above 4,100 c.m.s. during high water and above 

 3,400 c.m.s. during low water are discharged 

 through the spillway (flows through the fish 

 ladders are an insignificant portion of the total 

 flow). 



Columbia River chinook salmon migrations 

 may be separated into spring, summer, and fall 

 runs by their time of appearance at Bonneville 

 Dam. For the purpose of this discussion, the 

 spring run is defined as occurring in April and 

 May, the summer run in June and July, and the 

 fall run in August and September. Only a few 

 chinook salmon migrate past Bonneville Dam 

 before April or after September. 



The three runs are characteristically exposed to 

 different river discharges at the dam. The sum- 

 mer chinook salmon run, the smallest of the three, 

 usually migrates upstream during the period of 

 peak river discharge. Less frequently the spring 

 run passes the dam during annual peak flows, al- 

 though both spring and summer runs are always 

 subjected to relatively high flows. During the 

 fall run, flows are relatively low and little water 

 is discharged through the spillway. 



Figure 9 shows the daily flows at the dam in 

 1946-55 for the periods when the central 75 per- 

 cent of the spring, summer, and fall chinook 

 salmon runs were counted. The 75-percent figure 

 was arbitrarily selected to include the major part 

 of each run; 12.5 percent was subtracted from each 

 end of the run to determine the 75-percent period. 

 The dashed line indicates the 7,000-c.m.s. flow 

 level. 



The 7,000-c.m.s. figure was arbitrarily selected 

 as the point above which heavy mortalities occur, 

 based primarily on 1955 information — the year 

 for which the most extensive data are available 

 (fig. 6). The reasons for establishing 7,000 c.m.s. 

 as the critical level are: In 1955, 75 percent of the 

 spring run passed the dam when total river flow 

 was low — between 4,200 and 5,100 c.m.s.; few 

 floating chinook salmon were observed on an in- 

 tensive search of the river during a large spring 

 chinook salmon run (172,000); the maximum 

 daily count past the dam was more than 13,000 

 fish. Therefore, flows under 5,100 c.m.s. were not 

 associated with a high mortality. Increasing num- 

 bers of floating chinook salmon became apparent 

 downriver from the dam by June 12, 1955, after 

 a period when the chinook salmon count was 



^ 



z::^^ 



FALL 



FLOW 



7.000 CMS 



■■7S-PERCENT 



PERIOD 



r r 



10 20 30 10 20 30 10 20 30 10 20 30 10 20 30 10 20 30 

 APR, MAY JUNE JULY aUG. SEPT 



Figure 9. — Columbia River flows (solid lines) at 

 Bonneville Dam during periods (horizontal bars) 

 when 75 percent of the spring, summer, and fall 

 chinook salmon runs passed the dam, 1946-55. 

 Dashed line at 7,000 c.m.s. indicates flow above 

 which serious mortality occurs. 



CHINOOK SALMON MORTALITY IN COLUMBIA RIVER NEAR BONNEVILLE DAM 



483 



