amount required to maintain the desired pond level, 

 the surplus water flows either over the top of the 

 dam or is released under lifting gates. Increased 

 flow raises the water depth just below the dam and 

 thereby decreases the velocity of water flowing under 

 the gates. Under the.se conditions, we saw sockeye 

 salmon swim upstream under the gates, especially 

 when the water depth on the wooden apron just 

 below the dam e.xceeded 12-18 inches. Conse- 

 quently, the number of fish passing through the 

 fishways was not always a reliable inde.x of the 

 number passing upstream. 



Sockeye salmon have been counted and tags ob- 

 served (when present) at Rock Island Dam since 

 1933 and at Rocky Reach Dam since 1961. Com- 

 plete counts are obtained at Rock Island Dam. 

 During midsummer, when sockeye salmon are 

 migrating, the counting gates near the exits of the 

 three fish ladders are open during daylight but closed 

 at night. At Rocky Reach Dam, on the other hand, 

 the counting gate near the exit of the single fish 



ladder is open 24 hours daily. Fish are counted 50 

 minutes per hour from 5 a.m. to 9 p.m. The 50- 

 minute counts are multiplied by 1.2 to estimate the 

 total hourly count. A nighttime correction factor 

 is obtained by counting 24 hours per day once a week. 

 All fish-count data from Rocky Reach Dam used in 

 this report have been corrected by both the hourly 

 and nighttime factors. 



STREAM FLOW AND TEMPERATURE 



The sockeye salmon migration between Rock 

 Island and Zosel Dams is marked by movement from 

 a larger, cooler river to a smaller, warmer stream. 

 Comparative data on stream flow and temperature 

 are, therefore, potentially important to this study. 

 Data provided by the annual surface-water reports 

 of the U.S. Geological Survey show, however, that 

 the flow of both the Columbia and Okanogan Rivers 

 generally decreases during July and August and has 

 no apparent effect on the migration of sockeye 

 salmon from Rock Island Dam to Zosel Dam. 



Figure 3. — Forebay trap, Rock Island Dam. 



MIGRATION OF SOCKEYE SALMON 



135 



