FISHERY BULLETIN: VOL. 70. NO. I 



Figure 1. — The Columbia River and locations important 

 to the present study. 



was provided with two pool- and weir-type fish 

 ladders, one adjacent to each bank. A third was 

 added near the middle of the dam in 1936. 



Rock Island Dam was modified during 1951- 

 53. Six new generating units were added to the 

 powerhouse (on the left side of the dam looking 

 downstream), the reservoir was raised about 

 3.7 m, and regulating lift gates were installed 

 in spillway bays 16-37 on the right side of the 

 dam (Figure 2). Turbine discharge was in- 

 creased from about 793 to 2,265 m^ per sec, 

 the fish ladders were altered to enable them to 

 function at the new reservoir level, and the at- 

 traction flow at the entrance to the left ladder 

 was increased to counteract the increased dis- 

 charge from the turbines. Although the fishery 

 agencies requested changes at the lower end of 

 the right ladder to provide better entrance con- 

 ditions and additional attraction flow, no imme- 

 diate action was taken to implement these re- 

 quests. 



The Federal Power Commission, in granting 

 permission for the modification of the dam, re- 



FiGURE 2. — Rock Island Dam showing locations of the 

 fish ladders, powerhouse, and spillway bays. 



served the right to require alteration of the low- 

 er end of the right-bank ladder if substantial 

 evidence were presented to show that alteration 

 was necessary to protect runs of anadromous 

 fish. Any such alteration was to begin before 

 December 1, 1960. 



To determine whether the dam caused loss or 

 delay to the runs and whether loss or delay was 

 associated with the right-bank fishway, tagging 

 studies were conducted in 1954-56. French and 

 Wahle (1966) summarized the results as fol- 

 lows: 



Point estimates of sockeye salmon losses ranged from 

 to 42 percent. Tagging results (one season only) 

 on spring chinook salmon indicated a loss of fish re- 

 leased below the right bank ladder, but no loss when 

 total tag returns from below and above dam releases 

 were compared; data failed to show the dam caused 

 losses of simimer chinook. Tagged salmon released 

 below the dam were delayed 2 to 4 days. Altering 

 the right bank fishway may cause more fish to use it, 

 but there is no clear evidence that such alteration 

 will reduce overall loss or delay.^ 



Although the evidence did not indicate that a 

 major overhaul of the right-bank ladder was 

 justified, one relatively minor change was made. 

 In 1956, a concrete wall was built at the entrance 

 to the ladder. This wall replaced a cyclone fence 



" Seasonal races of chinook salmon (0. tahawytcha) 

 in the Columbia River system are classified as spring, 

 summer, or fall chinook depending on the time of year 

 that the adults enter the river to spawn. 



126 



