87 



hydroelectric dams were completed and their reservoirs 

 were created. Seventy percent of the 471 miles from the 

 mouth of the Columbia River to Lewiston/Clarkston on the 

 Snake River has been converted from free-flowing river into 

 reservoirs. 



Juvenile salmon (called smolts when they are ready to 

 migrate seaward) must find their way through the reservoirs 

 and past the dams. Smo!t-to-adult survival rates of Snake 

 River salmon stocks have declined to the point where they 

 equal about one-fourth the rates observed in the 1960s, and 

 migration time for smolts from the Snake River to the 

 Columbia River estuary has doubled. The longer the smolt 

 migration time, the greater the risks of predation and 

 disease. 



Smolts may pass the dams through different routes; those 

 passing over the spillway or using the bypass systems have 

 higher survival rates than those that pass through the 

 turbines. Improvements in bypass systems at several of the 

 dams have helped make passage safer for the fish. Before 

 the dams were built, flows in the spring were higher and 

 even in years of low flow, there was sufficient current to 

 carry the smolts to the sea. Then, as now, most of the 

 smolts migrated at night which might be a way of avoiding 

 predators. 



Not only did the dams slow the rate of downstream travel 

 for smolts, they increased the amount of habitat favorable to 

 predator species and provided physical conditions that 

 improved the predators' abiUty to detect and capture 

 juvenile salmon. Hatchery fish and exotic species have also 

 been introduced into the system, and they compete with, and 

 prey on, chinook and sockeye. 



The construction of hundreds of large and small reservoirs 

 has physically altered the original hydrology of the 

 Columbia River Basin. The flows for the lower Snake and 

 Columbia rivers have been changed from natural conditions 

 to those considered more desirable for human uses. The 

 large late-spring and early-sununer runoffs and small late- 

 autumn and winter flows have been changed so that now 

 there is a much smaller spring-summer runoff. A complex 

 management system has been developed to manage the 

 basin's reservoirs and flows. The system was primarily 

 developed to control floods, provide water for irrigation, and 

 serve the hydropower system by storing water for use when 

 power demand is high. 



In addition to the sections of the rivers that have dams, the 

 146 mile section of free-flowing Columbia River 

 downstream from Bonneville Dam, and the Columbia River 

 plume that extends into the Pacific Ocean have been altered 

 in ways that may reduce the survival of smolts migrating to 

 sea. Little research has been conducted on the effects of 

 reduced flows and turbidity on the survival of smolts in the 

 lower Columbia River and the estuary. However, it seems 

 likely that reductions in flows and turbidity in the spring 

 have not only increased the smolts' travel time, but also 

 added to their risk of predation on their way to the estuary. 

 Reductions in flow have also decreased the size of the river 

 plume extending into the ocean (an environmental factor 

 which offers the smolts some concealment from predators 

 when they first reach the ocean). 



Little is known about survival rates of salmon once they 

 enter the Pacific Ocean, but there is growing evidence that 

 survival rates are not uniform from year to year and that 

 variations in ocean productivity may affect stocks over a 



John Day Dam 



(Corps of Engineers Photo) 



