ducing levels of supersaturation; however, fish pas- 

 sage through the bulkheads resulted in a high (50%) 

 mortality rate and their use was discontinued 

 during periods of downstream salmonid migration. 

 • Spillway deflectors to reduce plunging 

 were designed and tested. These have resulted in a 

 substantial reduction of saturation levels below 

 the spillways, and the deflectors are planned for in- 

 stallation at several dams in the next few years. The 

 deflectors are designed to break the fall of spilling 

 water and deflect the water and entrained air across 

 the surface of the stilling basin rather than per- 

 mitting it to plunge to depths where hydrostatic 

 pressure forces the air to dissolve into the water. A 

 constraint on the design was to permit normal 

 plunge basin operation during very high flow years 

 to dissipate the energy of the spilled water and pre- 

 vent structural damage to the dam. Thus, the de- 

 flectors were designed for a 1-in-10-yr flood. Tests 

 indicated up to a 15% reduction in saturation values 

 when the forebay was normally saturated and a 10 



to 20% degasification when the forebay was super- 

 saturated at Lower Monumental Dam. Tests of coho, 

 chinook, and steelhead survival passing over 

 normal spillway bays and those with deflectors 

 indicated results ranging from no significant dif- 

 ference to greatly improved survival with the 

 deflectors. 



A final method of mitigation which was dis- 

 cussed was transportation of salmonids around the 

 dams. In this scheme, which has been tested with 

 good results, screening devices are installed before 

 the turbines of an upstream dam and downstream 

 migrants transported by truck to a location below 

 Bonneville Dam. Initial tests are favorable with a 

 greatly increased rate-of-return. Estimates are that 

 40% of the migrants could be captured at a given 

 Snake River dam, meaning that about two million 

 chinook and two to three million steelhead 

 would be transported. Research on screening con- 

 figuration is continuing to minimize capture dam- 

 age (primarily scaling). 



Round Table Discussion 779 



