Thermal Enrichment 



To offset the undesirable effects of thermal pollution, some 

 electrical companies have announced their intention to install 

 cooling towers. The water would then be recycled within 

 the power plant and would not be discharged into the river. 



Eventually (in salt water) the problem of thermal pollu- 

 tion may disappear into one of thermal enrichment, leading 

 to extensive aquaculture. The matter is complex, however. 

 Meanwhile, the Bureau is racing with time to save the cold- 

 water species of fish from needless sacrifice. 



MIGRATIONS OF ADULT FISH 



Adult salmon and trout migrating upstream are con- 

 fronted with many natural and manmade obstacles (fig. 22). 

 Nearly every dam on the Columbia River has a fishway (or 

 fish ladder) consisting of a series of stepped pools through 

 which the fish can swim over the dam (fig. 23). Because 

 fishways are costly to construct and because delays at dams 

 can be harmful to the fish, our research seeks ways to im- 

 prove the efficiency of fish passage. Some studies are made 

 at dams and along the river; others are made with models 

 in laboratories. 



FIGURE 22.— Ice Harbor Dam on the Snake River. Some 

 races of migrating salmon and trout must pass over many dams 

 to reach spawning areas. 



FIGURE 23.— Chinook salmon ascending the fish ladder at Ice 

 Harbor Dam, Snake River. 



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