The size of the chinook salmon escapements 

 passing Rocky Reach dannsite was estimated 

 by applying to the Rock Island Dam count the 

 ratio of spawners observed in spawning streams 

 above and below Rocky Reach. First, totals 

 of peak Chinook counts for the Wenatchee 

 River system and the river systems above 

 Rocky Reach were compiled, and a ratio 

 calculated of the number of spawners found 

 in the two areas. Then this ratio was applied 

 to the escapements counted at Rock Island 

 Dam to obtain an estimate of the numbers 

 of fish passing Rocky Reach. 



To estimate the number of sockeye salmon 

 passing Rocky Reach damsite, we subtracted 

 the number of sockeye passing Tumwater Dam 

 on the Wenatchee River from the Rock Island 

 counts. This procedure was adopted in prefer- 

 ence to using stream surveys for obtaining 

 the ratios of sockeye salmon in the areas 

 above and below Rocky Reach. 



Chinook Salmon 



Chinook salmon runs passing Rock Island 

 Dam show a multimodal distribution according 

 to the time of arrival (fig. 2). These modes 

 are generally associated with the arrival of 

 different seasonal groups (called races) of 

 Chinook salmon. At Bonneville Dam (the lower- 

 most dam on the Columbia River) three races 

 are identified by their time of migration-- 

 springs, summers, and falls. At Rock Island 

 Dam, Fish and Hanavan (1948) in reporting 

 on the bimodal distribution of the arrival 

 of chinook salmon over the years, separated 

 the spring and summer races on July 9; they 

 did not connment on the existence of a fall 

 run. Meekin (1963) and Pacific Northwest 

 Power Company (1959) identified fall runs 

 at Rock Island. Tagging studies (French and 

 Wahle, 1960) have established that two and 

 possibly the three races at Rock Island pass 

 the dam at different times and that they are 

 separated on the spawning grounds (spring 

 chinook in smaller more rennote tributaries, 

 summer chinook in lower main tributaries, 

 and probably fall chinook in the main Columbia 

 River). From aerial surveys we found most 

 spawning in the Columbia River occurred 

 above Rocky Reach damsite, near the con- 

 fluence of the Okanogan River. 



The exact time of separation of the three 

 races varies from year to year. For the 

 purpose of estimating the abundance of the 

 various races at Rock Island, we have desig- 

 nated spring chinook salmon as those arriving 

 by June 18 to July 9 (the exact date depend- 

 ing on the time of division between modes), 

 sumnner chinook salmon fronn the end of the 

 spring run to the week ending August 27, 

 and fall chinook salmon after August 27. 



7 14 21 28 4 II 18 25 2 9 16 23 30 6 13 20 27 3 10 17 24 I 8 IS 22 

 Mor Jun« July Auoutl S«pt«nb*f October 



Figure 2. — Weekly chinook salmon counts at Rock Island 

 Dam, 1954-60. 



Fall chinook salmon apparently were much 

 more abundant in 1960 than in the previous 

 6 years (fig. 2). This was surprising in view 

 of the generally decreased escapement of this 

 race in other areas. The effects of the new 

 pool formed behind Priest Rapids Dam (lo- 

 cated about 56 miles below Rock Island) may 

 have displaced some fall chinook formerly 

 spawning there and caused them to move up- 

 stream to spawn. 



More spring chinook salmon (61 to 78 per- 

 cent of the total passing Rock Island) were 

 calculated to pass to tributaries above Rocky 

 Reach than to the Wenatchee River (table 1), 

 and conversely, more summer chinook salmon 

 were found in the Wenatchee River than in 

 tributaries above Rocky Reach. 



