actually occur because in- 

 creasing the depth of the lake increases its 

 thermal capacity and results in colder deep water 

 (Hutchinson, 1957). No correction was made 

 for the cooling eflFect of deeper water. 



The actual surface elevation of the reservoir 

 and therefore the depth of the tunnel intake 

 would depend on operational requirements of 

 the power plant and the flow of water into Lake 

 Grace. The fixed tunnel intake at the 125.0-m 

 elevation could be under 6.4 to 27.4 m of water 

 because the proposed active reservoir elevation 

 varies from 131.4 to 152.4 m. We allowed for 

 these fluctuations in using the temperature-depth 

 profiles as an indication of water tempei-atures 

 at tunnel intake depth. 



The project development plan for Lake Grace 

 included a graphic model of simulated reservoir 

 water surface elevations for each month from 



588 



