EFFECT OF ENCROACHMENT OF WANAPUM DAM RESERVOIR ON 

 FISH PASSAGE OVER ROCK ISLAND DAM, COLUMBIA RIVER 



Richard L. Majors and Gerald J. Paulik° 



ABSTRACT 



The filling of Wanapum Reservoir in 1964 flooded the lower sections of the three fish ladders at Rock 

 Island Dam, 61 km upstream from Wanapum Dam on the Columbia River. To maintain fish passage 

 under the new hydraulic conditions, the lower portions of the center and left-bank fish ladders of Rock 

 Island Dam were rebuilt and a new sequence of spill patterns inaugurated. The effectiveness of these 

 modifications was evaluated by comparing results from a series of tagging experiments conducted in 

 1964-65 on spring chinook salmon (Oncorhynchus tshawytscha) and sockeye salmon (0. nerka) with the 

 results of similar experiments in 1954-55 before Wanapum Dam was built. These comparisons indicated 

 fish passage over Rock Island Dam had improved substantially between 1954-55 and 1964-65; tagged 

 fish traveled over the dam in a shorter time, and higher percentages of the tagged groups were sighted 

 passing over the dam under postencroachment conditions. 



Successful reproduction of Pacific salmon {On- 

 corhynchus spp.) and steelhead trout (Salmo 

 gairdneri) requires that sufficient numbers of 

 adults in suitable physical condition reach the 

 spawning grounds. Serious consequences can 

 result from delays en route. Thompson (1945) , 

 for example, showed that very few of the sockeye 

 salmon (0. nerka) that were delayed more than 

 12 days by the Hell's Gate rock slide (Fraser 

 River, British Columbia) reached their spawn- 

 ing grounds. Thompson also suggested that 

 shorter delays reduced the reproductive capa- 

 bility of the survivors. Similarly, man-made fa- 

 cilities such as hydroelectric dams, even though 

 equipped with fish-passage facilities, can act as 

 barriers and thus delay or otherwise interfere 

 with the migratory behavior of salmonids on 

 their way to the spawning grounds. 



One of the primary goals of the agencies re- 

 sponsible for conserving the fish resources of the 

 Columbia River is to seek ways of minimizing 

 the eflfects of dams on the migration and spawn- 

 ing success of the river's populations of salmo- 

 nids. Although a variety of solutions to the 



^ National Marine Fisheries Service Fisheries North- 

 west Center, 2725 Montlake Boulevard East, Seattle, 

 WA 98102. 



" Center for Quantitative Science, University of Wash- 

 ington, Seattle, WA 98195. 



problem of dams impeding the passage of mi- 

 grating spawners have been proposed and a 

 number of these have been tried in the field, the 

 pool type of ladder has proven to be the only 

 practical means of passing large numbers of 

 adult salmonids over the dams on the Columbia 

 River. Many research studies aimed at improv- 

 ing fish passage have been conducted over the 

 past several decades. One result of this research 

 has been the introduction of a number of im- 

 provements in design and operation of the pool- 

 and-weir ladder. In some cases fish passage over 

 ladders can be substantially improved by modi- 

 fication of spill patterns (Leman and Paulik, 

 1966). 



The present study was designed to evaluate 

 the eflfectiveness of modifications in the fish lad- 

 ders at Rock Island Dam and changes in the 

 spill pattern which were made after the lower 

 portions of the ladders were flooded by the res- 

 ervoir of Wanapum Dam. This type of problem 

 is apt to become more common as all existing 

 sites for hydroelectric dams are utilized and the 

 reservoir of one dam begins to encroach on the 

 tailrace of the dam immediately above. 



Rock Island Dam, completed in 1934, was the 

 first dam built on the Columbia River. It is in 

 central Washington and about 725 km above the 

 river's mouth (Figure 1). Originally, the dam 



Manuscript accepted July 1971. 



FISHERY BULLETIN: VOL. 70, NO. 1, 1972. 



125 



